[Music] welcome to toomamina teaching this is your second grade nine natural science lesson for term two in this lesson we will be discussing balancing of chemical equations rust and the reaction of metals with oxygen the key term for today is combustion it is the process of burning in the presence of oxygen it is a rapid chemical reaction that releases large amounts of energy in the form of light and heat let us do a quick revision regarding compounds so when we look at carbon dioxide we notice that there are two elements that are bonded together carbon and oxygen there is a subscript in this compound you will see that there's a small two behind the oxygen this means that there are two oxygens bound to carbon this means there is a ratio of one to two between carbon and oxygen when we write an equation for the chemical reaction that results in carbon dioxide we must remember that the reactants will go on their left hand side and the products will go on the right hand side in this instance we know that carbon dioxide is our final product and therefore it will always be on the right hand side when we write a word equation we write out all the words so it will be carbon plus oxygen with an arrow not an equal sign carbon dioxide but when we use a chemical equation we use the symbols that we use on the periodic table so instead of writing carbon now we will simply use the symbol c and instead of writing oxygen we will use the symbol o but please remember in the previous lesson we discussed diatomic molecules and remember there are seven diatomic molecules of which oxygen is one and therefore oxygen must always get a subscript of two when written alone let us take a closer look at another compound water one molecule of water consists of hydrogen and oxygen we will notice that the hydrogen will have a subscript of two meaning that there are two hydrogen atoms bound to one oxygen atom please notice that oxygen does not have a subscript because he's not featured alone in this case the ratio of hydrogen to oxygen is two to one when we write the word equation it will simply be hydrogen plus oxygen with an arrow water please remember that the arrow indicates that a chemical reaction has taken place that means a whole new product was formed it has new properties which makes him completely different from the reactants it's like baking a cake you put in raw eggs but you get a solid cake at the end but now it gets tricky when we write the chemical equation for water we must remember that hydrogen and oxygen are both diatomic molecules so when we write the symbols it must be h2 plus o2 with the arrow pointing to h2o but now when we look at this equation we will notice that there are two hydrogens on the left side and two hydrogens on the right side two oxygen atoms on the left side but only one oxygen atom on the right side this means this equation is not balanced remember when a chemical reaction takes place atoms cannot be lost or gained they are simply rearranged that means if i started with two i must end with two it is important to remember that the subscript indicates the number of atoms of the specific element present in the formula when there are numbers in front of the compound or element it indicates the ratio to which the elements compounds or molecules react with one another so back to the equation for water we will remember that water is formed by hydrogen reacting with oxygen of which both are diatomic molecules when we attempt to balance equations it's best to always start with the oxygens and then we will focus on the hydrogens so when we look on the left hand side we see the two oxygen atoms and on the right hand side only one oxygen atom i cannot simply add a subscript behind the oxygen because that will change the formula into a different compound it will no longer be water if i put a 2 in front of the formula for water i will change the number of atoms present in the compound because there's only one oxygen in the compound it will be two times one resulting in two oxygen atoms in the compound please remember that the big two in front says that there are two molecules of water in my formula right now meaning each molecule has one oxygen but because i placed a two in front of the compound i now have two oxygen atoms on the right hand side which equals to the number of oxygen atoms on my left hand side therefore the oxygen atoms have now balanced but this change now also changed the number of hydrogens on my right hand side because h2o has a subscript of 2 by the h it is the big 2 in front times the subscript resulting in 4. now i have 4 hydrogen atoms on my right hand side with only two on my left hand side to counteract this change i must now add a big two in front of the hydrogen on my reactant side on the left hand side if i do this it will be 2 times the subscript of 2 resulting in 4. so let's quickly take a look to see whether we are balanced on the left hand side we will have four hydrogen atoms and on the right hand side we will have four hydrogen atoms so those are balanced on the left hand side there should be two oxygen atoms remember the subscript indicates that there are two oxygen atoms on my reactant side on the right hand side there should be two oxygen atoms the big two in front of the compound of water make sure that we have two oxygen atoms therefore this equation is now balanced when we add heat to a chemical reaction we use it to make the reaction happen faster but when we write the equation we must indicate that we used heat now remember heat can be anything you can use a bunsen burner or a spirit spurner but remember this will always be done in the presence of your teacher please do not attempt to do any reactions at home without proper supervision so how do we write in our equation that we used heat to speed up our reaction we simply add a small triangle below the arrow remember the arrow indicated that there is a chemical reaction taking place and the triangle will mean that i have used heat to speed up the reaction in our third example we will use magnesium that will burn in the presence of oxygen remember this is a combustion reaction we will use a triangle below the arrow to indicate that we have used heat the product that will form is a white powder its name is magnesium oxide let's head over to high school stellenbosch where ms lumpracht will be conducting the lab experiment accompanying this lesson magnesium can be found in magnesium lint which is a solid condensed form of magnesium or it can be found in a powder both will have the same reaction so if we look at the reaction magnesium plus oxygen will form magnesium oxide this reaction will result in a white bright flame as a safety measure be aware of the white flame don't look into it for too long we will be using the bunsen burner so i'll open the gas and then we light up our flame okay okay the magnesium lint will be held with the thumb and i like to twirl it around a bit just so the magnesium doesn't break off so easily then i hold it in the flame it takes a while to heat up but once it's heated it burns a quite nice flame and magnesium oxide is formed which is a white powdery substance which can be seen on the thong as well as the magnesium limb the breath in the second reaction i will show you this exactly the same i will just be using the powder so the powder is sprinkled over the flame and instead of one large flame it forms and spocks magnesium is a metal and it is located on the periodic table look at group two period three so when magnesium burns in the presence of oxygen it means that the magnesium is reacting with the oxygen so on the left hand side we can see that there's one magnesium atom that reacts with a diatomic oxygen molecule therefore it is one to two in the reactant side when we look at the product we will see that the formula is mg for magnesium connected to the o for oxygen there are no subscripts in this formula therefore there is one magnesium and one oxygen on my right hand side this equation is not balanced because on the left hand side i have two oxygens and on the right hand side i only have one oxygen please remember that we are not allowed to add any subscripts to the product we are only allowed to add in front of the formula i suggest that we place a 2 a coefficient of 2 in front of the magnesium oxide the product this means that there are now two magnesiums and two oxygens but on the left hand side we only have one magnesium which means i also need to add a coefficient in front of the magnesium if i place a two there i will have two magnesiums on both sides of my equation i will have two oxygens on both sides of my equation making this equation balanced so now let's quickly look at what happens when iron reacts with oxygen you will know what happens because rust is what forms that orange brown substance often covering a metal surface so rust forms when iron reacts with moisture that contains oxygen iron is a metal located in group eight period four so the word equation is iron plus oxygen with the arrow iron oxide and iron oxide is a part of rust that we see in moist air the oxygen dissolves into the water vapor when the metals are exposed to this the iron that forms part of the metal reacts with the oxygen and begins to rust that is why we often see rusted bicycles and cars close to the ocean the air is always moist rust itself is an orange brown compound that flakes easily and when it flakes off the metal beneath is also exposed to the moist air and that too will begin to rust this is a corrosive process that weakens the metal and as we know iron is very expensive and therefore we need to prevent rust from forming there are four commonly used methods painting is the cheapest of them all it prevents the air from reaching the surface of the metal but when the paint begins to peel the metal will be exposed to the damp air and the rusting process will begin and therefore we need to paint often which decreases the value of the metal another method is to use oil or grease that is water repellent to protect the moving parts of a machine such as your bicycle chain another way to prevent rust from happening is galvanizing in this method we cover the metal with a layer of zinc this method is often used in water pipes and metal garden furniture zinc is also a metal and also oxidizes in the presence of oxygen however it doesn't react as easily as iron our final method for preventing rust is called electroplating in this method we add a layer of chrome or zinc by means of electrolysis the chrome adds a layer of shine and that's why we often use it with our taps and our door handles so let's get back to that iron oxide when i burn iron in the presence of oxygen i will form iron oxide the metal and oxygen reaction i will be demonstrating is iron which can be found in steel wool and there's a lot of oxygen involved when burning an open flame so i will open the gas to the bunsen burner and then light it now we have an open flame and i will use a thong to hold the steel wool so i don't have to get too close when bringing the steel wool to the flame it glows orange if the steel wool didn't contain iron we wouldn't see the orange flame in summary this reaction is where iron and oxygen reacts to form iron oxide if we look at this chemical equation it's clearly not balanced on the left hand side there's only a single ion atom reacting with a diatomic oxygen molecule therefore the ratio is one to two in the reactants the formula that represents iron oxide is f e with a subscript two big o with a subscript three on the left hand side i have two oxygen atoms or a single oxygen molecule but on the right hand side i now have three oxygen atoms when we balance equations we always start with oxygen and it will get tricky now so try to focus do you remember what the lowest common multiple is yes i'm starting to use mathematics in natural science the lowest common multiple of two and three is six this means i must have six atoms of oxygen on both sides on the left hand side we see the subscript is two now two times three will equal to six and that is why i will place a coefficient of three in front of the oxygen atom on the right hand side i have the compound iron oxide the oxygen has a subscript of three and three times two will give me six therefore i will place a coefficient of two in front of the entire compound now my oxygen atoms are balanced on both sides when i place that final two in front of iron oxide i also changed the number of iron atoms in the compound i now have two times the subscript of two which results in 4. remember the big two tells me that there are two molecules of iron oxide on my right hand side and if i look at the two molecules and i count all the ion atoms i will find four on my left hand side there is a single ion atom i must get that one to four atoms and the only way to do this is to place a four in front of the ion now my equation is balanced so just a reminder metals react with oxygen in a combustion reaction remember combustion is another word for burning in the presence of oxygen this way it releases a lot of energy in the form of heat and light when magnesium burns in the presence of oxygen i get a bright white light when iron burns in the presence of oxygen i get a bright yellow orange light in both cases we found that the product was a metal oxide remember magnesium oxide iron oxide so when a metal reacts with oxygen it produces a metal oxide the metal oxide that forms is usually a solid this is a good time to consolidate what we've learned test your knowledge by answering these few questions [Music] do [Music] [Music] do what do we call the small number behind an element to indicate the number of atoms involved in the compound we call it a subscript write down the symbol to indicate that heat was added during a chemical reaction [Music] a triangle underneath an arrow is the correct answer state the name of the compound as well as the ratio in which the elements occur [Music] do our first example is magnesium oxide the ratio of magnesium to oxygen is one to one in our second example we see that it is zinc chloride the ratio of zinc to chloride is one to two our third example has one of those groups that you had to study the nitrate this is sodium nitrate so the ratio for this compound is one sodium to one nitrogen to three oxygens let's practice the skills required to write down the chemical reactions that occur between metals and oxygen when the metal sodium reacts with oxygen the product formed is sodium oxide the reaction can be represented as follows can you write down a word equation for this reaction [Music] my sodium plus oxygen an arrow indicating that a chemical reaction occurred and the product sodium oxide now write down the chemical equation for this reaction [Music] do remember when we use the chemical formula we must only write down the symbols for the elements or compounds is this chemical equation balanced can you motivate your answer do no it is not on my left hand side there is a single sodium atom but on my right hand side there are two sodium atoms and similarly on my left hand side there are two oxygen atoms but on my right hand side only one oxygen atom therefore this equation is not balanced so lastly let's attempt to balance this equation if you've added a 4 in front of the sodium and a 2 in front of the sodium oxide you have mastered balancing equations if you are unsuccessful please watch the lesson again that marks the end of our second lesson see you next time when we will be discussing how non-metals react with oxygen we hope you benefited from this video lesson our vision at 2 when we are teaching is to accelerate access to quality education and our dream is to see learners across south africa flourishing academically we are a non-profit company and we are entirely dependent on public support we produce lessons at 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