hello in this video we are going to begin our conversation of the major categories of chemical reactions so when i put things together what kind of change do i expect and so for that we're going to emphasize recognizing patterns these are all of the chemical reaction patterns that we're going to discuss some of them some may sound familiar because we've talked about them a bit earlier on in the course so these are synthesis or combination reactions decomposition single displacement also called single replacement double displacement also called double replacement and combustion in this video we're going to focus on synthesis and decomposition reactions so we are familiar with synthesis because we talked about ionic compounds and making them already and so this is a little bit of review synthesis in general is when two or more reactants make one product so where synthesis means make um so any time we're combining um two things to make one thing that is going to be a synthesis the major examples that we have um are from our ionic compound chapter so synthesis of oh sorry for that of binary ionic compounds we have sodium and sulfur solid sodium and solid sulfur what does that make well that's going to be metal and non-metal make ionic compound that's remember from that chapter which ionic compound well sodium and sulfur are going to make sodium sulfide what is the formula for that well we can show our work in the margin of our paper by saying okay we remember how to do the formulas of ionic compounds we put the charges 2 minus we crisscross that's going to give us na2 s and then what phase of matter is this solid so pure ionic compounds come out solid because they have strong forces holding them together they are the solid phase now now here's where we add what we've been learning in previous videos balance do we need to do it yes there are two sodium here so there needs to be two sodium there two sodium and one sulfur make one sodium sulfide and this is in terms of either atoms or the unit moles because they're both counting two moles of sodium and a mole of sulfur make a mole of sodium sulfide now how about this one again you should also be comfortable writing a chemical equation as a from a word problem calcium metal reacts with oxygen gas go ahead pause the video and determine what does this mean as far as ingredients go and in this context what is our product okay this is what i get so far so calcium metal reacts so it's changing with oxygen gas remember oxygen is a diatomic 7 o2 gas all right makes and now we should be comfortable doing this what does calcium and oxygen make metal [Music] non-metal they make ionic compound so we're gonna do crisscross calcium has a plus two o is minus two crisscross but then we reduce because these are empirical formulas ionic compounds are always empirical formulas so we reduce cao our final step is check balance is it balanced is there the same atoms on each side no we need a two here because of the oxygens two oxygen now two oxygen calcium we have two so this actually needs to be a 2. so what should we take away from this whenev what is the pattern here metal and non-metal make ionic compound this is synthesis this is an example of synthesis you should be comfortable if it's two substances becoming one recognizing that it's synthesis if it is a ionic compound synthesis you should be comfortable predicting the product and then writing the balanced chemical equation um what about the phases so just a reminder we've talked about this before let's go ahead on this periodic table and highlight the different um different sort of categories or phases of matter that happen at room temperature so that we know which subscripts phase labels to put for each one so first oh before we do that which ones are our diatomic seven again we should be really comfortable with those because when we're making chemical changes we're doing it at room temperature in the natural form of the substance and the natural state of seven elements is as a diatomic okay so i've highlighted our diatomic seven remember it's hydrogen and there's sort of a little seven that happens there nitrogen through iodine make a seven and that means their formulas are always going to be x 2 whatever the symbol is with a little 2 subscript um how about which elements are gases at room temperature so go ahead think about it do you remember which ones they are okay and here they are so it's most of the right corner there's a little funny shape that happens here actually most of the diatomic seven is a is uh in the gas phase so hydrogen nitrogen oxygen fluorine chlorine are gases but bromine and iodine are not and then don't forget the noble gases that should be a sort of giveaway the name even says noble gases how about which ones are liquids at room temperature go ahead and think about it so these are our liquids there are only two bromine and mercury are the only two liquid elements at room temperature the solids the rest of the elements are solid that is how we're going to know which phase to put at room temperature we're going to know our corner ish area and hydrogen are gases there are only two liquids and the rest are going to be solids as far as writing the formulas the elements themselves are just their element symbol except we need to remember the diatomic seven those are going to be the element symbol with a little two subscript decomposition is the opposite of synthesis it's when two or uh one substance splits into two or more substances and so for every synthesis that we can have we can have its decomposition as well so for example let's write the equation for solid mercury two oxide decomposes back to its elements at room temperature so mercury 2 mercury is hg what does the two mean two plus oxide so this is an ionic compound so if we don't know the formula don't be afraid to actually do the crisscross so that means this is hg oh and now it's solid it says that it's solid decomposes means turns into so there's the arrow back to its element so what is the elemental form of mercury and oxygen h g liquid plus oh to gas what is our final step that we just learned a few a video ago check for balance are there the same atoms on each side so no we're gonna need a two here for the oxygens and now a two here for the mercury so this is the decomposition of mercury two oxide two mercury two oxide become two mercury and one oxygen molecule there is a special kind of decomposition that we need to be aware of especially if you're going on into chem 1a because you'll study it in a lab when heated metal carbonates decompose into co2 and a metal oxide what does this mean well let's look at it through an example cuc3 is what is its name copper ii carbonate so when heated metal carbonates so this is an example it's carbonate and a metal so copper two carbonate decompose turn into it always has the same pattern a metal carbony decomposes into co2 and the metal oxide so notice the cu remains with the leftover oxygen two of the oxygens left one remained as far as phases co2 is a gas but ionic compounds are always solid how can we sort of rationalize what's happening well when we heat um a carbonate there's literally one carbon and three oxygens and these atoms are going to leave as the co2 gas so what we can do is sort of do a sort of subtraction method to figure out what's going to happen if i take these atoms cu and then co3 and i chop off carbon and two oxygens what am i left with well the co2 has left and we're left with c u and o so that's kind of one way that i i sort of advise students to sort of help them as they study to predict the product um you can always get it because we're going to make the metal oxide so copper ii oxide would have this formula but another way to do it is if you took the formula cuco3 and you subtracted co2 from it because co2 is leaving then what's left over it's kind of like well if i take away a c then these go away if i take away two oxygens i'm left with one so what's left c u and o and that's how we would get our answer why is this an important reaction um well i mentioned that you'll see it again in chem 1a but also um this reaction this kind of reaction is um a big part of the um concrete making process there's actually a lot of concrete in our world there's concrete sidewalks concrete buildings concrete lots of things as concrete dries it actually undergoes this kind of reaction only it uses oops sorry about that it uses a calcium carbonate so this is in concrete so it uses calcium carbonate as the calcium carbonate dries over time and even after it's completely dry it's still reacting it's actually losing carbon dioxide during this process so the calcium carbonate goes through just replace the cu with ca and that's what happens in concrete now this is a big problem because there's a lot of concrete in the world and this concrete is slowly emitting co2 and as we may know co2 is actually a problem because it's a greenhouse gas so maybe um this is this is sort of an interesting new fact for you but the concrete production concrete industry is a major emitter of co2 so one sort of goal that we have for ourselves as you know communities is um to sort of look for alternative building materials other than concrete because concrete is actually negatively impacting the climate change sort of scenario that's happening right now so just a fun fact about this to sort of give a give a reason why this is an important reaction it's happening right now and it's not necessarily a good thing but it's an important thing because you know we needed concrete for buildings for many generations so we can think this reaction and then sort of say okay we should try to look for things that don't do this reaction as well but i digress what should you know synthesis elements coming together to make art to make um a new compound decomposition compound breaking apart it doesn't have to be elements like in this example but it just means breaking apart well synthesis is forming or making something you should be comfortable how to predict when metals and non-metals come together what ionic compound they make that's the synthesis reaction that we're going to focus on and then when they decompose they go back to their elements that's something you should know carbonate decomposition is a special reaction that is of importance to our world and you should know what happens just by me saying um calcium carbonate is heated what will it do that's all i should be able to say and you'll know that it turns into co2 and then the leftover in this case it would be cao if it helps try the subtraction method if i take away a co2 what is what formula am i left with that might be helpful to figure out the answer however you can always get it by thinking what's the metal oxide in this case so copper 2 oxide would be co uo