Rates and Equilibrium

Question 1

Equilibrium

Answer 1

A state of balance. In chemical reactions, the state when the concentrations of reactants and products are constant and the rate of forward and reverse reactions are equal.

Ammonia Example:
N2(g) + 3 H2(g) -> 2 NH3(g)
*This reaction occurs naturally in a closed container, but goes faster if heated.
* During the reaction, the reactants, H2 and N2, are consumed, so their concentrations gradually decrease. The concentration of NH3, the product, will gradually increase.
* After a period of time, however, the concentrations of N2, H2 and NH3 no longer change.
* The concentrations of the reactants (H2 and N2) decrease at first, while the concentration of the product (NH3) increases. Then, before the reactants are used up, all concentrations become constant.

Question 2

Reversible Reaction

Answer 2

• When a reaction results in an almost complete conversion of reactants to products, chemists say that the reaction goes to completion - but most reactions do not go to completion.
• The reactions appear to stop because they are reversible.
• A reversible reaction is a chemical reaction that can occur in both the forward and reverse directions.

Question 3

Reversible Equation

Answer 3

• Forward: N2(g) + 3 H2(g) -> 2 NH3(g)
• Reverse: 2 NH3(g) -> N2(g) + 3 H2(g)
• Chemists combine these two equations into a single equation that uses a double arrow to show that both reactions occur.
• N2(g) + 3 H2(g) <->2 NH3(g)
• The reactants in the forward reaction are on the left of the arrows. The reactants in the reverse reaction are on the right of the arrows.
• In the forward reaction, hydrogen and nitrogen combine to form the product ammonia.
• In the reverse reaction, ammonia decomposes into the products hydrogen and nitrogen.
• Reversible reactions occur when the reaction has a low activation energy in both the forward and reverse direction.

Question 4

Concentration Effects

Answer 4

• How does the reversibility of this reaction affect the production of ammonia?
• In the beginning, no NH3 is present, therefore only the forward reaction can occur.
• As H2 and N2 combine to form NH3, their concs. decrease. The rate of a reaction depends on the concentration of the reactants, so now the forward reaction is slowing down.
• As soon as NH3 is present, the reverse reaction can occur, slowly at first, but at an increasing rate as the concentration of NH3 increases.
• As the reaction proceeds, the rate of the forward reactions continues to increase until the two rates are equal.
• At this point, NH3 is produced at the same rate that it is decomposed, so the concentrations of N2, H2, and NH3, remain constant.

Question 5

Dynamic Equilibrium

Answer 5

• A state of balance where the forward and reverse reactions continue at equal rates, so there is no net change in the amounts of reactants and products present in the mixture.

Question 6

Equilibrium and Physical Processes

Answer 6

• Dynamic equilibrium is sometimes more easily understood by visualising physical processes.
• Consider a sealed bottle containing both liquid water and air containing water vapour.
• Given sufficient time, this system will come to equilibrium, a state where the rate of vapourisation of the liquid water and condensation of water vapor is equal. (Dynamic Equilibrium)

Question 7

Equilibrium Expressions for Chemical Reactions

Answer 7

• Consider the following generalised chemical reaction. The substances are indicated by symbols A, B, C and D, and a, b, c, and d represent the numerical coefficients in the balanced equation:
  aA + bB <-> cC + dD
• The generalised equilibrium expression is:
• Kc = [C]c [D]d/ [A]a [B]b (Products over reactants) ([A]a is A to the power of a)
• Kc > 1 equilibrium will lie to the right and favour the products.
• Kc < 1 equilibrium will lie to the left and favour the reactants.
• Kc values close to 1 imply that significant concentrations of
  reactants and products are present at equilibrium.

Question 8

Writing Equilibrium Law Expressions

Answer 8

• The equilibrium law expression does not include solids or pure liquids as their concentrations do not vary from one reaction to the next.

Question 9

Reaction Quotient

Answer 9

• Sometimes a chemist needs to know if current conditions (concentrations) will shift toward the product side or the reactant side in coming to equilibrium.
• To make this determination we will need to calculate a quantity called the reaction quotient (Qc)
• The formula for Qc is mathematically the same as that for the equilibrium constant, but we use current concentrations rather than equilibrium concentrations.
• In general, if Qc < Kc, the reactions will shift to the right. If Qc > Kc, the reaction will shift to the left.