Equilibrium I summary questions - Chemistry A Level

What is a dynamic equilibrium?

The point in a reversible reaction where the rate of the forward reaction equals the rate of the backwards reaction and the concentration of each substance remains constant (REMINDER: the concentrations are constant, not the same!). 

Only occurs in closed systems. 

What is Le Chatelier’s Principle?

If a reaction at equilibrium is subjected to change in pressure, temperature or concentration, the position of the equilibrium will shift to counteract the change. 

How will the equilibrium shift if the concentration is changed? 

If we increase the concentration of a reactant or product, the equilibrium will shift to try and reduce the concentration (the opposite will happen if concentration is decreased). if conc. of a reactant is increased, equilibrium will shift to the right to reduce concentration by using up the reactant, producing more product. 

How will the equilibrium shift if the pressure is changed? 

If we increase the pressure, the equilibrium will shift to try and reduce the pressure. If we increase the pressure, equilibrium will shift to the side with the fewest number of total gas particles. The opposite happens if pressure is decreased. 

How will the equilibrium shift if the temperature is changed? 

If we increase the temperature, equilibrium will shift to try and reduce the temperature. This means that the equilibrium will shift in the endothermic direction to try and reduce the temperature. The opposite happens if temperature is decreased. 

How will the equilibrium shift if a catalyst is added?

Catalysts have no effect on the position of equilibrium as they speed up the rate of the forward and backward reaction equally. Catalysts speed up the rate that equilibrium is reached but have no effect on yield. 

What is the equilibrium constant and how can it be worked out? 

When 2A(g)+B(g) ⇌ 2C(g)+D(g), then K_c = [C]²[D] / [A]² [B] , however if the reaction is heterogeneous, the solid (or pure liquid) is not included in the expression for K_c

Evaluate and explain the necessity, for many industrial processes including making ethanol, to reach a compromise between the yield and the rate of reaction. 

Reaction: C₂H_{4 (g)} + H₂O_(g) -> C₂H₅OH_(g)  ΔH= -46kJ mol^-1

Temperature: Decreasing the temperature would result in a shift to the right, hence more ethanol would be produced, as the reaction is exothermic. However, decreasing temperature lowers the rate of reaction, so ~300^oC is a compromise between yield and rate

Pressure: High pressure would result in the equilibrium shifting to the right, producing more ethanol, and also increases the rate of reaction. However, maintaining high pressure is expensive (due to need for thicker vessels and pipes), so ~60 atm is a compromise between yield and cost