Chemical Equilibria

Question 1

What are reversible reactions?

Answer 1

Reactions where he products can be reacted to make the original reactants. They are in constant dynamic equilibrium

Only completely dynamic when it is isolated
(Nothing put on or taken out)

Question 2

What are he properties of dynamic equilibrium?

Answer 2

The concentration of the products and reactants is constant

The rate of the forward reaction is equal to the rate of the backwards reaction

Question 3

What is la chateliers principle?

Answer 3

When a system is in dynamic equilibrium it is subject to change. The position of equilibrium will shift to minimise the change.

Question 4

What happens to the position of equilibrium when we change temperature?

Answer 4

Equilibrium depends on the enthalpy of the reaction.

If the forwards reaction is exothermic (negative) and the temperature is increased, equilibrium will shift to oppose this change and move in the endothermic direction (backwards) and therefore favour the reactants

Question 5

What happens to the position of equilibrium when we change concentration ?

Answer 5

Equilibrium depends on the concentration of products or reactants. ( number of molecules present)

Increase the concentration will cause equilibrium to move to the side with the lower concentration (fewer molecules)

Question 6

What happens to the position of equilibrium when we change pressure?

Answer 6

Only affects reactions involving gases

Has the same effect as changing the concentration so increasing the pressure will shift equilibrium to the side with the fewer molecules

Question 7

What happens to the position of equilibrium in the presence of a catalyst?

Answer 7

Does not change the position of equilibrium

Only affects the rate of the reaction

Both the forwards and backwards reactions equally.

Question 8

What is partial pressure?

Answer 8

The pressure that a single gas exerts on its own is the partial pressure

Total pressure is the sum of partial pressures

Question 9

What is mole fraction?

Answer 9

The fraction of mixture that is made from a particular gas

Question 10

How do you calculate mole fraction?

Answer 10

Number of moles of substance A / total number of moles of all substances

Question 11

How do you calculate partial pressure?

Answer 11

Partial pressure = mole fraction X total pressure

Question 12

What is the equation for the Haber process?

What are the optimum conditions?

Answer 12

N2 +3H2 2NH3

Optimum conditions

Low temperature
High pressure

Question 13

What are the issues with the optimum conditions for the haber process?

Answer 13

High pressure: potentially unsafe and expensive

Low temperature: decreases rate of reaction

Question 14

What are the industrial conditions used for the haber?

Answer 14

Temperature:
400-500 degrees
High enough for a good rate of reaction
Suitable yield

Pressure:
200 atm
High enough to maximise yield and maintain safety

Catalyst:
Iron catalyst
Increased rate of reaction

Question 15

What is Kc

Answer 15

Gives a measure of where equilibrium lies

Kc = [products] / [reactants]

Question 16

Explain which reaction is favoured when Kc = 1. Kc < 1. Kc > 1

Answer 16

Kc = 1
1:1 ratio

Kc > 1
Favours products

Kc < 1
Favours reactants

Question 17

What is Kp?

Answer 17

Used for equilibrium reactions involving gases we can write expressions in terms of partial pressures and not concentration

Question 18

What is the equation for Kp?

Answer 18

Kp = P(products) / P(reactants)

For Kp expressions where more than 1 state is present we only include gases

Question 19

How does temperature affect K?

Answer 19

Whether increases or decreases depends on whether the reaction is exo or endothermic

Forwards reaction endothermic:
increases in temperature pushes equilibrium towards the products and therefore K increases

Forwards reaction exothermic:
increase in temperature pushes equilibrium towards the reactants and therefore K decreases