STPM Chemistry Form 6 Notes – Chemical Equilibrium (Part 3)

by BerryBerryTeacher

in Berry Reference (Notes)

In the previous posts in this series on Chemical Equilibrium for STPM Chemistry (Tingkatan 6), we learnt about reversible reactions, dynamic equilibrium, equilibrium law, equilibrium constant (and more importantly how to express it), homogeneous equilibrium and heterogeneous equilibrium. Those topics are theoretical concepts on chemical equilibrium. In this part 3 of the series from Berry Berry Easy, we’ll be looking at usage of them in industrial applications such as in the Haber Process. You’ll also learn about the Le Chatelier’s Principle (it’s a term students can easily memorise, due to the exotic nature of the name, at least to our Malaysian ears). Knowing the name is one, understanding it and applying it is another. Make sure you are well-verse with the latter rather than just the former.

STPM Chemistry Form 6 Notes – Chemical Equilibrium (Part 3)

Le Chatelier’s Principle: more (or less) product

  • change the concentration of a reactant or product
  • change the temperature
  • change the pressure on a system that contains gases

A) The effect of concentration on equilibrium

  • Add more of a reactant or product to an equilibrium system, the reaction will shift to the other side to use it up.
  • Remove any of a reactant or product in an equilibrium system, the reaction will shift to the other side to replace it.

Example:

Haber process: N2(g) + 3H2(g) <—-> 2NH3(g) ΔH = – 92.2 kJ

Add nitrogen gas (increase the concentration of nitrogen gas), equilibrium is reestablish and reaction shifts to the right (forward reaction occurs / shifts from left to right) which forming more ammonia (product).

B) The effect of temperature:

i) On the position of equilibrium

  • Increase the temperature (heating a reaction) causes an equilibrium to shift to the endothermic (heat adsorbing)
  • Decrease the temperature (cooling a reaction) causes an equilibrium to shift to the exothermic (heat releasing)

Example:

Haber process: N2(g) + 3H2(g) <—-> 2NH3(g) ΔH = – 92.2 kJ

Heat the reaction (increase the temperature of the reaction), equilibrium is reestablish and reaction shifts to the left (reverse reaction occurs / shifts from right to left) which forming more nitrogen gas and hydrogen gas (reactants)

ii) On the position of equilibrium constant

  • Van’t Hoff equation: ln K = – (ΔH/RT) + C where K is equilibrium constant, ΔH is the heat of forward reaction, R is gas constant (8.31 J mol-1 K-1 and C is a constant of the particular reaction.
  • Van’t Hoff equation: ln K2/K1 = (ΔH/R)(1/T1 – 1/T2)
  • Endothermic reaction, ΔH is positive
  • Exothermic reaction, ΔH is negative

C) The effect of pressure on equilibrium that contains gases

  • Reactant and/or product must be gases
  • Increase the pressure on an equilibrium causes the reaction to shift to the other side containing the fewest number of gas molecules.

Example:

Haber process: N2(g) + 3H2(g) <—-> 2NH3(g) ΔH = – 92.2 kJ

Increase the pressure (by making the container smaller or by putting in non-reactive gas), equilibrium is reestablish and reaction shifts to the right (forward reaction occurs / shifts from left to right) which forming more ammonia (product) – reduce the number of gas molecules in the container.


Chemical Kinetics

Chemical kinetics is a way to change reaction speeds.
There are 6 factors that affecting the speed of a reaction (same in Reaction Kinetics)

  • A) Complexity of the reactants – reactants which are large and complex molecules will need to take longer time for the molecule to react in a reaction. They may be a lot of collision occur but only the collision at the reactive site leads to chemical reaction. Sometimes the reactive site in a molecule is totally block off, as the result no chemical reaction occurs.
  • B) Particle size of the reactants – the greater the total surface area, the higher the reaction rate.
  • C) Concentration of the reactants – the more reactant molecules collide, the higher reaction rate.
  • D) Pressure of gaseous reactants – the higher the reactant pressure, the higher the reaction rate because the increased number of collisions.
  • E) Temperature – 10˚C increase in temperature, the reaction rate is doubled.
  • F) Catalysts – with catalysts increase the reaction rate by lowering the reaction’s activation energy

Now that you have learnt about the Le Chatelier’s Principles and Chemical Kinetics, make sure you also log back again soon for topics on ‘Factors affecting the rate of reaction, rate constant, equilibrium constant and composition of equilibrium mixture’ and Perturbations and shifts in the final part in the series on Chemical Equilibrium.

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August 3, 2012

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