Physical Chemistry

Question 1

What is the enthalpy change of a process?

Answer 1

The heat transferred in the process at a constant pressure.

Question 2

Draw the energy profile diagrams for:

• endothermic reactions
• exothermic reactions

labelling reactants, products, Δ H and E_a

Answer 2

Question 3

What is the standard enthalpy change of a reaction?

Answer 3

Enthalpy change when reaction occurs in the molar quantities shown in the equation, under standard conditions.

Question 4

What is the standard enthalpy of formation?

Answer 4

Enthalpy change when 1 mol of a compound is formed from its elements in their standard states, under standard conditions.

Question 5

What is the standard enthalpy of combustion?

Answer 5

The enthalpy change when 1 mol of a substance undergoes complete combustion in oxygen, under standard conditions.

Question 6

What is the standard enthalpy of neutralisation?

Answer 6

The enthalpy change when an acid and an alkali react together under standard conditions to from 1 mole of water (always -57 kJmol^-1)

Question 7

What is activation energy?

Answer 7

The minimum energy required for a reaction to occur by breaking the bonds in the reactant molecules.

Question 8

What is bond enthalpy?

Answer 8

The average energy needed to break 1 mol of bonds in gaseous molecules.

Question 9

What are the sources of heat loss in calorimetry experiments?

Answer 9

• Heating surroundings and equipment
• Incomplete combustion

Question 10

What conditions need to be met for two particles to react?

Answer 10

• They need to be orientated correctly when they collide.
• They need to collide with the minimum amount of kinetic energy required for. the reaction (activation energy).

Question 11

How does increasing the temperature affect the rate of a reaction? Draw two Boltzmann distributions on the same axes to aid your answer.

Answer 11

• Increased T - particles have more KE so will move faster on average.
• Greater proportion of particles will have at least the activation energy and be able to react.
• Molecules will also collide more often, therefore increasing the frequency of effective collisions.

Question 12

Explain the effect of increasing the concentration/pressure of reactants on the rate of a reaction.

Answer 12

• The particles with be closer together on avg.
• They’ll collide more frequently, meaning the frequency of effective collisions also increases
• The rate of reaction increases

Question 13

Explain the effect of adding a catalyst on the rate of a reaction. Draw an enthalpy profile diagram and Boltzmann distribution to aid your answer.

Answer 13

• Lower activation energy by providing a different pathway for the reaction to proceed by, meaning more particles will have enough energy to react.
• They therefore increase the rate of reaction without being used up.

Question 14

What are homogenous and heterogenous catalysts?

Answer 14

Homogenous: same physical state as reactants. Combines with reactants to form an intermediate species which then reacts to form the products and reform the catalyst.

Heterogenous: in a different phase/state from the reactants. Provides a surface on which the reaction takes place, therefore increasing the surface area of the catalyst increases the number of molecules that can react at the same time, increasing the rate of reaction.

Question 15

What are the benefits of using catalysts in industrial processes?

Answer 15

• Increases rate of reaction without requiring an energy input, meaning reasonable rates can be achieved without high temps./pressures so less energy is required, making the process less expensive. Also less fuel burnt so lower greenhouse gas emissions.
• Catalytic converters in cars decompose toxic carbon monoxide and nitrogen (II) oxide into carbon dioxide and nitrogen gas, which are less harmful to the environment.
• Catalysts reduce waste by allowing a different reaction to be used with a better atom economy.

Question 16

Describe the change in mass method of investigating reaction rates.

Answer 16

• Start a stop clock when reaction starts
• Take mass measurements at regular intervals
• Reaction is finished when mass doesn’t decrease over 3 successive time intervals
• Method is accurate but not suitable if gas released is toxic

Question 17

Describe the volume of gas produced method of measurig the rate of a reaction.

Answer 17

• Connect gas syringe to top of conical flask via delivery tube
• Take volume measurements at regular intervals
• Accurate but vigorous reactions can blow plunger out of syringe

Question 18

When does a dynamic equilibrium exist?

Answer 18

When the rate of the forward reaction is the same at the rate of the backward reaction so that the concentrations of products and reactants don’t change.

Question 19

What is le Chatelier’s principle?

Answer 19

In a dynamic equilibrium (i.e. a closed system), if there is a change in concentration, pressure or temperature, the equilibrium will shift to counteract the change.

Question 20

What effect do catalysts have on the position of equilibrium?

Answer 20

They have no effect - they increase the rate of the forward and backward reactions by the same factor - they don’t increase yield but do decrease the time taken for equilibrium to be reached.

Question 21

Describe how you can investigate the equilibrium position with changing temperature.

Answer 21

• Make up 2 sealed tubes containing an equilibrium mixture of brown NO₂ gas and colourless N₂O₄ gas.
• Place one in a warm water bath and one in a cool water bath.
• Tube in warm water will change to darker brown as equilibrium shifts to the left (N₂O₄ → 2NO₂, ΔH = +ve).
• Tube in cool water will turn a paler brown as equilibrium shifts to the right.

Question 22

Describe how you can investigate the equilibrium position with changing concentration.

Answer 22

• Fe₃⁺_(aq) + 3SCN^-_(aq) ⇌ Fe(SCN)_3(aq)
• yellow, colourless, blood red
• Add equal amounts of equilibrium mixture to 4 test tubes.
• Tube 1 - control (keeps inital yellow-red colour)
• Tube 2 - add iron (iii) nitrate, eqm. shifts right, mixture turns deep red
• Tube 3 - add potassium thiocyanate, eqm. shifts right, mixture turns deep red
• Tube 4 - add iron (iii) thiocyanate, eqm, shifts to left, mixture turns yellow.