7.4 Predicting the Direction of a Reaction

SCH4U – Grade 12 Chemistry, University PreparationMs. Papaiconomou & Ms. Lorenowicz

• Equilibrium Constant (Kc) and the Reaction Quotient (Qc)• Le Châtelier’s Principle(pp.354-370)

• Law of Chemical Equilibrium• At equilibrium there is a constant ratio between the concentration of

reactants and products in any change.

• Using ICE charts and the equilibrium equation we can solve for the:• Concentrations at: I = initial, C = change, E = equilibrium

• Equilibrium constant• Initial or Equilibrium concentrations of chemical species in a reaction• May need to use the quadratic equation to solve problems. • There may be more than one solution, but only one of the solutions will make sense.

• The equilibrium constant (K) describes the extent of a reaction.• K > 1 means equilibrium favours the products (more products made than reactants)• K = 1 means at equilibrium there are equal concentrations of products & reactants.• K < 1 means equilibrium favours the reactants (less products made than reactants)

We learned in previous lessons that…

How do you know a reaction is at equilibrium?

How can you predict the direction in which the reaction must proceed to reach equilibrium?◦ The reaction quotient (Qc) is an expression that

is identical to the equilibrium constant expression, but its value is calculated using concentrations that are not necessarily those at equilibrium.

Reaction Quotient, Qc

c d

c a b

[C] [D]Q =[A] [B]

aA + bB cC + dD

Q < K denominator is large◦ system attains equilibrium by moving to the right, move

towards making products Q = K system at equilibrium Q > K numerator is large

◦ system attains equilibrium by moving to the left, move towards making reactants

Reaction Quotient (Qc)

What happens if we change the concentration of a chemical entity when

a reaction is at equilibrium?

A dynamic equilibrium tends to respond so as to relieve the effect of any change in the conditions that affect the equilibrium.

Predicts the way that an equilibrium system responds to change

Involves:◦ Initial equilibrium system◦ Shifting “non-equilibrium” state◦ New equilibrium state

Le Châtelier’s Principle

Henry-Louis Le Châtelier, 1850-1936, a French chemist and engineer

Change concentration of Reactants

Increase concentration of reactants

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\Nelson Chemistry 12 Textbook\Chem_12\Attachments\d)_Animations\16M11AN1.mov

Decrease concentration of reactants

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\Nelson Chemistry 12 Textbook\Chem_12\Attachments\d)_Animations\16M11AN2.mov Video:

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\McGrawHill Chemistry 12 Textbook\elp\unit4\4_1.mov

Change concentration of Products

Increase concentration of products

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\Nelson Chemistry 12 Textbook\Chem_12\Attachments\d)_Animations\16M11AN3.mov

Decrease concentration of products

F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\Nelson Chemistry 12 Textbook\Chem_12\Attachments\d)_Animations\16M11AN4.mov

Common ion effect◦ Involves adding an ion to a solution in which the

ion is already present in solution (concentration effect)

◦ Equilibrium shifts away from the added ion

Effect of Ions in Aqueous Equilibrium Systems

Video:F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\McGrawHill Chemistry 12 Textbook\elp\unit4\4_3a.avi

The value of the equilibrium constant changes with temperature, because the forward and reverse reactions are affected

Le Châtelier’s Principle still holds. The sign of the enthalpy change for the reaction is

important.◦ Endothermic Change (ΔH >0)

↑ temperature, shift equilibrium to right, forming products & Kc increases

↓ temperature, shift equilibrium to left, forming reactants, & Kc decreases

◦ Exothermic Change (ΔH <0) ↑ temperature, shift equilibrium to left, forming reactants & Kc

decreases ↓ temperature, shift equilibrium to right, forming products, & Kc

increases

Effect of Temperature on Equilibrium

Video:F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\McGrawHill Chemistry 12 Textbook\elp\unit4\4_3b.avi

Video:F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\McGrawHill Chemistry 12 Textbook\elp\unit4\4_3c.avi

Pressure and Volume are inversely related at a constant temperature.

Pressure is caused by gas molecules striking the walls of a container, so you change pressure by changing the number of gas molecules.◦ if ↓volume, ↑pressure so, shift to

more gas molecules◦ if ↑volume, ↓pressure so, shift to

less gas molecules◦ What if same number of gas

molecules on both sides of equation? Volume & pressure have no effect on

the position of equilibrium.

Effect of Volume & Pressure Changes on Equilibrium

Catalyst A catalyst speeds up the rate of a reaction. Overall effect is to lower the activation energy, which increases

the rate of reaction, of both the forward and reverse reactions. Catalysts do not affect the position of equilibrium, only the

time taken.

Changes that do not affect the position of Equilibrium Systems

Inert Gases (N2 & Noble Gases) Inert gases do not react with other gases.

This will not be part of the equilibrium system. The equilibrium position of the system will not

change.◦ The presence of the inert gas changes the probability of

successful collisions for both the reactants and products equally, resulting in no shift in the equilibrium system.

Changes that do not affect the position of Equilibrium Systems

Video:F:\Courses\SCH4U1 - Chemistry Gr12 Univ\~ Resources ~\McGrawHill Chemistry 12 Textbook\elp\unit4\4_2.avi

Chickens and Equilibrium

Please re-read Section 7.4 (pp.354-370) and answer:◦ p.356 Q.26-28◦ pp.366-367 Q.29-33◦ p.370 Q.1-5

Homework