Equilibria

Question 1

When is a reaction at equilibrium?

Answer 1

• A reaction is at equilibrium when a balance of amounts of reactants and products is reached.
  
  • The rate of the forward reaction equals the rate of the backward reactions.
  • The concentrations of reactants and products do not change.

Question 2

When the backward and forward rate balance, the concentrations of reactants and products on never equal.

Answer 2

• If the concentration of reactants is greater than that of the products, the equilibrium is left.
• If the concentration of reactants is less than that of the products, the equilibrium is right.

Question 3

When does this work?

Answer 3

• This only works if it is a closed system - the chemicals do not ‘get out’.
  
  • Initially, the forward reaction rate is fast but then it slows has the reactants are used up.
  • At the same time, the backward reaction rate increases as more is available to react.
  • Eventually the backward reaction is as fast as the forward reaction. Equilibrium has been reached.

Question 4

As the temperature increases in this reaction.

Answer 4

• Less product is made as the equilibrium is moving to the left.
• At 40 °C there is 52% product.

Question 5

As the pressure increases in this reaction.

Answer 5

• More product is made as the equilibrium is moving to the right.
• At 15 atm there is 75% product.

Question 6

The reaction to produce dinitrogen tetroxide is

Answer 6

• 2NO_2(g) ⇔ N₂O_4(g)
• If N₂O₄ removed as it forms, the equilibrium moves to the right giving a higher yield as more is made to replace it and if no NO₂ is introduced, the equilibrium goes to the right.
• The equation shows 2 moles of NO₂ produce 1 mole of N₂O₄, so increasing the pressure move the equilibrium to right to reduce the pressure as there are fewer gas particles on the right.
• Increasing temperature moves the reaction in the direction of the endothermic process.
• These are examples of Le Chatelier’s principle, which state that if a Change is made in a closed system, the equilibrium will shift in a way to reduce the effect of the change.

Question 7

Contact process word equation

Answer 7

sulfur dioxide + oxygen ⇔ sulfur trioxide

Question 8

Contact process symbol equation

Answer 8

2SO_2(g) + O_2(g) 2SO_3(g)

Question 9

To obtain the best most economic yield

Answer 9

• The reaction is carried out at
  
  • about 450°C
  • At atmospheric pressure
  • Using a catalyst vanadian pentoxide V₂O₅

Question 10

These conditions achieved the most economic yield because:

Answer 10

• The forward reaction is exothermic, so high temperatures drive the equilibrium to the left, but if the temperature is too low the rate of reaction is too slow. High temperatures increase the rate of reaction. This temperature is a compromise
• There are 3 gas molecules on the left of the equation and 2 on the right, so high pressure increases the yield - it forces the equilibrium further to the right. The equilibrium already lies to the right anyway, so high pressures are not needed.
• Catalyst do not affect the position of the equilibrium. They make the reaction go faster