6. Equilibria

Question 1

Define a reversible reaction

Answer 2

• All reversible reactions reach a dynamic equilibrium state

Question 3

State examples of reversible reactions

Question 4

Define dynamic

Answer 4

• Both forward and backward reactions are occurring simultaneously

Question 5

State two features of dynamic equilibrium

Answer 5

• Forward and backward reactions are occurring at equal rates
• The concentrations of reactants and products stay consent

Answer 6

We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture.
If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.

Question 7

State Le Chatelier’s Principle

Answer 7

• If an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it)

Question 8

State the four things that can effect equilibrium

Answer 8

• Concentration
• Temperature
• Pressure
• Catalysts

Answer 9

• Increase concentration of a reactant
• Decrease concentration of a product
• Shift to the right

Question 10

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Answer 10

• Increase conc of product
• Decrease conc of reactants
• Shift to left

Answer 11

Increasing the concentration 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. The position of equilibrium will shift towards the right, giving a higher yield of I- and IO-. ( The colour would change from brown to colourless)
Adding H+ ions reacts with the OH- ions and reduces their concentration so the equilibrium shifts back to the left giving brown colour

Question 12

Describe what happens to the equilibrium when you increase pressure in a reversible reaction

Answer 12

• Cause equilibrium to shift towards the side with fewer moles of gas to oppose the change the change and thereby reduce the pressure
  (Increase = want to decrease, less moles, less collisions, shift to less moles so more)

Question 13

Describe what happens to the equilibrium when you decrease pressure in a reversible reaction

Answer 13

• Cause the equilibrium to shift towards the side with more moles of gas to oppose the change, and thereby increase the pressure
• (Decrease = want to increase, more moles, more collisions, shift to more moles, so more)

Question 14

Describe the effect of pressure on equilibrium when the no of moles are the same

Answer 14

• If the no of moles of gas is the same on both sides of the equation, then changing pressure will have no effect on the position of equilibrium

Answer 15

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 pressures)

Answer 16

Typical exam question: What effect would increasing pressure have on the yield of methanol?
CO +2H ⇌ CHOH
(g) 2(g) 3 (g)
Exam level answer : must include bold points
If pressure is increased the equilibrium will shift to oppose this and move towards the side with fewer moles of gas to try to reduce the pressure . The position of equilibrium will shift towards the right because there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a higher yield of methanol.

Question 17

Describe the effect of increase in temp on equilibrium in a reversible reaction

Answer 17

• Equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.
• (increase = we want to decrease, endothermic takes heat from surrounding, shift to endo so less heat)

Question 18

Describe the effect of decrease in temp on equilibrium in a reversible reaction

Answer 18

• Equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.
• (Decrease = we want to increase, exothermic releases heat, shift to exo so more heat)

Answer 19

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.

Answer 20

Typical exam question: What effect would increasing temperature have on the yield of ammonia?
N2 +3H2 2NH3 H=-veexo Exam level answer : must include bold points
If temperature is increased the equilibrium will shift to oppose this and move in the endothermic, backwards direction to try to decrease temperature. The position of equilibrium will shift towards the left, giving a lower yield of ammonia.

Question 21

Describe the effect of catalysts on equilibrium

Answer 21

• No effect on position of equilbrium
• Will speed of the rate at which the equilibrium is achieved

Question 22

Explain why catalysts do not have an effect on equilibrium

Answer 22

• It speeds up the rates of the forward and backward reactions by the same amount

Question 23

Haber Process

Answer 23

• N2 + 3H2 ⇌ 2NH3
• T= 450oC, P= 200 – 1000 atm, catalyst = iron
  Low temp gives good yield but slow rate: compromise temp used
  High pressure gives good yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure
  *

Question 24

Contact Process

Answer 24

Contact process
Stage 2 SO + 1⁄2 O ⇌ SO H = -98 kJ mol 2 (g) 2 (g) 3 (g)
Stage 1 S (s) + O2 (g)SO2 (g)
-1
Low temp gives good yield but slow rate: compromise moderate temp used
High pressure only gives slightly better yield and high rate: too high a pressure would lead to too high energy costs for pumps to produce the pressure

Answer 27

In all cases high pressure leads to too high energy costs for pumps to produce the pressure and too high equipment costs to have equipment that can withstand high pressures

Answer 28

Recycling unreacted reactants back into the reactor can impr. ove the overall yields of all these processes.

Answer 29

Both methanol and ethanol can be used as fuels.
If the carbon monoxide used to make methanol in the above reaction was extracted from the atmosphere then it could be classed as carbon neutral.
It would only be carbon neutral, however, if the energy required to carry out the reaction was not made by combustion of fossil fuels .

Question 30

Define carbon neutral

Answer 30

• An activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere