Chemistry- Kinetics, Chemical Equilibria and Redox

Question 1

What is activation energy?

Answer 1

The minimum amount of energy required to break bonds between reactants.

Question 2

What is a catalyst and how do they work?

Answer 2

A substance which speeds up the rate of reaction without being used up in the reaction. They work by providing an alternative route for the reaction to take place where the activation energy is lower.

Question 3

What is the Maxwell Boltzman Distribution?

Answer 3

All molecules of a particular compound/element have the same mass so their kinetic energy is only dependant on the speed of its particles.

Question 4

What effect does temperature have on the maxwell boltzman distribution?

Answer 4

Increase in temperature, the molecules will on average have more kinetic energy and move faster. A greater proportion of molecules will have energies greater than the activation energy.

Question 5

What effect does a catalyst have in the maxwell boltzman distribution?

Answer 5

A catalyst provides an alternative route for the reaction with a lower activation energy. More molecules have high enough energy to react upon collision therefore in a certain amount of time more molecules react and hence the rate of reaction is increased.

Question 6

How can you calculate a reaction rate for a chemical reaction?

Answer 6

Rate = ΔConcentration/ΔTime

Question 7

On a maxwell boltzman distibution curve what happens to the curve when the temperature is increased?

Answer 7

• Curve moves to the right
• most probably energy lowers
• The rest of the curve is higher (more molecules) than the previous curve.

Question 8

On a maxwell boltzman distibution curve what happens to the curve when a catalyst is present?

Answer 8

• The activation energy line and shaded region moves to the left.

Question 9

What is a Heterogenous catalyst?

Answer 9

A catalyst which is in a different state to the reactants.
(e.g a solid catalyst would have liquid/gas reactants)
They work by providing a surface for the reaction to take place on.

Question 10

What is a Homogenous catalyst?

Answer 10

A catalyst which is in the same state as the reactants.

They work by forming intermediates.

Question 11

What is the contact process?

Answer 11

Process that produces sulphuric acid in high concentration needed for industrial processes.
Catalyst - Vandium oxide V2O5

Question 12

What is the Harber Process?

Answer 12

Used to make ammonia for fertilisers.

Catalyst - Iron Fe

Question 13

What is a reversible reaction?

Answer 13

A Chemical reaction where the reaction can go both ways.

i.e The reactants produce the products when they react and the products can react to produce the original reactants.

Question 14

What is Chatelier’s Principle?

Answer 14

The position of equilibrium shifts to try to cancel out any changes made.

Question 15

What effect does temperature have on equilibrium?

Answer 15

Depends of whether the forwards reaction is exothermic or endothermic:
If the temperature is changed then equilibrium will shift in the opposite direction to counteract the change.
E.g if the temperature is increased, then the reaction equilibrium will shift in the endothermic direction to decrease the temperature.

Question 16

What effect does concentration have on equilibrium?

Answer 16

When the concentration of the products or reactants is increased the equilibrium will shift to the side which will decrease the concentration. Same happens if the concentration is decreased.

• Add molecules to the left and equilibrium system moves right.
• Add molecules to the right and the equilibrium system moves to the left.

Question 17

What effect does pressure have on equilibrium?

Answer 17

Pressure only effects equilibrium if the number of moles on either side of the reaction is unequal.
More moles on the right side of the reaction, if pressure is increased then equilibrium will move to the left, and vice versa.

Question 18

What effect does a catalyst have on equilibrium?

Answer 18

Catalysts do not affect Kc or the postition of equilibrium.

It only affects the rate of reaction as the rate of the forwards and backwards reaction is increased equally.

Question 19

What is Equilibrium law?

Answer 19

Tells us the relative proportions of reactants and products present at equilibrium.

Question 20

What is the Equilibrium law equation and what must also be calculated?

Answer 20

For reaction: aA +bB cC + dD
Kc=( [C]^c [D]^d) /( [A]^a [B]^b)
Must calculate the units in mol^-1dm^3

Question 21

What is the constant Kc used for?

Answer 21

Kc is a constant which represents how far the reaction will proceed at a given temperature.
Temperature is the only factor which will affect the Kc Value.
When:
kc = 1 Reaction is almost equal
Kc > 1 Reaction favours forwards direction (products)
Kc < 1 Reaction favours backwards direction (reactants)

Question 22

What is Kp?

Answer 22

The equilibrium constant calculated from partial pressures for a system at a constant temperature.

Question 23

Define Partial pressure?

Answer 23

The pressure that a gas in a mixture of gases would exert if it alone occupied the available volume.

Question 24

What is a Homogenous system?

Answer 24

When all the components in the system / equation are the same state (e.g Gas /liquid / solid)

Question 25

What is the Kp formula?

Answer 25

For reaction: aA(g) +bB(g) cC(g) + dD(g)
Kp = [PC]^c [PD]^d / [PA]^a [PB]^b
Work out units as well.

Question 26

How do you calculate partial pressure?

Answer 26

partial pressure of gas = Mole fraction of gas x Total pressure

Question 27

What are the rules for oxidation numbers?

Answer 27

1) As an ion. oxidation number = ion charge
2) Elements oxidation number = 0
3) Oxidation number of monotonic ions (one atom) is equal to the charge of the ion.
4) O2 oxidation number = -2 except in compounds of peroxide.
5) H2 oxidation number = +1 except in a metal hydride when it = -1
6) F oxidation number = -1. All other Halogens = -1 except from Cl with O2
7) sum of oxidation number =0
8) Sum of oxidation numbers in a polyatomic ion = charge of ion
9) Hydroxide ion is -1 charge.

Question 28

How do you balance Ionic half equations?

Answer 28

1) Write out balanced equation for species
2) work out oxidation states (before and after)
3) balance states with electrons
4) if charges do not balance add H+ ions to one side to balance.
5) if equation still isnt balance add H2O to one side.

Question 29

How do you combine half equations?

Answer 29

1) multiply electrons so no of electrons in each is the same.
2) add the 2 equations and cancel out electrons either side of the equation
3) If necessary cancel out any other species that appear either side of the equation.