2.08b How Changes Shift the Equilibrium

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Do curso por University of Kentucky

Química Avançada

405 classificações

University of Kentucky

Química Avançada

405 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

This problem shows us a reaction, shows us a system at equilibrium and

what we need to do is figure out if we make these changes how is that equilibrium

going to be shifted.

Are we going to shift towards the formation of more products or

are we going to shift towards the formation of more reactives?

And to do this, we use Le Chatelier's Principle,

which tells us that if we have some change to a system,

the system will respond to minimize that disturbance or minimize that change.

So we can look at all different types of changes and

figure out which way the equilibrium will shift,

whether it will be towards the right or towards the left.

Now, the first two we look at are increasing and decreasing temperature.

And in order to do that, we have to know something about the enthalpy

of the reaction or change in enthalpy.

Here we have a negative delta H value, which tells us it's an exothermic

reaction, which tells us that we can look at heat as if it were a product.

So I'm actually going to go over here and write +heat

with my reaction just as a reminder that I'm treating it like a product,

because I'm losing energy as this reaction proceeds.

So if I increase my temperature, that's the same as adding heat to the reaction.

And so what I'm going to see is that's going to drive the reaction

towards the lift, towards the formation of more reactants.

If I decrease my temperature, I can look at that as the same as I'm taking away

heat and removing one of those products.

And so as a result, that's going to draw the reaction towards the right.

Now knowing that we're doing an increase and a decrease of temperature,

we would expect different responses in the way that their reaction shifts

towards the left and the right.

We can also look at increasing the volume or decreasing the volume.

And in order to do that, what we have to worry about is the moles of gas.

And so when I look at my reactive side,

I look at all my substances or gases so I can account everything.

On the left side, I have 4 moles of gas and

on the right side, I just have 2 moles of gas.

And so if I increase the volume, what I'm going to see

is that the reaction is going to favor the side that has more moles of gas.

We're increasing the volume, it decreases the pressure.

But if I reincrease then,

the moles of gas, I'm going to get a shift towards the left side.

Likewise, if I decrease the volume without any other changes,

that would be increasing the pressure.

So the reaction is going to shift to decrease some of

that pressure that we've gotten from decreasing the volume.

And so I go from 4 moles of gas to 2 moles of gas on the right.

And so I'm going to see that it's going to favor the right side and

favor the fewer moles of gas.

Now note that we always look at the moles of gas here.

If we have solids or liquids, we don't include those in those calculations.

Okay, again, we increased the volume, we've decreased the volume,

we've done opposite things.

We see opposite responses within the reaction.

Now I'm going to look at adding nitrogen or hydrogen.

And here what I'm looking at is I see that nitrogen and hydrogen are both reactants.

And so if I add more reactant, the system goes, wait a minute,

we've got too much reactant.

I need to get rid of some that reactant and the reaction is going to need to shift

towards the right to minimize the effect of that added reactant.

If I add NH3, it's going to say, wait a minute.

Now we've got too much NH3, I need to get rid of some of that NH3.

And the way it gets rid of that excess NH3 is to shift towards the left,

toward the formation of more reactant.

We see just the opposite happening when we remove N2 or H2 or remove NH3.

If I remove N2 or H2, one of my reactants,

I see that I now don't have enough of my N2 or H2 to be at equilibrium.

And the reaction will proceed in order to make some more N2 or

H2 to make up for that loss, so it will shift towards the left.

If I remove NH3, I now don't have enough NH3 to be at equilibrium, so

the reaction is going to shift to try to replace some of that lost NH3.

So it will shift to the right.

The last change we can look at is the addition of argon, which in this case is

an inert gas, meaning it doesn't react with any substances in the mixture, okay.

Because of that, it actually has no effect on the equilibrium.

And so we will see that it will not shift towards the left or

towards the right, because the partial pressures of my nitrogen, hydrogen and

NH3 are not affected by the addition of this argon gas.

So, we can take any reaction and

figure out how it will be affected by these different changes.

If we had an endothermic reaction where delta H was greater than zero,

had a positive value, then we will see a different effect because of the increase

in temperature and decrease of temperature.

Likewise, if we have a different reaction with different moles of gas on

the reactant and product side, we may see a different shift in that

equilibrium as we increase or decrease the volume.

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