0:16

So when we look at reactions,

Â we're actually talking about two different things.

Â We talked about the chemical reaction.

Â And this is the actual process that's actually happening.

Â The changes we're actually seeing in our container, and so

Â in this case, we actually have vinegar reacting with baking soda.

Â And so, this is the actual chemical reaction.

Â A chemical equation is what we use to represent that reaction, so

Â it uses chemical symbols to show what's going on.

Â It shows us both our reactants and our products.

Â And shows how many of each of them we need.

Â So, we tend to use these two terms, reaction and equation interchangeably.

Â But when we want to be specific, we want to make sure we

Â call the reaction the actual reaction that occurs in the equation,

Â is the representation of that reaction.

Â 1:02

So when we look at equations, what we can see is there are lots of

Â different ways to represent what's happening.

Â We can build models and

Â show what these things would look like if we built the models of them.

Â Here we have a molecule of methane reacting with two molecules of oxygen,

Â forming a molecule of CO2 and two molecules of water.

Â We can also write it out in words and describe those.

Â We can also write it in symbols,

Â which is our chemical equation that we'll talk about most frequently.

Â And we can also look at our atoms because that's what we're going to have to

Â do when we balance an equation,

Â is make sure we have the right number of atoms on either side of the equation.

Â because here we have 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms,

Â combining and reacting to form substances that contain 1 carbon atom,

Â 4 hydrogen atoms, and 4 oxygen atoms.

Â And all of these, are just an expanded version of our sentence we have here at

Â the top, that methane reacts with oxygen, to produce carbon dioxide in water.

Â So all of these are completely reasonable ways to report an equation.

Â The one that we will see most often is using the chemical symbols.

Â 2:24

The arrows themself means to yield.

Â So reactants yield.

Â And then on the right side, we have our products over here.

Â So reactants yield products.

Â The other thing we have to add in is our coefficients.

Â And that's what these numbers are here, and

Â what they do is make sure that we have a balanced chemical equation, and

Â that makes sure that when we have our number of atoms on the left side, and

Â the right side of each element, that we have the same number of each.

Â 2:57

So, let's look at how we would balance this equation.

Â Okay?

Â The first thing is, we want to write our equation, so that we have CH4, which is

Â the formula for methane, reacting with oxygen to form carbon dioxide and water.

Â Now, we've already seen what the rebalanced equation looks like, but

Â we're going to show the process by which we actually balance equations.

Â So, the first thing I want to do,

Â is I want to make a list of all the elements that I see in the reaction.

Â Note, that the elements on the left must be the same elements on the right.

Â And then I'm going to basically tally up how much I have of each atom.

Â So when I look at my equation, I look at the reactant side on, here on the left,

Â and I see that I have 1 carbon, I have 4 hydrogens, and I have 2 oxygens.

Â I do the same thing on the product side, where I have 1 carbon, I have 2 hydrogens,

Â and I have 3 oxygens, because I have 2 oxygens and 1 oxygen there.

Â Note that our subscripts, remember, tell us how many we have of

Â that particular element in a unit of that molecule.

Â So now, go back to my tally sheet at the bottom, and I notice that my carbon is

Â the same, which is great, but I notice that my Hydrogens are not.

Â I have 4 Hydrogen atoms on the left, and only 2 Hydrogen atoms on the right.

Â So I need to add a coefficient, so that I have the same number on both sides.

Â Now there's not any particular place that's the best place to start,

Â we can actually start with any of our elements.

Â I usually go in order from the, a, as the way I wrote them on my tally sheet, but

Â if you have trouble with one element, skip it and

Â come back to it after you've balanced the other elements.

Â So now, and for my hydrogen, I need to add a 2 in front of the H2O,

Â because that gives me 2 times 2, which is 4 hydrogens.

Â But note this, also change the number of oxygens, so now I have 2

Â oxygens in the water, but I also have 2 oxygens here in the carbon dioxide.

Â So, I now have a total of 4 oxygens.

Â So, while I fixed my hydrogens, I did not fix the oxygens, because now I

Â have 4 oxygens on the right side of the equation, and only 2 on the left.

Â So now, I need to go back and put a 2 in front of O2,

Â and when I do that, the only thing that changes is the number of oxygens, and

Â I see that I have 4 oxygens on the left, and 4 oxygens on the right.

Â Now note, that it's always a good idea to go back at the very end and

Â recheck everything.

Â I have 1 Carbon on the left,.

Â 1 carbon on the right.

Â 4 hydrogens on the left.

Â 2 time 2, so 4 hydrogens on the right.

Â 2 times 2, 4 oxygens on the left.

Â And I have 2 oxygens here, and 2 oxygens there for a total of 4.

Â So I see that all of my elements are balanced.

Â So now, I have a balanced chemical equation.

Â 5:47

Now, I can look at the phases of substances and add those in.

Â So, in order to have a more complete equation,

Â what I want to do is know what phase each reactant or product is in.

Â I can look at four different options,

Â I have a gas, I can have a liquid, a solid, or aqueous.

Â Now, something that is aqueous means it's dissolved in water,

Â now it is also in the liquid state, but because it's a mixture,

Â it's a solution of something dissolved in water, we call that aqueous.

Â A liquid means a pure liquid.

Â 6:20

Most commonly, when we're dealing with water, nothing dissolved in it,

Â we call that pure liquid.

Â So now, I can go back and add my phases, so

Â we have methane gas, oxygen gas, CO2 gas, and liquid water.

Â Now for some substances, you'll probably know what the phase is.

Â For oxygen, generally we're talking about a gas, or carbon dioxide, or

Â carbon monoxide.

Â Other times it won't be as obvious.

Â Sometimes we have to make an educated guess about what we think about it,

Â based on what we know about the compound, and

Â sometimes we're given other information in the problem that allows us to

Â determine what that phase should be, or what that state of matter is.

Â 7:02

Now, just a quick review of the steps for

Â balancing equations before we look at another example.

Â First, we're going to write the formulas for

Â reactants and products on the appropriate side of the equation.

Â This is going to require that we pull in our information about nomenclature, so

Â that we correctly match the formula with the name of the compound.

Â We are going to then count how many of each element we have in

Â the equation as written.

Â So before we put any coefficients in it, that's what our starting point is.

Â 7:54

Now, let's look at an example here.

Â We have silicon, chloride plus water, yielding silicone dioxide and HCL.

Â So the first thing I want to do,

Â is basically take an accounting of what I have.

Â So I look on the left side of the equation, I have 1 silicone,

Â I have 4 chlorines, I have 2 hydrogens, and 1 oxygen.

Â Now, I go to the product side, which is the right side of the equation, and

Â I see I have o1 silicone, I have 1 chlorine, 1 hydrogen and 2 oxygens.

Â So now I have a complete accounting of all the elements I have, and

Â I notice that the only thing that balances right now, is my silicon.

Â So, the first thing I'm going to do is decide which element to start with.

Â It doesn't matter specifically where you start.

Â Just pick something and

Â if you're struggling with how to balance it, try starting with a different element.

Â I'm going to start with my chlorine.

Â I see I have 4 chlorine atoms on the left.

Â I need 4 on the right.

Â So I can put a 4 in front of HCl.

Â When I do that, it does change my chlorines to 4, but

Â it also changes my hydrogens to 4.

Â So now I have 4 chlorines and 4 hydrogens.

Â 9:04

I need to look at my hydrogens, because I have 4 on the right side and

Â only two on he left.

Â The only place I have hydrogen is in water, so

Â I need to add a 2 in front of H20.

Â When I do that, that changes my hydrogens to 4 and it changes my oxygens to 2.

Â Every time I put a coefficient in, I'm going to go back through, and

Â check to make sure that any changes that resulted from

Â that coefficient are reflected in my tally at the bottom of the page.

Â So now, I see that by balancing the hydrogens and

Â chlorines, I've actually already balanced my oxygen atoms.

Â At the end, I'm always going to go back through and

Â check just to make sure I have 1 silicone, one silicone, 4 chlorines,

Â 4 chlorines, 4 hydrogens, 4 hydrogens, 2 oxygens, and 2 oxygens.

Â So now I have a balanced chemical equation.

Â 10:05

So the correct answer is 4.

Â The temptation is just to say, well,

Â I've got 2 irons on the right, I must have a 2 in front of the iron on the left.

Â However, we actually have to look through and

Â determine what those coefficients could be, because I have to

Â make sure all elements are balanced, not just one particular element is balanced.

Â So let's look at the equation and see how we would balance it.

Â The first thing I'm going to do,

Â is take accounting of everything I have in the equation as written.

Â What I see is that for iron, I have 1 iron on the left, and 2 oxygens on the left.

Â On the right, I have 2 irons and 3 oxygens.

Â Now if I start with my iron, I see that I need a 2 in front of the iron in order to

Â have the iron add up to balance, because that would change that to a 2.

Â So the iron is balanced at this point, however, the oxygen is not.

Â And in order to report coefficients, we have to balance the entire equation.

Â So now, I need to look at my oxygens, and I say that if I

Â have 6 oxygens on both sides, I can have a balanced number of oxygens.

Â So now I need to put a 3 in front of my O2.

Â So now I have 6 oxygen on the left.

Â But I also need to change the coefficient in front of the iron oxides, so

Â that I can have 6 oxygens on the right.

Â When I add my 2 in front of the Fe2O3,

Â I end up changing the number of irons to 4, and the number of oxygens to 6.

Â So now, my Iron is back out of balance, and so although my oxygens are balance,

Â I'm going to have to revisit the irons to make sure it's balanced.

Â And what I see, is that if I change that 2 that's in front of the iron to a 4,

Â that would change my number of iron atoms to 4.

Â It doesn't change anything else.

Â And what I see now, is that my irons atoms match, and that my oxygen atoms match.

Â I'm always going to go back to the equation.

Â I have 4 irons on the left, 4 irons on the right,

Â 6 oxygens on the left, and 6 oxygens on the right.

Â So now I have a balanced chemical equation, and

Â 4 will be the coefficient for iron.

Â