Transcript for:
Understanding Group Two Elements

our group two elements right so group two elements are here so the elements such as burum magnesium calcium strontium barium and of course radium ending with FR wait yeah radium let end with radium right so radium right would be the last one there these are all our group two elements and we know that our group two elements are part of the S block right right there are s block Els so let's start off here the atomic numbers from berum going to radium right so we have verum being being four magnesium is 12 calcium is 20 strum is 38 barium is 56 and radium is 88 and let's look at these properties now while we look at this table and scan this table to look at the trends because I want it to look and then for yourself you can figure out the trends because we can read with understanding right let's look at what the syllabus wants us to understand we must explain the variations in the properties of the elements right in terms of the structure and bonding so we must specifically speak about the atomic radi and the ionic radi right as well as ionization energies you must be able to describe the reactions of these elements oxygen water and dilute acids explain the variation of the solubility of their sulfates explain the variation of the in the thermal decomposition of carbonates and nitrat and we must also discuss the use of some of these compounds such as magnesium well the compounds of magnesium and calcium right um we have to look at those uses right so we know that magnesium and calcium can magnesium oxide and calcium oxide can and as well as their hydroxides and carbonates can use for many important or many daily practices let's say that many daily practices all right so that's what we're supposed to be looking at so here we already have our metallic radius which are our atomic radius technically right and then we have our an radius here and our density of these comp BS or well these elements the melting points and the first ionization engines and I hope we've all read them and looked at the trends that we can see all right so the trends in atomic radius we know that atomic radius always decreases going down a group and well sorry increas is going down a group and decreas is going across a period so down every group a atomic radius must decrease well increase I'm mixing up my increase decrease but yeah it's increasing right it gets larger atoms get larger going down right ionic now we know the ions if the atoms get larger then the ions get larger that must make sense right that must make sense there guys all right so hold on I hear somebody joing the meeting but I'm not me on my phone give me a moment let me just let the real quick but yeah so we have that there right our density now we know that as we go down the group density increases we know what density is yes everybody should know what density is the density increases going down the right the melting point as well we know that metallic go ahead the milting point Bing point we have that there and then the first end energy is actually increasing going well decreasing going let's have a look at this right Atomic and metallic radius now the metallic and ionic radi right both decre increase right so we know why successive energy levels right um we have screening effects we have the fact that the orbital shielding is different all of that do we have to explain this the first thing we we explained this before in group four right have to go back through that you don't have to so just know the atomic radi increase going down all right lovely density now the density depends on the mass of the atoms and the way they are packed in a in the ltis right so at higher levels of chemistry then we're going to look at um packing right FCC and PCC packing and stuff like that right but at this level we just talk about the fact that okay they're P differently all right so from calcium to Barium there's an increasing density then from burum and magnesium have higher densities than calcium because they are packed more um efficiently in the lattice with less wasted space right so that's that's what's happening I won't get into what packing is it is not necessary at this level all right so it's just that's just information there ionization energy now the ionization energy is the energy needed for the complete removal of a mole of electrons from the outermost shell of a mole of atoms to produce a mole of cations right so this onc understanding the group the number of shells increase therefore um the outer shells are more shielded from the nucleus and less closely held right and easier to remove therefore the ionization energy of the atoms decreased down the group the first ionization energy right so we have that there we know that already metallic Point melting points no sorry rather about that metallic Point um melting points right the stronger the metallic bond the harder it is to separate the cations so what does this mean this means that the melting point and hardness are higher if the metallic bond is stronger or the metallic bonding is stronger so on descending the group the size of the cations increase and the charge of the of the cations remain constant right so the charge density does decreases and the attraction between the cat and delocalized electrons actually decrease thus there's less energy required to melt structure the melting points and hardness therefore decreases right so that's what we have there hoping that we're following right so we know that the m so for example melting point of radium is actually 700° C and the melting point of barium is actually 730° C so note how the melting point is decreasing because the charge density decreases right so the charge density decreases the attraction to local deiz electrons actually decrease and that means that the metallic structure itself is weaker and that also translates the metallic so the to the melting points of the hardness um decreasing right so that's what happened so we have that there we're now as look at the chemical reactions let's look at the sub of these things right what's happening now go ahead Sur can you explain why the melting point of magnesium is less than the melting point of calcium please all right so the melting point of magnesium okay hold on there's a difference there hold on did I not catch that aha okay I I forgot about that right so the melting point of magnesium right it's actually smaller does anybody know how to explain that what is the reason s m go ahead so is it because magnesium has a different um L structure it because Mages L structure all right so that is one approach okay does it do it is true it is true all right so it is a part of the packing right that we were talking about the density of the elements right so burum and magnesium has higher densities right than that of calcium right so there's a difference here right so the strength of the bonds are important with the melting points here right so although we we know that um magnesium right has a stronger um metallic bond than calcium all right well let's look at this now the Magnesium atoms are actually Loosely packed compared to calcium right so magnesium atoms are Loosely packed compared to calcium right so what does that mean h there's less energy needed to break the metallic bond in magnesium and therefore we would know that the lower melting point must be attributed um to it right so this would have a lower melting Co than calcium notice that it's lower than that of calcium right why is that right magnesium atoms are actually Loosely packed compared to that of calcium so since their packing structure their ltis structure is different as the person rightly stated right it's going to be easier to isolate magnesium atoms rather than calcium atoms and if it's easier to isolate them that means that the melting point is actually going to be lower right than compared to the energy required to isolate the calcium atom right so the melting point of magnesium should be lower than that of calcium because of the structure of the latice all right magum is less dense and calcium all right okay hopefully that covered it was it a ask is that F milon yes sir all right lovely anything yeah if anything else becomes foggy just ask okay so okay so we know that if we have a negative um e not we will lose electrons more we know that right so we're looking at the well we know what enots are we know done module too long time no but all that is about right so so we're here here saying that burum is actually so notum barium is actually the element that is most reactive so the reactivity of group two elements increase as we go down the reactivity all right so the reactivity actually increases as we go down so Barum is more reactive than strum which is more reactive than calcium which is more reactive than magnesium sorry yeah magnesium all right so let's look at this now so since the reactivity increase is going down we can see some Trends here with the reaction of oxygen water and and acid right so with what with oxygen now first of all group two elements what is the name of group two the traditional name for group two these are what alkaline earth metals the alkaline earth metals lovely gery right so Al earth metals right so therefore right when we what therefore they're going to show Regular reactions with oxygen right but different with water so let's just have a look at this now we can see that magnesium itself right will actually burn with a bright brilliant bright flame right White Flame actually so it's a white flame keep in that mind right with oxygen to form magnesium oxide right all burn readily oxygen right so the reactions are highly exothermic and reactivity increases down the group so all the group of elements tarnish in the air as a layer of oxide is formed on the surface of the metal right so if we are to I don't let me see if this will make sense give me one moment let me look at something all right so if we are to really look at that we're going to have a quat okay I want I what I wanted to really look at is whether or not you guys required a picture but if you guys Google right any picture of any of these um any like calcium or magnesium right you should actually see right you should actually see um the fact that when you cut it right it's much shinier when we cut it right and then after a while our oxide layer will cover it right and that is one of the things that we can tell it's reactivity from right so it tarnishes in air the oxides are basic in character so upon reacting these oxides with water they will form hydroxides right so calcium oxide would actually form lime water right cuz calcium hydroxide is lime that's why it's called lime right so lime as in limone in calite and those type of minerals we find them so lime in water is just lime water that's basically so calcium oxide in water gives us calcium hydroxide lovely right reactions of the metals with water directly with water not its oxides but itself in water will actually give us hydroxides because they're alkaline earth metals we know that they will form basic o b basic hydroxides in water all right calcium strong to amarium all well let's look at this magnesium actually reacts very slowly in cold water so it's reacts very very slowly when the water is um cool but react reacts rapidly in Steam to form magnesium oxide and hydrogen gas so that's what's happening here so in the water here right at a low temperature and water here a high temperature how do we know the difference in temperature this here is gaseous and this here is liquid right so it's reacts in Steam to form magnesium oxide and hydrogen gas right but in water reacts slowly regular cold water reacts slowly to form magnesium hydroxide right so you react slowly with water producing that hydroxide there so this reaction soon stops as the magnesium hydroxide is insoluble and then there's a barrier between the Magnesium preventing further reaction so after it drops into the water it's going to form hydroxide quoting and then remain there right with calcium strum and barium they all re in water increas in increasing Vigor right to produce metal hydroxides and hydrogen gas right so the hydroxides are not very soluble so we know the solubility of the group two hydroxides decreases as we go down the group the solubility of the group two hydroxides decree um sorry increases right if they are not really solubility increases my bad right so it actually increases going down the group so here the solubility the group hydroxides increases the right so when we create these metal hydroxide hydroxides are not very soluble themselves you know right but barium hydroxide would be more stable that well more soluble rather than strontium hydroxide and Etc going up the group right so the solubility of hydroxide increas is going down the group th less precipitates are seen right down the group because more hydroxides dissolve in water so white precipitate of calcium hydroxide that is formed when calcium reacts um with water and it is dissolved right to form a weakly alkaline solution all right so that's what we have there so the this the resulting Solutions are more alkaline going down the group of course so basicity also increases going down the group so basicity increase going down the group solubility in of the hydroxides increase going down the group reactivity increases going down the group so lot of them tend to increase going down the all right reactions with acids now right so a metal and acid will give you hydrogen gas right so let's look at this all the groups of metals would produce hydrogen gas and a salt right so let's look at this if it's reacting magnesium with um the hyic acid it's going to give you magnesium chloride and hydrogen right and we know that reactivity increases down the group therefore calcium will be more rigorous than that of something like magnesium okay so calcium reacts more rigorously with acids while barium reacts violently right so we're looking at stuff like that with sulfuric acid reacts with barium in Sol barium sulfate forms right which quotes the metal and then prevents it from reacting right so we have stuff like that happening so generally what this is saying is that generally when we react with our all our acids with hydrogen chloride or hydrochloric acid it will form a chloride and hydrogen gas right and when we're going to react most of our Metals with sulfuric acid would create a sulfate and hydren gas but something like a burum right burum sulfates are generally more soluble because solubility of suat actually um decrease going down the group okay so if we're looking at this now the solubility for the group two compounds let me see if this is relevant information great lce ions release energy hydrate this is the explanation for the solubility right um specifically so this is in terms of the um lce energy and hydration energy so with no solubility low solubility results when the ltis energy exceeds the hydration energy I hope you guys remember that I'm not sure if we did in solubility product right but when the lce energy is larger than that of the hydration energy then you're going to see that the substance is actually insoluble right so solid ability product is actually somewhat proportional to the lce energy all right so this is just information explaining I'm going to send you guys the document I can pause and look at it right so this just an explation explanation of it specifically but you won't necessarily use a lot of this information right um when answering the group two questions right but we will make mention to um to the en latis enthalpy and eny of hydration right we're going to make mention so ltis enthalpy is great and when the L enthalpy exceeds the enthalpy of hydration the substance is insoluble right and when the enthalpy of hydration exceeds the L energy the substance is soluble hopefully we can remember that from um solubility product all right the sulfates now right the lat energy decreas is going down the group excuse me can you repeat that please okay can you repeat that please okay so let's repeat that so the lce energy if the lattice energy is larger than the enthalpy of hydration of the substance right what we're going to have is the substance being less soluble that is because what is lce energy can somebody Define that what is lat energy there what's that the energy to break the lce into ions okay so the energy required to break the lce into its constient ions lovely right but what is the enthalpy of hydration the energy the ion releases when it's already in it ion all right so the energy I energy releases when the ions are fully dissolved right so what we're saying you know if the energy right if L energy is greater than that of length of the have hydration then when the thing is dissolve is going to be put into water right L energy is going to be way greater than the energy required to actually dissolve the substance therefore it will be insoluble but what happens when we have an enal of hydration um being actually greater than that of the lce energy we're going to actually have the substance dissolving right so it's going to be soluble so that's what we're talking about so if the lce energy right so as we stated here the lce energy specifically attraction of those where is it where is the statement the Lattis energy exceeds the hydration energy then the substance is actually more insol but if the hydration energy exceeds the lce energy then we're going to have the substance being solid all right and we can make reference it here if the lce energy decreases down the group then we can note the solubility of the sulfates right because as you can see here the solubility of the sulfates actually decrease going down the group so barium sulfate is more is more insol than strontium sulfate which is more insoluble than calcium sulfate which is more insoluble than magnesium sulfate all right hopefully that makes sense so we decrease in that's enthalpy right going down the group so thecase actually Fair small fairly small right so there's not really a large difference in the solubility but you know the solubility changes right so the size of cat increases how this in well increases is however this increase in the size is Nega compared to the large size of an an all right so we saying is know is that the en the enthalpy is fairly small because it is determined more by the size of the large sulfate ion more so than the C right remember the size of the ions can be used to determine right our ltis energy remember we spoke about that and we did that in the first marathon we spoke about that the size and distance of ions determine the lce energy but in this case we have a low lce energy right because more so because of the large size of the sulfate ion more so than the the cation that we deal with right so bullet points now the enthalpy change of hydration also increases in the same order it becomes more positive because if we know this now if we have a decrease in Lattis energy we are going to have an increase in hydration energy or the enal of hydration right so this decrease so we're going to have that right it becomes more positive right however this this increase is relatively larger because it is determined more by the size of the cation than the anion so what we're looking at you know hydration is more likely determined by the size of the cation right and ltis energy is more likely determined by the size of the annion and not the cation right so as the size of the cation increases going down the group The Charge density decreases and this decreases the force of attraction between the ions and the and with the polar water molecules right so the charge density is decreasing as we go down the right cuz the larger the ion is the larger the atom is right the larger the ion rather right the larger the ion the smaller the charge density in this case right as we go down the group charge density decreases and charge density can Define how much electrostatic attraction we have right between the anion and the cation so if our charge density decreases our force of attraction will decrease right so the enthalpy of solution gets more endothermic going down the group and we know what the enthalpy of solution is the enthalpy of solution speaks to what go ahead so the energy change sir the the energy change in which occurs when one Mo of aute disolves completely all right go ahead formin solution understand that okay so when completely dissolves in the solution right so we're just keeping it at that concept here just for this all right so when it dissolves right so the complete energy change when it dissolves right if that now becomes more endothermic right specifically endothermic are positive values right we're actually going to act um decrease the solubility right so if the energy required to dissolve the solution becomes so great right there's going to be a point in which we cannot overcome it right therefore it will remain insoluble right so it's same thing with solubility constants right when we speak about KSP we have to increase the temperature of the solution in order to dissolve so we need to increase the temperature so we can match the enthalpy of solution so we can allow the enthalpy of hydration to surpass the lce energy so the thing can dissolve right so what this is really speaking about is the main reason why things dissolve because if we put like sodium chloride in water we just take it for granted that it's dissolved but what's happening with the Sodium Chloride is that the en hydration surpasses a l energy of sodium chloride and there's um there's enough um kinetic energy right within the solution for the en Your solution to be reached therefore the salt dissolves right so it's all of that that takes place right that remember that chemistry is a study of these things study of why these things happen right so the solubility of the group two sulfates right actually decreases going down the group and in conversely it increases going up the group simple and that's that's a solubility we have there right we know that solubility speaks to the mole per 100 G of water right the mole that the mole of substance increase um in the solution per 100 G of water right so that's what solubility all right will speak to in this case all right so we know it decreases going down the group for the solubility of our carbonates now um Sol soluble carbonates um solity of the carbonates actually decreased going down the group right it's generally the same thing here so the decreasing solubility is similar to the sulfate so we have to speak about the ltis energy exceeding the hydration energy and the enthalpy of solution becomes more positive going down the group therefore carbonates decrease in solubility going down the group so which will be more soluble are strum carbonate or magnesium carbonate which is more soluble strum carbonate or magnesium magesium you're saying the strum carbonate magnesium Mages be magnesium right cuz magnesium is above strum in the series right in thep all right and for hydroxides now right solubility of hydroxide increased going down the group and we look at the hydroxide I right the hydroxide ion is actually very small right compar compared to sulfates and carbonates therefore the L energy actually decreases significantly as we go down the group because the large um positive ion the cation would actually be the thing that we are looking at right here so the so the size of the catalon will actually be which oh will actually have more significance right will actually have more significance compared to the hydroxide ion so if you look at it you know the larger the anion the more likely the thing is to be insoluble right because a small cation and a large anion all right small cation can actually polarize the orbitals existing around the large anion right hopefully that it's expressed here okay it's expressed in thermal stability right I think it's expressed in termal stability ofit right but it's similar to what was happening up there hopefully let me see if there's a diagram so the diagram here will try will attempt to explain so I'm going to explain polarizability down there all right it's something um similar to what was happening in our um attractions force of attractions right so thermal stability of the group two compounds now we did stated in qualitative analysis earlier that carbonates tend to be thermally um well not they tend not to be therm table right but they have different thermal stabilities so let's look at this now right thermal stability basically speaks to the fact right how easily right are these compounds decomposed at certain temperatures right so is it that if we change the temperature to 300° is it going to just decompose into carbon dioxide oxygen and and its parts like that or is it going to remain stable at high temperatures right so nitrates now the nitrates of group two undergo thermal DEC composition to give the metal oxide and nitrogen dioxide gas so we know if we Heat this group two metal oxides this is what we get this is literally a reaction that we can see coming out from qualitative analysis right it is used in the determination of our nitrates right so going down the group The nitrates become more stable so higher temperatures are required to decompose them so the thermal stability of nitrates increases going down the right so as we go down the group the size of the cations right themselves decreases right therefore there's a polarizing effect that occurs right so the smaller highly charged cations are more polarizing and tend to attract electrons clouds right of the nitrate ion enough to weaken the structure and break the bonds right so looking at that let me let me actually change this let me look at that for a second give us one give me one moment let me look over this all right polarizing effect small all right that should be fine so if we're looking at the size of the cation right the cation can actually um what what we call polarize the molecule right so the smaller it is right the higher the charge density so it tends to increase the poles right the strength of the poles on the molecule itself all right it's giving us one second here just pausing for a second all right so we don't want to run across so I believe there is a huh this seems so much like an interesting all right so we have the increasing size atomic size going down the group so there's a decrease in katonic size right so therefore so we're looking at the polarizing effects specifically right so the more a substance is polarized the easy it easier it is right to actually um break the bond and that's because the degree of Co valency actually decreases right so what does Co valency speak to we know that calent bonds right are are um will arise from situations in which compounds right or elements actually share electrons right if we continue to increase the polarization on the molecule and one becomes increasingly positive and the other becomes increasingly negative the electrons are going to actually be um held right much closer to themselves right so what's going to happen here right if we have two strongly positive well strongly charged right strongly charged molecules or particles right what's going to happen is that electron affinity and ionization energy will take um hold right so the one that should be losing right will achieve it its electron um affinity and the one that should be positive will achieve its ionization energy and then they will actually switch electrons so electrons will swap our Redux reaction will occur right and we're going to have the creation of ions right so we know that ionic substances are substances that are held together by electrostatic forces so if we have a calent molecule and we're increasing the charge right increasing positive charge or increasing negative charge right is going to increase so much so it acts like two completely charged particles and no electrostatic forces will act to ensue so the more we polarize the molecules the more they behave like ionic compounds right so they lose what we call the co valency so it decreases their their competency for Co valent bonds and ionic substances specifically right um yeah so it actually increases um their bond specifically right so it actually decreases Co valent bonds right the strength of Co valency the coent bonds and the bonds what they're making now would actually be ionic in nature right so it strengthens their bonds specifically right so I hope you guys get that part though it sounds kind of murky but what we're seeing is that the compounds get so much charge right so polarized one become so positive and other become so negative it behaves like it's ionic right so it co valency will decrease therefore majority of the energ energ keeping the substances together will be electrostatic right and because the distance between them are going to be so small right it's going to I'm hoping if people if you guys do physics it's going to be similar to coolum's law the two charge part particles are going to be so close together the the energy required to move them apart will be way more right would be greater than if it was Co valent so it's going to have strong ionic bond characteristics right rather than its regular coent Bond characteristics right and that makes the compounds more stable because we know that ionic compounds are more stable thermally than coent compounds so the more ionic you are the more polarized you are as a coal compound the stronger your bond and that's what's happening with the nitrates right so the nitrates stability right actually increases right as we are actually um going down the gr because that charge density is now increasing right and you're able to polarize the molecule better so your molecule loses its Co valency and has more electrostatic interactions allowing the bond to be way more stronger and not easily thermally decomposed hopefully that wraps it up there right same thing for our carbonates the carbonates of group two undergo thermal decomposition to give us carbon dioxide gas and our oxide right so let's look at this group two catons have the same charge 2 plus but why um do they change why does thermal decomposition change or thermal stability change as we down the group as we go down the group the size of the metal C increases right the smaller the cat the better it is at distorting the electron cloud right of the larger carbonate the smaller ions are better polarizers of larger ions so you have the polarizing effect right so group two carbonates with small ions of a greater degree of Co valency right in the ionic bonding so the greater the degree of Co valency the less energy required to break the bond so hopefully that makes sense so what's happening here now we have a group two metal right here specifically right and we have the localized electrons existing in our carbonate because we know our carbonate actually has a has a resonance structure so have the localized electrons here so the oxygen atom is well on it way to become an oxide atom right why because there's going to be some electrostatic interaction between these two so two differently charged molecules right so the smaller molecule the atom the not the atom the cation is the more polarizing it is right and it allows for this bond to lose its Co valency and become more ionic all right so that's what's happening there now for the use of group two elements now we know that magnesium is used commonly in group two right usually for the creation of lightweight Alloys right so um magnalium right so magnalium is an example of alloy between magnesium and aluminum right so we can use magnalium as an alloy right it is used in the manufacturer of cars and aircrafts right and we also have magnesium oxides right which are used in refin lining of furnaces right at high melting points and low because of high melting point and low reactivity right so we have that there so we know the melting point and reactivity of our substances so we should be able to relate them right it is using medication to relieve heartburn so usually you see um magnesium in some amount of tablets right so magnes sufate as well is used as a powerful laxative right so I have some examples there and you guys should know um generally magnesium sulfate like epom salt magnesium sulfate heah hydrate that is what epom salt is Right magnesium sulfate hydr it's a coent it's a complex compound right calcium strum and barium have less uses because of their High re activity right carb calcium carbonate though right actually is used in the construction industry so building materials some marble exit um or using marble right or an ex ingredient in some other type of building material so cement Etc right it is used widely in the medicine medical industry so dietary calcium supplements are also there and it can act as an ant acid right calcium carbonate itself right um and calcium oxide and calcium hydroxide are also used in the agricultural industry to contract soil acidity right so when you have runoff and those type of things due to higher rainfall right we can actually replenish the alkaline substituents in the soil right and we know what's happening there right cuz if we have run off of minerals which are alkaline right the microorganisms in the soil will continue to create acids right and increase the soil acidity so what we need to do is counteract that by adding these compounds these alkaline earth metals back into the Earth right so these are also used in water treatment so it reduce a acidity as in flocculant flocculant is something that we use in the treatment of water right in the indust in the industry specifically they are also used as scrubbers to duluze waste gases and to neutralize acidic effluence right so in the creation of sulfuric acid via the contact process Etc we use these in what we call in the tubes right that release the air right that release um um gases from industry so usually when they have those huge Towers in industrial complexes releasing all that what we call smoke into the air we can use these compounds to actually trap the acidic substances right so we prevent acid rain from being an issue right because acidrain is sulfuric acid when the sulfates go up in the air react with water to create sulfuric acid so if we can use these substances as what we call scrubbers we can desulfurize the gases so we have a a smaller chance of acid right calcium oxide is also what we call Quick lime and reacts exothermically in water to create calcium hydroxide right and it is sufficient to heat or ignite combustible materials in some instances right so we have that there so we can use it to um we can also use calcium hydroxide um to actually um extinguish fires when react it with water as well right we also have barium sulfate right um it can be eaten as a part of a barium meal right so it can be consumed because it's really unreactive in sulfate because we know that barium sulfate is really um insoluble right it can be eaten right and it's good at absorbing xrays right therefore what are we looking at right if magnesium sulfate is actually consumed right it can outline Parts in the guts right um by um using x-rays right so what it's going to do when it goes through the gut you can actually use x-rays to see the path of the barium right sulfate right so although barium ions are very toxic these techniques is harmless because barium sulfate is completely insoluble right so we have that there and we know that our elements here group two elements are using fireworks as well because of the flame test Barum gives a green flame right strum is a Crimson flame right these different types of colors are given out in um Fireworks so when you go to Fireworks shows and see fireworks um exploding when you see the red it is most likely some strontium or lithium right when you see green it's most likely um it's going to be less likely copper because why copper it's going to most likely be barium right so those different brilliant colors that you see in firor displays our metals are being used right on the high temperatures all right so that is basically it for the review of the two metals either in question