6. Chemical Equilibrium

Question 1

Effective Collisions

Answer 1

• collisions between reactant molecules or ions that lead to a net chemical change ie. chemical reaction
• 3 conditions necesary for a reaction to occur
  1. ‘activation energy’
  2. particle orientation
  3. collision frequency

Question 2

Rate of Reaction

Answer 2

• how quickly reactants disappear + products are formed
• expressed as moles/litre/second
• not constant over time

Question 3

Chemical Kinetics

Answer 3

• study of the speeds of reactions

Question 4

1. Activation Energy

Answer 4

• colliding particles must possess a certain minimum amount of energy in order to break the bonds in the reactants so that new bonds may form
• minimum energy necessary for a reaction to occur is ‘activation energy’ + is dependent on relative speeds (kinetic energy) of colliding objects + on their angle of approach
• the lower the activation energy, the faster the reaction
• the higher the activation energy, the slower the reaction
• therefore the activation energy is inversely related to the rate of reaction

Question 5

2. Particle Orientation

Answer 5

• colliding particles must come together in the correct orientation ie. an atom or ion must collide with another that it is attracted to eg. –> +ve -ve

Question 6

3. Collision Frequency

Answer 6

• collisions between particles must be frequent, so that enough collisions occur with sufficient energy and proper orientation of particles within a given time for a reaction to take place
• speed of a reaction is proportional to the probability of effective collisions

Question 7

Factors Affecting Activation Energies + Reaction Rates

Answer 7

• nature of reactants
• concentration of reactants
• reaction temperature
• presence of a catalyst

Question 8

1. Nature of Reactants

Answer 8

• generally reactions that take place between ions in aqueous solution are extremely rapid; activation energy is low because no covalent bonds need to be broken
• conversely, reactions between covalent molecules are much slower as the bonds need to be broken
• particle size also affects the reaction rate: powder will react more quickly than the same mass in a solid block due to the increased surface are of the powder
• physical state of the reactants will affect the reaction rate: liquid state reactants will react faster than solid state reactants, and gaseous state reactants will be faster again

Question 9

2. Concentration of Reactants

Answer 9

• usually the reaction rate increases when the concentration of one or more reactants is increased because there are more particles present for successful collisions to occur in a shorter space of time
• often a direct relationship exists between concentration + reaction rate

Rate = k[compound]

where’k’ is the rate constant and [compound] is the molar concentration of the compound shown in brackets

• rate constant is a proportionality constant between the molar concentrations of reactants + the rate of reaction + is usually calculated from the initial rate of reaction + corresponding initial concentrations

Question 10

3. Temperature of the Reaction

Answer 10

• almost always, an increase in temperature causes an increase in reaction rate
• often, an increase of 10°C doubles the rate of reaction
• due to two factors:
  1. as temperature increases, particles move more rapidly and therefore collide more frequently
  2. with an increase in temperature comes a different distribution of speeds. The number of very fast particles increases much more than the number of average speed. As a consequence, the number of effective collisions rises even more than the total number of collisions
• so not only do more collisions take place, but the percentage of collisions that have sufficient activation energy also rises. This factor is mainly responsible for the sharp increase in reaction rates with an increase in temperature

Question 11

4. Presence of a Catalyst

Answer 11

• a substance that alters the rate of a chemical reaction by providing a lower activation energy path that leads from reactants to products, and is not itself used up in the reaction, is a catalyst
• a heterogeneous catalyst is in a different state from that of the reactants
• a homogeneous catalyst is in the same phase as the reactants (solid, liquid or gas)

Question 12

Reversible vs Irreversible Reactions

Answer 12

• some reactions, such as combustion, are irreversible ie. once the reactants form products, the products cannot be converted back into the reactants
• a reversible reaction is one that can go in either direction ie. the reactants form products and the products can be converted back into the reactants
  eg. Combination and decomposition:

A + B –> AB (combination) is the forward reaction, AB –> A + B (decomposition) is the reverse reaction

Question 13

Dynamic Equilibrium

Answer 13

• a situation whereby two opposing processes are occurring at equal rates
• as a reversible reaction progresses, the concentration of reactants decreases, while the concentration of products increases
• also, as the reactants are being used up, the rate of the forward reaction decreases, while the rate of the reverse reaction increases as more product is formed
• eventually the two rates become equal and at this point the process is in dynamic equalibrium
• by definition then, it follows that if the rates of reaction are equal, then the concentrationof each substance will remain unchanged
• Reactants Products

aA + bB cC + dD

where the capital letters symbolise substances and the lowercase letters are the coefficients of a balanced equation

Question 14

The Equilibrium Expression

Answer 14

K = [C]^c [D]^d / [A]^a [B]^b

where K is the ‘equilibrium constant’: the ratio of product concentrations to reactant concentrations (with exponents that depend on the coefficients of the balanced equation)

• K does not change as long as temperature remains constant
• pure solids and pure liquids are not included when calculating K as their concentrations do not vary
• if K > 1, there is more product than reactant
• if K < 1, there is more reactant than product
• for large values of K (above 10³) the reaction is said to have gone to completion
• for small values of K (10^-3) almost no reaction has taken place

Question 15

Le Chatelier’s Principle

Answer 15

• when a system that is in dynamic equilibrium is subjected to a disturbance that upsets the equilibrium, the system undergoes a change in direction that counteracts the disturbance and restores equilibrium
• conditions affecting equilibrium
  1. addition or removal of a component
  2. change in temperature
  3. change in pressure
  4. using a catalyst

Question 16

1. Addition or Removal of a Component

Answer 16

• when the equilibrium of a system is disturbed by the addition or removal of a component, the equilibrium will shift away from the substance added or toward the substance removed
• regardless of what happens to the individual concentrations, the value of the equilibrium constant remains unchanged

Question 17

2. Change in Temperature

Answer 17

When the equilibrium of a system is disturbed by a change in temperature, the value of the equilibrium constant will change and the equilibrium position will shift as follows:

• an increase in temperature drives an exothermic reaction toward the reactants and an endothermic reaction toward the products
• a decrease in temperature drives an exothermic reaction toward the products and an endothermic reaction toward the reactants

Question 18

3. Change in Pressure

Answer 18

• the equilibrium of a system is only disturbed by a change in pressure if one or more components are gaseous
• an increase in pressure shifts the reaction towards the side with fewer moles of gas
• a decrease in pressure shifts the reaction toward the side with more moles of gas
• when the moles of gas are the same on each side of a balanced reaction, an increase or decrease in pressure results in no shift of the reaction
• the addition of a non reactive gas to the mixture has no effect on the equilibrium position

Question 19

4. Using a Catalyst

Answer 19

• for reversible reactions, catalysts always increase the rates of both forward and reverse reactions to the same extent. Therefore, the addition of a catalyst has no effect on the position of the equilibrium, only the speed at which it gets there