LeChatelier’s Principle



Chapter 14 Worksheet 3 (ws14.3)

Effect of concentration, temperature, pressure, and catalyst on equilibrium; LeChatelier’s Principle

A thorough understanding of chemical equilibrium allows the chemist to figure out how to GET MORE PRODUCT by changing the reaction conditions.

For the reaction aA + bB ⇌ cC + dD:

Q = [pic][pic] Q is called the reaction quotient

Keq = [pic]

(For gases, Q and Keq can be written in terms of partial pressure.)

When thinking about the problems below, imagine that the reaction is already at equilibrium. In other words:

Q = Keq which means that ratef = rater

Anything that changes either Q or Keq (or the relative reaction rates) will disturb the equilibrium. IMMEDIATELY after the change decide whether:

Q < Keq (ratef > rater) So reactant is consumed and product is made! (→)

or

Q > Keq (ratef < rater) So “product” is consumed and “reactant” is made. (←)

Reaction continues until equilibrium is re-established.

A. A chemist can make the following changes. Do they change Q or Keq?

Change Q? Change Keq?

1. Add or remove some reactant or product

2. Change the temperature

3. Change the volume

(Tricky! This changes the concentration/pressure

of all reactants and products.)

4. Add a catalyst

Think about what each of these changes does to the rates of the forward and reverse reactions.

Effect of changing the concentration of reactants or products

REMEMBER THIS: ADDING A REACTANT OR PRODUCT SHIFTS THE EQUILBIRIUM IN THE DIRECTION THAT CONSUMES IT. REMOVING A REACTANT OR PRODUCT SHIFTS THE EQUILIBRIUM IN THE DIRECTION THAT PRODUCES IT.

Consider the following reaction:

Fe3+(aq) + SCN-(aq) ⇌ FeSCN2+(aq)

(dark red)

B. Write the reaction quotient (Q) and equilibrium constant expressions (Kc) for this reaction.

C. Let’s reason out what should happen if KSCN is added. Before the change, the reaction is at equilibrium. What is the relationship between Q and Kc? What is the relationship between ratef and rater?

E. Just after adding KSCN but before reaction can occur, what is the relationship between Q and Kc? What is the relationship between ratef and rater? In which direction must the reaction proceed; left to right or right to left? What should we see?

G. What do you expect to observe if Fe(NO3)3 is added?

H. The items above, involved adding more reactant. Suppose you could remove some reactant, for example, with a chemical ‘iron (III) remover’. What would you expect to observe? Why?

I. What would you expect to observe if some FeSCN2+ was added? If some was removed? (Describe the color immediately after the change, and after equilibrium is reestablished.)

( Hint: equlibrium is usually reestablished before the disturbance is totally counteracted.)

Effect of changing temperature

Adding or removing reactants or products shifted the position of equilibrium because Q was changed (but Keq was not changed). In contrast, changing the temperature shifts the position of equilibrium because Keq changes (do you remember why?). The direction of shift (right or left) depends on whether the reaction is endothermic or exothermic.

REMEMBER THIS: INCREASING THE TEMPERATURE SHIFTS EQUILIBRIUM IN THE ENDOTHERMIC DIRECTION. LOWERING THE TEMPERATURE SHIFTS EQUILBRIUM IN THE EXOTHERMIC DIRECTION. (The equilibrium constant for an endothermic reaction increases with increasing temperature. The equilibrium constant for an exothermic reaction increases with decreasing temperature.)

Consider the reaction:

Co(H2O)62+ (aq) ⇌ Co(H2O)42+ (aq) + 2H2O

(pink) (blue)

A. Do you think this reaction is endothermic or exothermic?

B. Is the reverse reaction endothermic or exothermic?

C. The tube contains an equilibrium mixture of the reactants and products above. What do you expect to observe if the tube is heated? Cooled?

D. There is a trick that makes it very easy to predict which direction the equilibrium shifts when you change the temperature.

1. Rewrite the balanced equation but include heat as either a reactant or a product. (Is heat consumed or produced by the reaction?)

2. Use the same logic as you did for deciding the effect of adding or removing reactants or products. The reaction will proceed in the direction that consumes added heat (endothermic direction) or that regenerates removed heat (exothermic direction).

Effect of changing volume

REMEMBER THIS: REDUCING THE VOLUME SHIFTS EQUILIBRIUM IN THE DIRECTION THAT PRODUCES FEWER MOLES. INCREASING THE VOLUME SHIFTS EQUILIBRIUM IN THE DIRECTION THAT PRODUCES MORE MOLES. (Q changes if there are more moles on one side of the equation. Keq does not change.)

Consider the reaction:

N2(g) + 3H2(g) ⇌ 2NH3(g)

A. Write the equilibrium constant expression (Keq) for this reaction.

B. What happens to the partial pressure (concentration) of each gas if the volume of the reaction vessel is halved? What happens to the total pressure (concentration)?

C. Just after changing the volume and before the equilibrium shifts, what is the relationship between Q and Keq? Which way does the equilibrium shift?

D. Now consider this reaction: Br2(g) + Cl2(g) ⇌ 2BrCl(g). Which way will the equilibrium shift if the volume is halved? (What is the relationship between Q and Kp after the volume change?)

There are two ways to increase the total pressure of a gaseous system without changing the volume.

1. Add more of a gas involved in the reaction (N2, H2, or NH3).

2. Add an inert (unreactive) gas (for example, He).

We already know how to deal with number 1 since it simply increases the partial pressure (concentration) of the added gas.

E. How does adding an inert gas affect the partial pressures of the reactants and products? How does adding an inert gas effect the position of equilibrium?

Le Chatelier’s principle

WARNING! LE CHATELIER’S PRINCIPLE IS A MEMORY TRICK AND NOTHING MORE THAN A MEMORY TRICK. UNDERSTANDING HOW TO USE IT DOES NOT MEAN THAT YOU UNDERSTAND THE CHEMISTRY BEHIND IT!!

Le Chatelier’s principle: If a change is made to a reaction that is at equilibrium, the equilibrium will shift in the direction that counteracts the imposed change.

Add a reactant or product: Reaction will consume the added substance.

Remove a reactant or product: Reaction will produce the removed substance

Add heat by raising the temperature. Reaction will consume the added heat by going in the endothermic direction. (Keq increases in the endothermic direction.)

Remove heat by lowering the temperature. Reaction will produce heat by going in the exothermic direction. (Keq increases in the exothermic direction.)

Decrase the volume of a reaction vessel. This increases the total concentration/pressure of particles (reactants + products). Reaction will shift in the direction that has fewer molecules (decreases the total concentration/pressure).

Increase the volume of a reaction vessel. This decreases the total concentration/pressure of particles (reactants + products). Reaction will shift in the direction that has more molecules (increases the total concentration/pressure).

Note: Usually the imposed change is only partially counteracted.

Review

Consider the reaction:

2NO2(g) ⇌ N2O4(g) ΔH = -58.0 kJ/mol

Name 4 ways to increase the yield of N2O4.

1.

2.

3.

4.

Which do you think is the most economically feasible?

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