CHEM 2 - Equilibrium

  • Created by: Franklin
  • Created on: 07-04-14 17:01

Dynamic Equilibrium

Imagine you are on a downward-moving escalator and you want to remain at a fixed point between two levels. You can achieve this by climbing upwards at exactly the same speed as the people move downwards. Chemical equilbrium is dynamic.

E.g. Br (l) ---> Br (g)

Br liquid from the surface evaporates and then the least energetic particles condense. When the colour of the Br remains constant dynamic equilibrium has been reached

  • Wherever the starting position, the mixture will tend towards the position of equilibrium
  • The conc. of products and reactants do not change after dynamic equilibrium has been reached

- In saturated AgI (radioactive) the solid ions and the dissolved ones will eventually exchange at the same rate and after a few hours later the substance will be completely radioactive

  • In equilibrium concentration of products and reactants remain constant
  • Equilibrium can only be acheived in a closed system

Le Chatelier's Principle: If a system in equilibrium is subject to change the equilibrium position tends to shift as to minimise the change.

  • If pressure is increased the equilibrium will shift to the side with less moles/species of gas
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Le Chatelier's Principle

Changing concentration

  • If concentration of reactant(s) is increased LCP says that the system will shift in the direction as to reduce the concentration of reactant therefore converting it to product.
  • If some of the product was removed, more of the reactant would be used to increase the concentration of the product

Changing temperature

  • Reversible reactions that are exothermic in one direction are endothermic in the other
  • Increasing the temperature moves the equilbrium such that the system is cooled (endothermic direction)

Ammonia Synthesis

N2 + 3H2O ---> 2NH3

According to LCP, synthesis should be carried out at low temperature and high pressure since the product is made in the exothermic direction and there are less moles of gas on the product side. Reactions are slow at low temp. so a compromise has to be made between yield and rate

Catalysts - They don't change the position of equilibrium but increase the speed at which it is reached

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Equilbria in Industry

Certain factors have to be taken into account when deciding which process to  use in order to create the desired product:

  • Availability of starting materials
  • Equilibrium conditions required for good yield
  • Rate of reaction
  • Cost - electricity, materials, work plant
  • Hazardous effects to the works and environment

Ammonia Yield

Comprise conditions -

  • Temp is high enough for reaction to occur at realistic rate and not give minimal yield. 
  • Catalyst used to increase rate of reaction.
  • Pressure is not tooo high as to be impractical
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