Chapter 15 Learning Objectives

Learning objectives for Chapter 15: Chemical Equilibrium

Refers to the point in a reaction where the rates of the forward and reverse reactions are equal, resulting in no further change in the concentrations of the reactants and products.

What is equilibrium?

Established when the reaction's forward and backward processes happen at the same speed, so the amounts of reactants and products stay the same.

How is equilibrium established?

Using aA+bB\rightleftharpoons cc+dD

K_{c}=\dfrac{\left[ C\right] ^{c}\left[ D\right] ^{d}}{\left[ A\right] ^{a}\left[ B\right] ^{b}}

What is the equilibrium expression?

(l) and (s) are NOT included because they have fixed densities and volumes and do not change significantly.

Why are liquids and solids left out of equilibrium expressions?

It indicates that at equilibrium, the concentration of products is greater than that of reactants, favoring the formation of products (lies to right).

What does it mean if K > 1?

It indicates that at equilibrium, the concentration of reactants is greater than that of products, favoring the formation of reactants (lies to the left).

What does it mean if K < 1?

The value of K becomes the reciprocal of the original K value, indicating that the equilibrium position shifts to favor the reactants.

How does K change if a reaction is written in the reverse direction?

The value of K is raised to the power of that number, reflecting a change in the stoichiometry of the reaction.

How does K change if a reaction is multiplied by a number?

The equilibrium constant K for the overall reaction is the product of the equilibrium constants for the individual reactions, indicating how the individual reactions contribute to the overall reaction's position.

If a series of reactions are added to give an overall reaction, how does K for the overall reaction relate to the K_s for the individual reactions?

The K value increasing

A temperature change causing a shift towards the products results in…

The K value decreasing

A temperature change causing a shift towards the reactants results in…

K_c uses concentration (molarity) of products and reactants.

K_p uses partial pressure (atm) of products and reactants.

What is the difference between K_c and K_p?

When the reaction involves only gases.

When can you express an equilibrium as either K_c or K_p?

When dealing with concentrations of reactants and products in molarity (M).

If aqueous, K_c must be used.

When do you have to express an equilibrium as Kc?

When you are given a balanced equation, at least one initial amount, AND equilibrium amount or value of K for a reaction.

OR

When you are asked to solve for the equilibrium constant (K) or the equilibrium amounts of reactants and products.

How do you know when you should use an ICE table to solve a problem?

The reaction quotient (Q) measures the relative amount of products and reactants during a reaction at a particular point in time.

What is the reaction quotient?

K is the equilibrium constant, that is a ratio of reactants to products at equilibrium, while Q represents the ratio of products to reactants at any point in a reaction.

How is the reaction quotient (Q) different from K?

How are K and Q the same?

Both K and Q are ratios that compare the concentrations of products to reactants, reflecting the direction of a reaction.

If K>Q, the reaction will proceed in the forward direction

If K=Q, its already at equilibrium

If K<Q, the reaction will proceed in the reverse direction

How can you use the relationship between Q and K to determine the direction in which a reaction will proceed?

Forward: -x in the reactants, +x in the products (using reactants to make products)

Reverse: +x in the reactants, -x in the products (using products to make reactants)

How are the signs of the values in the “Change” line of an ICE table affected when a reaction proceeds in the forward direction vs the reverse direction to reach equilibrium?

If a system at equilibrium is disturbed, the system will shift its equilibrium position to counteract the effect of the disturbance in order to get back to equilibrium.

What is Le Chatelier’s principle?

The system will push back against the added reactant or product to establish equilibrium.

What happens when there is a change in concentration of the reactants or products (be careful about solid or liquid reactants and products!)?

If the volume increases the pressure decreases, the system will respond by shifting to the side with less moles/molecules of gas.

If the volume decreases the pressure increases, the system will respond by shifting to the side with more moles/molecules of gas.

What happens when there is a change in pressure or volume?

Increase temperature, the system will respond by decreasing the temperature/heat, shifting away from the heat in the chemical equation.

Decrease temperature, the system will respond by increasing the temperature/heat, shifting towards the heat in the chemical equation.

What happens when there is a change in temperature?

The system will shift to use up that reactant or product.

Which way will the system shift if there is an increase in concentration?

The system will shift to make more of that reactant or product.

Which way will the system shift if there is an decrease in concentration?

Adding a catalyst has NO EFFECT on the position of equilibrium.

What is the effect of a catalyst on the position of an equilibrium?

K will increase and the system will shift towards the products

For an endothermic reactions, what happens to K when the temperature increases and which way will the system shift?

K will decrease and the system will shift towards the reactants.

For an endothermic reactions, what happens to K when the temperature decreases and which way will the system shift?