2.01 Dynamic Equilibrium

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Química Avançada

353 classificações

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University of Kentucky

Química Avançada

353 classificações

A chemistry course to cover selected topics covered in advanced high school chemistry courses, correlating to the standard topics as established by the American Chemical Society. Prerequisites: Students should have a background in basic chemistry including nomenclature, reactions, stoichiometry, molarity and thermochemistry.

Na lição

Chemical Equilibrium

This unit introduces the concept of chemical equilibrium and how it applies to many chemical reactions. The quantitative aspects of equilibrium are explored thoroughly through discussions of the law of mass action as well as the relationship between equilibrium constants with respect to concentrations and pressures of substances. Much of the discussion explores how to solve problems to find either the value of the equilibrium constant or the concentrations of substances at equilibrium. ICE (initial-change-equilibrium) tables are introduced as a problem-solving tool and multiple examples of their use are included. From a qualitative standpoint, Le Châtelier’s principle is used to explain how various factors affect the equilibrium constant of a reaction along with the concentrations of all species.

2.01 Dynamic Equilibrium6:28

2.02 Law of Mass Action7:34

2.03 Law of Mass Action for Combined Reactions4:36

2.04 Relationship Between Kc and Kp6:51

2.04a Relationship Between Kc and Kp2:34

2.05 Calculating the Equilibrium Constant13:09

2.05a Finding Kc5:04

2.06 Reaction Quotient5:55

2.06a Predicting Reaction Progress with the Reaction Quotient1:55

2.07 Calculating Equilibrium Concentrations12:54

2.07a Finding Equilibrium Concentrations, part 14:59

2.07b Finding Equilibrium Concentrations, part 23:20

2.07c Finding Equilibrium Concentrations, part 34:22

2.08 Le Chatelier's Principle Part A9:55

2.08a Le Chatelier's Principle Part B19:50

2.08b How Changes Shift the Equilibrium5:18

Conheça os instrutores

Dr. Allison Soult
Lecturer
Chemistry
Dr. Kim Woodrum
Sr. Lecturer
Chemistry

in this unit we will be talking about chemical equilibrium.

The first thing, we need to understand, is what we mean by the term dynamic equilibrium.

So that is the goal of this unit, is to understand what we mean

and specifically how that relates to the forward and reverse reactions.

So when we looked at kinetics, what we were worried is how fast the reaction proceeds.

How long, how much time does it take

to produce a certain amount of product.

How fast are our reactants consumed.

In equilibrium, we are worried about how far the reaction proceeds

because when we first introduce th idea of chemical reactions.

We kind of assume that they go from all reactants to forming all products.

But in reality, for many reactions that is not in fact what happens. We kind of assume that they go from all reactants to forming all products.

But in reality, for many reactions that is not in fact what happens.

What we get is some portion of the reactants are converted to products But in reality, for many reactions that is not in fact what happens.

What we get is some portion of the reactants are converted to products

and then we don't see the formation of products continue to increase.

So this is when we get to the point of chemical equilibrium.

What we have to look at is how do we know we are at equilibrium?

How does that differ between different reactions.

We need understand something about that ratio

of products to reactants.

So we are going to use the term dynamic equilibrium.

The reason we use this, is that we need to make sure

clear that there is change happening. The reason we use this, is that we need to make sure

clear that there is change happening.

Because the word dynamic means just that, it means change.

But we are also looking at equilibrium, so something else also has to be equal.

The things that are equal are the rates of the

forward and reverse reaction.

When we have a reaction that is at chemical equilibrium

so say we have A goign to B.

We use this equilibrium arrow

to represent that in fact A is being converted to B

and B is being converted back to A.

If we are at the point of equilibrium and B is being converted back to A.

If we are at the point of equilibrium

the rate of the forward process, A going to B, is equal to

to the reverse process, B going back to A.

When I get to some concentration of A, we are going to find that it

doesn't change once we are at equilibrium

neither does the concentration of B.

They don't necessarily have to be the same and

in fact they are usually not the same.

So the concentration of A is constant in fact they are usually not the same.

So the concentration of A is constant

the concentration of B is constant

but the individual molecules

that are going back and forth are going to be different.

So what I see is that some molecules are

at A are being concerted to B.

At the same rate that some molecules of B

are being converted back into A.

So we have seen this idea of equilibrium a coupl of times before.

We have looked at thermial equilibrium, if I have two objects

same mass, same substance

at different temperatures, we put them in thermal contact with one another.

what we see is they go to some temperature in the middle.

In this case, because it is the same mass and the same substance they go to what we see is they go to some temperature in the middle.

In this case, because it is the same mass and the same substance they go to

a temperature halfway in between.

Different substance, different temperature

but we see that they get to this point of equilibrium, there is a transfer

in this case, of heat from one object, the hotter one but we see that they get to this point of equilibrium, there is a transfer

in this case, of heat from one object, the hotter one

to the colder one, but then it gets to the point where we

don't see any net change in the temperature.

So what we look at with this, is we look at our process

going on, we look at the different panels to represent what is going on. So what we look at with this, is we look at our process

going on, we look at the different panels to represent what is going on.

We can put a time with this, this time is going to be different for every reaction.

We are not worried about the rate at which this is happening we are just

saying over time a substance reaches equilibrium.

And so I look at my reactants here, we have H2

reacting with Cl2

and I form HCl

and know that there are two molecules of HCl

because I have to have a balanced chemical reaction.

When I look at each panel I can look down here

and in Panel A I see that I have H2

and I have Cl2.

But notice I have none of my products

present initially, because I have combined my reactants.

Now, after some time passes present initially, because I have combined my reactants.

Now, after some time passes

what I see, is that I still have H2 and Cl2

but now I also have

4 [and I am going to call this moles]

so each molecule is representing a mole of product

so I have 4 moles of my HCl2 being produced.

I got onto my panel C

I still have HCl2 I still have H2

but now I have 8 moles

of my HCl

and I continue on to my panel D

now I have 12 moles of my HCl

again I still have H2 and Cl2 present

and I get to my panel E and I still have 12 moles of HCl.

So what has happened, is when I got to panel D

I am now at chemical equilibrium.

So we can say at this point

we are at equilibrium.

Because after we reach this point, we do not see any increase in the concentration of product

we do not see any decrease in the concentration of reactants. Because after we reach this point, we do not see any increase in the concentration of product

we do not see any decrease in the concentration of reactants.

We have reached a constant concentration of all our reactants and products. we do not see any decrease in the concentration of reactants.

We have reached a constant concentration of all our reactants and products.

Now if I were flag a particular Hydrogen molecule.

Say we look at this particular hydrogen molecule

here in panel D is shows us H2 Say we look at this particular hydrogen molecule

here in panel D is shows us H2

perhaps over here

that hydrogen molecule has split and formed

into HCl and there is some other

H2 that has form from other HCls

so we see individual molecules going back and forth

and that is where we get the dynamic part of this

but the concentration of H2 is constant

the concentration of Cl2 is constant

and the concentration of HCl is constant.

But that doesn't mean that they are all exactly the same.

It just means they are constant, their concentrations are not changing.

We can look at this in a tabulate form, we can look at here and again we have our reaction

we can see we start out with H2 and Cl2

and no HCl.

Over time we are losing H2 and no HCl.

Over time we are losing H2

losing Cl2

and we are gaining out HCl

and then again when we get to our point D and E what we see

is that our concentrations become constant.

We still have to acknowledge to Stoichiometry so

as we lose 2 moles, or two molecules of H2

we are gaining 4 molecules of HCl. as we lose 2 moles, or two molecules of H2

we are gaining 4 molecules of HCl.

because that is what the Stoichiometry of our reaction tells us.

We have looked at this in terms of the

qualitative assessments of what is going on

now what we need to be able to do, is to look at the law of mass action.

and actually quantify

these ratios between products and reactants.

That is what we will do int eh next unit.

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