The Reaction Quotient Bozeman Science English (auto-generated) English

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hi it's mr. Andersen and this is

chemistry essentials video 63 it's on the reaction quotient one of my favorite

Hi. It's Mr. Andersen and this chemistry essentials video 63. It's on the reaction quotient. One

games when I was a little kid was hide-and-seek but lots of times at the

of my favorite games when I was a little kid was hide and seek. But lot's of times at the

end you'd have one person who's still hidden and the person who's searching

end you would have one person who's still hidden and the person who's searching for

for him can't find him and so we would always play a game called hotter or

them can't find them. And so we would always play a game hotter or colder. As they get

colder as they get closer you would say you're getting warmer you're getting

closer you would say you were getting warmer, you're getting hotter. And as you're moving

hotter and as you're moving away you're getting colder and that'd be an easy way

away you're getting colder. And that would be an easy way for them to find that last

for them to find that last person and so in a reversible reaction we have something similar and so we identified K

person. And so in a reversible reaction we have something similar. And so we identify

as the equilibrium constant which is going to be a ratio of products to

K as the equilibrium constant which is going to be a ratio of products to reactants in

reactants in a reversible reaction but that only occurs when we reach equilibrium when we find what we're

a reversible reaction. But that only occurs when we reach equilibrium. When we find what

looking for but in science we have a reaction quotient Q which is going to be

we're looking for. But in science we have a reaction quotient, Q, which is going to

the same thing it's calculating the same way but it's where we are before we

be the same thing. It's calculated the same way, but it's where we are before we actually

actually reach equilibrium so it allows us to see how far we are away it's like

reach equilibrium. So it allows us to see how far we are away. It's like playing hotter

playing hotter or colder and so in a reversible reaction the reactants become

or colder. And so in a reversible reaction the reactants become products, products become

products products become reactants and over time they eventually reach equilibrium and when we reach

reactants and over time they eventually reach equilibrium. And when we reach equilibrium

equilibrium we can calculate the equilibrium constant which is simply K equal to the concentration of products

we can calculate the equilibrium constant which is simply K equal to the concentration

divided by the concentration of the reactants but along that pathway we

of products divided by the concentration of the reactants. But along that pathway we could

could also calculate Q which is going to be the concentration of products to

also calculate Q which is going to be the concentration of products to reactants at

reactants at any given time and over time Q is going to approach and eventually become K once we reach

any given time. And over time Q is going to approach and eventually become K, once we

equilibrium and so a model kind of works like this imagine if we have a free

reach equilibrium. And so a model kind of works like this. Imagine if we have a free

energy diagram and this ball is going to roll back and forth but let's say it

energy diagram and this ball is going to roll back and forth. But let's say it eventually

eventually stops right here at the bottom is going to be K and where it's

stops right here. At the bottom is going to be K. And where it's at now would be Q. So

at now would be Q so we know that it's going to move towards the left if we let it roll a little bit more

we know that it's going to move towards the left. If we let it roll a little bit more

it eventually pauses now we have a new Q now we know that it's going to move

it eventually pauses. Now we have a new Q. Now we know that it's going to move right

right and eventually it's going to reach K or that equilibrium and so if we use

and eventually it's going to reach K or that equilibrium. And so if we use an energy diagram

an energy diagram like this K is going to be right here and Q is going to be any way along that path and so here I've

like this, K is going to be right here. And Q is going to be any way along that path.

got a model which is essentially a reversible reaction model on the left

And so here I've got a model which is essentially a reversible reaction model. On the left side

side we've got a bunch of molecules we'll call them a and as they bounce

we've got a bunch of molecules. We'll call them A. And as they bounce over to the right

over to the right side to become B and so what I'm doing is simply keeping track

side they become B. And so what I'm doing is simply keeping track of the number of reactants

the number of reactants and products reactants are going to be green and products are going to be red and I'm

and products. Reactants are going to be green and products are going to be red. And I'm

keeping track of that every 50 seconds and you can see that it varies at the

keeping track of that every 50 seconds. And you can see that it varies at the beginning.

beginning at the beginning you can see here that we had 100 reactants and zero

At the beginning you can see here that we had 100 reactants and 0 products. But right

products but right now as we get 350 seconds in it's starting to stabilize and so I could gather data in a model

now as we get 350 seconds in it's starting to stabilize. And so I could gather data in

kind of like that now I'm going to take this data and I'm going to graph it and

a model kind of like that. Now I'm going to take this data and I'm going to graph it.

we're going to have a graph that looks like this so at the beginning we have

And we're going to have a graph that looks like this. So at the beginning we have all

all reactants and zero products but you can see over time it's eventually reaching this equilibrium and so let's

reactants and zero products. But you can see over time it's eventually reaching this equilibrium.

say that at the end we reach equilibrium and again this is simply a model but how

And so let's say that at the end we reach equilibrium. And again this is simply a model.

would we calculate that equilibrium constant it's simply going to be the

But how would be calculate that equilibrium constant? It's simply going to be the concentration

concentration of products divided by the concentration of reactants and so if I

of products divided by the concentration of reactants. And so if I were to calculate that

to calculate that our products is 65 and our reactant since 35 and so let's say K is 1.9 and so at the end when it reaches

our products is 65 and our reactants is 35. And so let's say K is 1.9. And so at the end

equilibrium will say that our K value is going to be 1.9 well what's Q Q is going

when it reaches equilibrium we'll say that our K value is going to be 1.9. Well, what's

to be values along the way before we reach equilibrium and so what's going to

Q? Q is going to be values along before we reach equilibrium. And so what's going to

be our Q at that first point it's going to be zero how did I calculate that I

be our Q at that first point? It's going to be 0. How did I calculate that? I simply took

simply took the number of products which is zero divided by the reactants which

the number of products which is 0 divided by the reactants which is 100 and that's going

is a hundred and that's going to give me a Q value of zero now if I go to the next one so if I go to 50 how am I going

to give me a Q value of zero. Now if I go to the next one, so if I go to 50 how am I

to figure that out it's going to be my products which are 0.42 divided by 0.5

going to figure that out? It's going to be my products which are 0.42 divided by 0.58.

eight and so that's going to be 0.72 and now let's watch what happens to our Q

And so that's going to be 0.72. And now let's watch what happens to our Q value as we move

value as we move closer and closer to equilibrium you can see it stabilizes here but I simply don't have enough

closer and closer to equilibrium. You can see it stabilizes here. But I simply don't

products it's lower than that now I overshoot it and my Q value goes a

have enough products. It's lower than that. Now I over shoot it. And my Q value goes a

little bit too high and then it eventually approaches it and it eventually becomes K Q and K are exactly

little bit too high. Then it eventually approaches it. And it eventually becomes K. Q & K are

the same thing and so this is a model but it shows you what's going on in a

exactly the same thing. And so this is a model but it shows you what's going on in a chemical

chemical reaction K is what we have at the end is Q and Q is what we have along

reaction. K is what we have at the end and Q is what we have along the way. And so if

the way and so if we were to summarize a little bit in a reversible reaction if my Q value is less than my K value the

we were to summarize a little bit in a reversible reaction, if my Q value is less than my K

reaction has to move to the right what

value, the reaction has to move to the right. What does that mean, move to the right? I

does that mean move to the right I have to convert more of my reactants into products what happens if my Q value is

have to convert more of my reactants into products. What happens if my Q value is greater

greater than my K value then I have to move to the left and what do I have when

than my K value? Then I have to move to the left. And what do I have when my Q value equals

my Q value equals my K value I'm at equilibrium or they're going to be this ratio now sometimes they're hard to

my K value? I'm at equilibrium. Or they're going to be the same ratio. Now it's sometimes

remember this and so if you put it on a number line it helps you quite a bit if

hard to remember this. And so if you put it on a number line it helps you quite a bit.

I put my queue of my K values on a number line like this so K is where I

If I put my Q and my K values on a number line like this, so K is where I want to be

want to be in queue is where I am which way am I going to have to move along

and Q is where I am, which way am I going to have to move along this number line? I'm

this number line I'm going to have to move from the right to the left I'm

going to have to move from the right to the left. So I'm going to have to move left. I'm

gonna have to move left I'm going to have to convert more of my products into reactants so I can eventually reach K

going to have convert more of my products into reactants so I can eventually reach K.

now this only works in a homogeneous kind of a state in other words if we were to looking at liquids if we were to

Now this only works in a homogeneous kind of a state. In other words if we were to look

look at liquids and solids or gases - solids or gases - liquids this reaction

liquids and solids or gases to solids or gases to liquids, this reaction quotient is not

quotient is not going to work now if we apply it to an actual chemical reaction

going to work. Now if we apply it to an actual chemical reaction, I've got this reversible

I've got this reversible reaction up here two moles of these makes one mole of this and two moles of this I can use

reaction up here. 2 moles of these makes 1mole of this and 2 moles of this, I can use what's

what's called an ice table to look at the initial concentrations and then my

called an ICE table to look at the initial concentrations and then my final concentration

final concentrations at equilibrium why do they call it an ice table well you

is at equilibrium. Why do they call it an ICE table? Well you can see here that I have

can see here that I have my initial concentrations my equilibrium at the end

my initial concentrations, my equilibrium at the end and then I have my change or I

and then I have my change or my IC e and this is going to be measured in molarity

have my I C E. And this is going to be measured in molarity. And so if we were to calculate

and so if we were to calculate what my Q value is right here here's my Q value right here well you can see that since I

what my Q value is right here, here's my Q value right here, well you can see that since

have no products at this point my Q is going to be equal to zero now how do I

I have no products at this point my Q is going to be equal to 0. Now how do I figure out

figure out my K value if I'm given my my concentrations at equilibrium I could

my K value? If I'm given my concentrations at equilibrium I could calculate K. Now I'm

calculate K now I'm going to show you how to do that in the next video on equilibrium but let's just say it is 650

going to show you how to do that in the next video on equilibrium. But let's just say it

and again watch the next video on equilibrium if you don't know how to do

is 650. And again, watch the next video on equilibrium if you don't know how to do that.

that so what which way is this reaction going to move is it going to move to the right

So which way is this reaction going to move? It is going to move to the right? Is it going

is it going to move towards the left well you can see if I put them on a

to move towards the left? Well you can see if I put them on a number line, if I put K

number line if I put K on a number line and Q on a number line I have to move to the right

on a number line and Q on a number line I have to move to the right, which should make

which should make total sense I have to convert these reactants to products if

total sense. I have to convert these reactants to products if I'm ever going to approach

I'm ever going to approach equilibrium so it's going to move towards the right and so if I were to show you what those

equilibrium. So it's going to move towards the right. And so if I were to show you what

values are going to be so how did I get these values if I take this number

those values are going to be, so how did I get these values? If I take this number subtract

subtract that I'm going to get this value right here you should start to see

that, I'm going to get this value right here. You should start to see a pattern in that

a pattern in that change row and so if I look at it all of these values are going

change row. And so if I look at it, all of these values are going to be the same molarity

to be the same molarity change well why is that the same molarity change it's because we have the same moles up here

change. Well why is that the same molarity change? It's because we have the same moles

if we look at this one it's point zero one nine why is that this is going to

up here. If we look at this one it's 0.019. Why is that? There's only going to be 1 mole

only only be more Anmol right there and so stoichiometric is incredibly important we have to make

right there. And so stoichiometry is incredibly important. We have to make sure that that

sure that that change matches the stoichiometry of our equation and so did

change matches the stoichiometry of our equation. And so did you learn to determine the effect

you learn to determine the effect of chemical reaction manipulation on Q and K values I hope so and I hope that was helpful

of chemical reaction manipulation on Q and K values? I hope so. And I hope that was helpful.