3.2 Physical Chemistry

Question 1

Define enthalpy change of reaction

Answer 1

Energy change associated with a given reaction

Question 2

Define enthalpy change of formation.

Answer 2

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.

Question 3

Define enthalpy change of combustion.

Answer 3

The energy change that takes place when 1 mole of substance is completely combusted.

Question 4

Define enthalpy change of neutralisation.

Answer 4

The energy change associated with the formation of 1 mole of water from a neutralisation reaction

Question 5

What are standard conditions?

Answer 5

100kPa
298K
All substances in standard states

Question 6

What is the standard enthalpy change of the formation of an element?

Answer 6

0kJ/mol

Question 7

What is calorimetry?

Answer 7

The quantitative study of energy in a chemical reaction.

Question 8

What is specific heat capacity?

Answer 8

The energy required to raise the temperature of 1g of a substance by 1K.

Question 9

What is q in the equation for calorimetry?

Answer 9

Heat exchanged with the surroundings (J)

Question 10

What is m in the equation for calorimetry?

Answer 10

Mass of the substance heated or cooled (g)

Question 11

What is c in the equation for calorimetry?

Answer 11

The specific heat capacity of the substance that is heated or cooled (J/g/K)

Question 12

How do you reduce heat lost to surroundings in calorimetry?

Answer 12

Insulating equipment with draught shields.

Question 13

What is average bond enthalpy?

Answer 13

The mean energy needed for 1 mole of a given type of gaseous Bonds to undergo homolytic fission

Question 14

Examples of exothermic reactions

Answer 14

Combustion

respiration

Question 15

Examples of endothermic reactions

Answer 15

Photosynthesis

Thermal decomposition

Question 16

Use ideas about the enthalpy changes that take place during bond breaking and bond
making to explain why some reactions are exothermic

Answer 16

Bond breaking absorbs energy
AND bond making releases energy
More energy released than absorbed

Question 17

Suggest three reasons why standard enthalpy changes of combustion determined
experimentally are less exothermic than the calculated theoretical values.

Answer 17

Heat released to the surroundings
Incomplete combustion OR incomplete reaction
OR not everything burns
Non-standard conditions

Question 18

Name the catalyst used in C2H4(g) + H2O(g) C2H5OH

Answer 18

acid

Question 19

State le Chatelier’s principle.

Answer 19

When conditions on a system in equilibrium are changed

The equilibrium moves to minimise the effects of the change

Question 20

What is important when calculating kJmol-1?

Answer 20

Remember + or - sign

Question 21

It is very difficult to determine the standard enthalpy change of formation of
hexane directly. Suggest a reason why

Answer 21

many different hydrocarbons would form
OR activation energy too high
OR reaction too slow
OR they don’t react together

Question 22

Choose a reaction that you have studied that is catalysed by chlorine radicals

Answer 22

Cl + O3 → ClO + O2
ClO + O → Cl + O2
overall: O3 + O → 2O2
OR
Cl + CH4 → CH3 + HCl
CH3 + Cl2 → CH3Cl + Cl
overall: CH4 + Cl2 → CH3Cl + HCl

Question 23

What is a catalyst?

Answer 23

A substance that increases rate of reaction without being used up during the process.

Question 24

What is a homogenous catalyst?

Answer 24

A catalyst that is in the same phase as the reactants (eg liquid catalyst that is mixed with liquid reactants)

Question 25

Give an example of a homogenous catalyst

Answer 25

Enzymes in blood and saliva

Question 26

What is a heterogeneous catalyst?

Answer 26

A catalyst that is in a different phase from the reactants.

Question 27

What is an example of a heterogeneous catalyst?

Answer 27

A catalytic converter in a car.

Question 28

What catalyst breaks down the triple bond in nitrogen in the Haber process?

Answer 28

Iron

Question 29

What is the Boltzmann distribution?

Answer 29

The distribution of energies of molecules at a particular temperature, often shown as a graph.

Question 30

What does the area under the curve of a Boltzmann distribution graph represent?

Answer 30

The number of molecules

Question 31

Why does the Boltzmann curve start at the origin?

Answer 31

There are no molecules with 0 energy.

Question 32

Why does the Boltzman curve not touch the energy axis?

Answer 32

There is no maximum energy of a molecule

Question 33

When is a chemical system in dynamic equilibrium?

Answer 33

the concentrations of the reactants and the products remain constant the rate of forward reaction is the same as the rate of the reverse reaction

Question 34

How can the position of equilibrium be altered?

Answer 34

By changing: concentrations of reactants and products pressure in reactions involving gas temperature

Question 35

What does a catalyst do to equilibrium?

Answer 35

Speeds up the rate of the forward and reverse reactions equally Establish it faster

Question 36

Examples of important equilibrium systems?

Answer 36

Haber process | the conversion of sulphur dioxide into sulphur trioxide in the Contact process.

Question 37

Where do nitrogen and hydrogen come from in the Haber process?

Answer 37

Nitrogen is obtained from air by fractional distillation | Hydrogen is prepared by reacting methane and water

Question 38

What are the optimum conditions for the Haber process?

Answer 38

High pressure and low temperature Forward reaction produces less molecules The forward reaction is exothermic

Question 39

What are the drawbacks of the optimum conditions of the Haber process?

Answer 39

Low temperature means low rate | High pressure very expensive high concentration bad safety

Question 40

What are the actual conditions used on a modern ammonia plant?

Answer 40

450 degrees 200atm Iron catalyst

Question 41

Why do bond enthalpies have positive values?

Answer 41

Breaking bonds is endothermic and energy must be put in

Question 42

Define the term bond enthalpy

Answer 42

The energy needed to break 1 mile of bonds in the substance in the gaseous state

Question 43

How do catalysts improve sustainability?

Answer 43

``` Not used up in reaction Lower temperatures Lower Ea Less fuel Less CO2 Different reactions can be used Better atom economy Less hazardous chemicals Generate specific products ```