Chapter 7: Extent of Chemical Reactions

Question 1

Open vs closed systems

Answer 1

• Open – matter and energy can be exchanged with the surroundings
  
  • E.g. bushfires
• Closed – only energy is exchanged with the surroundings
  
  • E.g. a submarine under water

Question 2

Reversible vs irreversible reactions

Answer 2

• Reversible – products can be converted back to reactants
  
  • Can reach a state of equilibrium where concentrations of reactants and products don’t change over time
• Irreversible – products cannot be converted back to the reactants

Question 3

Dynamic equilibrium

Answer 3

• Forward and reverse reactions occur simultaneously at the same rate
• Can only be achieved in closed systems, not open

Question 4

During dynamic equilibrium…

Answer 4

• Reaction is ‘incomplete’ and all substances are present in the equilibrium mixture
• Bonds are constantly being broken and and formed

Question 5

Extent of reaction

Answer 5

• Describes how much product is formed when the system reaches equilibrium

Question 6

Homogeneous vs heterogeneous chemical systems

Answer 6

• Homogeneous – reactants & products in the same state
• Heterogeneous – reactants & products in different states

Question 7

Le Chatelier’s Principle

Answer 7

• If an equilibrium system is subjected to a change, the system will adjust itself to partially oppose the effect of the change

Question 8

Position of equilibrium

Answer 8

• Relative amounts of reactants and products at equilibrium
• Position of equilibrium may be changed by:
  
  • Adding/removing a reactant or product
  • Changing pressure by changing volume (for equilibria involving gases)
  • Dilution (for equilibria in solution)
  • Changing the temperature

Question 9

Equilibrium constant (K)

Determined using the equilbrium law

Answer 9

• Value of the concentration fraction for an equilibrium system
• This value indicates whether a reaction is at equilibrium

Question 10

Effect of temperature on equilibrium constant (K)

Answer 10

• K is temperature dependent (its effect varies if the reaction is exothermic or endothermic)
  
  • Exothermic: increased temperature = decreased K value
  • Endothemic: increased temperature = increased K value
• NOT affected by changes in concentration, pressure or catalysts

Question 11

Reaction quotient (Q)

Answer 11

• The value of the concentration fraction for a system that is not at equilibrium
• Can be calculated at any time during a reversible reaction
• Q = K when a reaction system at a particular temperature reaches equilibrium

Question 12

Effect of doubling the pressure of an equilibrium system

According to LCP

Answer 12

• When the pressure is doubled, the system will move to reduce the pressure
• The equilibrium position will move in the direction of the lower number of particles
• E.g. if there are 4 moles on the reactant side and 2 moles on the product side, the system will move in a forward direction

Question 13

Effect of increasing the pressure of an equilbrium system with equal moles of products and reactants

According to LCP

Answer 13

• Increasing the pressure will have no effect

Question 14

Effect of increasing the temperature of an endothermic equilibrium

According to LCP

Answer 14

• Will result in a forward reaction
• Endothermic reactions have a positive △H, indicating that the reaction needs heat in order to proceed (increasing temp results in more heat, on the reactant side, being added to the system)
• The system opposes this change by consuming the heat