Equilibria

Slide 1

• Equilibria
• Many reactions are reversible: N2+3H2--><--2NH3
• All reversible reactions reach a dynamic equilibrium state.
• Dynamic=Both the forward and the backward reaction occur simultaneously.
• 2 Features of Dynamic Equilibrium
• 1.) The forward and backward reactions occur at equal rates
• 2.) The concentrations of reactants and products stays constant.
• Position of equilibrium is used to describe the composition of the equilibrium mixture.
• If the position of equilibrium favors the reactants (aka to the left), then the equilibrium mixture will contain mostly reactants.
• LCP aka Le Chatelier's Principle
• Tells you how the position of equilibrium will change, if a condition changes:
• --If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change.
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Slide 2

• EFFECT OF TEMPERATURE ON EQUILIBRIUM
• If the temperature is increased, the equilibrium will shift to oppose this, and will move in the endothermic direction to try and reduce the temperature by absorbing heat.
• If the temperature is decreased, the equilibrium will shift to oppose this, and will move in the exothermic direction to try and increase the temperature by releasing heat.
• Low temperatures may give a higher yield of product, but will also result in slow rates of reaction.
• Often, a compromise temperature is used that gives a reasonable yield and rate.
• QUESTION: What effect would increasing the temperature have on the yield of ammonia?
• ANSWER: If the temperature is increased, the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease the temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.

Slide 3

• Effect of pressure on equilibirum
• Increasing Pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change, and therefore reduce the pressure.
• Decreasing the pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change, and therefore increase the pressure.
• If the number of moles of gas is the same on both sides of the equation, then changing the pressure will have no effect on the position of equilibrium.
• Increasing pressure may give a higher yield of product and will produce a faster rate.
• Industrially high pressures are expensive to produce--high electrical energy costs for pumping the gases to make a high pressure-- and the equipment is expensive--to contain the high pressure--.
• Question:What effect would increasing pressure have on the yield of methanol?
• Answer: If the pressure is increased, the equilibrium will shift shift to oppose this and will move towards the side with fewer moles of gas to try and increase decrease the pressure.
• The position of equilibrium will shift to the right, giving a higher yield of methanol. (3 moles of gas on left, 1 mole of gas on right.)

Slide 4

• Effect of concentration on equilibirum
• Increasing the conc of OH- ions causes the equilibrium to shift to oppose this and move in the forward direction to remove and decrease the concentration of OH- ions.