Equilibrium

• Reversible reactions occur in both the forward and backward directions
• A reversible reaction can reach equilibrium in a closed system

  • This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter
• Equilibrium can only be reached in a closed vessel which prevents reactants or products from escaping the system

  A) open

  B) closed

  C) forward direction

  D) reversible

  
• At equilibrium:

  • The rate of the forward reaction is equal to the rate of the reverse reaction
  • The concentration of reactants and products remains constant (given there is no other change to the system such as temperature and pressure)
• Equilibrium is dynamic

  • This means that the molecules on the left and right of the equation are changinginto each other by chemical reactions constantly and at the same rate
• An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process (1)

  • At the start of the reaction, only nitrogen and hydrogen are present
  • This means that the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest
  • As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease
  • So, the rate of the forward reaction will decrease
• An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process (2)

  • However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase
  • Ammonia will decompose to reform hydrogen and nitrogen
  • In a closed system, the two reactions are interlinked and none of the gases can escape
  • So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
• At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction

  A) decreases

  B) equilibrium

  C) equal

  D) increases

  
• The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction

  • This balance is framed in an important concept known as Le Chatelier's Principle,
• This principle states that when a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change
• The principle is used to predict changes to the position of equilibrium when there are changes in:

  • Temperature
  • Pressure
  • Concentration
• Knowing the energy changes, states and concentrations involved allows us to use the principle to manipulate the outcome of reversible reactions

  • For example, if pressure is increased, the position of equilibrium moves in the direction which has the smallest amount of gaseous molecules
• The position of equilibrium is said to shift to the right when the forward reaction is favoured

  • This means that there is an increase in the amount of products formed
• The position of equilibrium is said to shift to the left when the reverse reaction is favoured

  • So, there is an increase in the amount of reactants formed
• We can predict the effect of changes in temperature on systems in equilibrium

  • To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
• If the temperature is raised:

  • The yield from the endothermic reaction increases
  • The yield from the exothermic reaction decreases
• If the temperature is lowered:

  • The yield from the endothermic reaction decreases
  • The yield from the exothermic reaction increases
• When a change in temperature is made to a system, the system will oppose the change

  • E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature
  • It will do this by favouring the endothermic reaction
• If we add more product to a reversible reaction, this would shift the equilibrium towards the reactant side because it reduces the amount of product available to form further product.

  A) endothermic

  B) exothermic

  
• How pressure affects equilibrium

  • Changes in pressure only affect reactions where the reactants or products are gases
• We can predict the effect of changes in pressure on systems in equilibrium

  • To make this prediction, the balanced symbol equation must be known
• The effects of pressure on equilibrium

  A) least

  B) greatest

  
• The effect of changing concentration can be thought of as a balance, with the reactants on the left and the products on the right

  • If the concentration of a reactant increases, then the equilibrium shifts to the right to balance this balance
• The effects of concentration on equilibrium

  A) right

  B) left

  C) left

  D) right

  
• The presence of a catalyst:

  • Does not affect the position of equilibrium
  • Increases the rate at which equilibrium is reached
• The catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway requiring lower activation energy

  • As a result, the equilibrium concentration of reactants and products is the same as it would be without the catalyst
• Diagram showing the effect of a catalyst on the time taken for equilibrium to be established

  A) forward

  B) forward

  C) catalyst

  D) backwar

  E) catalyst

  F) backward

  
• When the conditions at equilibrium are changed, the system always responds by doing the opposite.

  • For example if the concentration is increased the system tries to reduce it by changing the direction of the reaction or if the temperature is increased the system will try to reduce the temperature by absorbing the extra heat.
• Closed system - where no more reactants are added and no more products are removed
• In a closed system, a point will eventually be reached when the rate of the forward reaction is equal to the rate of the backward equation

  • known as equilibrium or dynamic equilibrium
• In an equilibrium, the concentrations of reactants and products remain constant

  • don’t say that the concentrations are the same
• When the temperature increases, the equilibria position shifts to the endothermic direction

  • decreased temperature means the equilibria position shifts to an exothermic direction
• How temperature affects equilibrium

  A) exothermic

  B) endothermic

  C) reduce

  D) A+B

  E) C+D

  F) increase

  G) C+D

  H) A+B

  
• Increased pressure means equilibri position shifts to the side with the least number of moles of gas
• How pressure affects equilibrium

  A) fewer gas molecules on right

  B) more gas molecules on left

  C) reduce

  D) C+D

  E) A+B

  F) increase

  G) A+B

  H) C+D

  
• Increased concentration of one or all reactants means the position of equilibrium shifts to product side to use it up
• Decreased concentration of one or all reactants, position of equilibrium shifts to reactant side to break down products to make up for the lack of reactants
• A catalyst increases the rate of both of the forward and backwards reactions equally

  • doesn’t change the equilibrium position
  • once equilibrium is reach, position of equilibria doesn’t change