0:16

So when we look at reactions,

we're actually talking about two different things.

We talked about the chemical reaction.

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

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

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

And so, this is the actual chemical reaction.

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

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

It shows us both our reactants and our products.

And shows how many of each of them we need.

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

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

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

is the representation of that reaction.

1:02

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

different ways to represent what's happening.

We can build models and

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

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

forming a molecule of CO2 and two molecules of water.

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

We can also write it in symbols,

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

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

do when we balance an equation,

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

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

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

4 hydrogen atoms, and 4 oxygen atoms.

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

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

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

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

2:24

The arrows themself means to yield.

So reactants yield.

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

So reactants yield products.

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

And that's what these numbers are here, and

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

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

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

2:57

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

Okay?

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

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

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

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

So, the first thing I want to do,

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

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

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

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

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

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

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

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

that particular element in a unit of that molecule.

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

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

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

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

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

we can actually start with any of our elements.

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

if you have trouble with one element, skip it and

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

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

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

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

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

So, I now have a total of 4 oxygens.

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

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

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

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

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

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

recheck everything.

I have 1 Carbon on the left,.

1 carbon on the right.

4 hydrogens on the left.

2 time 2, so 4 hydrogens on the right.

2 times 2, 4 oxygens on the left.

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

So I see that all of my elements are balanced.

So now, I have a balanced chemical equation.

5:47

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

So, in order to have a more complete equation,

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

I can look at four different options,

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

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

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

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

A liquid means a pure liquid.

6:20

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

we call that pure liquid.

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

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

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

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

carbon monoxide.

Other times it won't be as obvious.

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

based on what we know about the compound, and

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

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

7:02

Now, just a quick review of the steps for

balancing equations before we look at another example.

First, we're going to write the formulas for

reactants and products on the appropriate side of the equation.

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

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

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

the equation as written.

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

7:54

Now, let's look at an example here.

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

So the first thing I want to do,

is basically take an accounting of what I have.

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

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

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

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

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

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

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

It doesn't matter specifically where you start.

Just pick something and

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

I'm going to start with my chlorine.

I see I have 4 chlorine atoms on the left.

I need 4 on the right.

So I can put a 4 in front of HCl.

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

it also changes my hydrogens to 4.

So now I have 4 chlorines and 4 hydrogens.

9:04

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

only two on he left.

The only place I have hydrogen is in water, so

I need to add a 2 in front of H20.

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

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

check to make sure that any changes that resulted from

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

So now, I see that by balancing the hydrogens and

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

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

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

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

So now I have a balanced chemical equation.

10:05

So the correct answer is 4.

The temptation is just to say, well,

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

However, we actually have to look through and

determine what those coefficients could be, because I have to

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

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

The first thing I'm going to do,

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

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

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

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

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

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

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

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

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

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

So now I have 6 oxygen on the left.

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

that I can have 6 oxygens on the right.

When I add my 2 in front of the Fe2O3,

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

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

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

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

that would change my number of iron atoms to 4.

It doesn't change anything else.

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

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

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

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

So now I have a balanced chemical equation, and

4 will be the coefficient for iron.