1.4 Energetics

Question 1

What energy change is breaking bonds associated with?

Answer 1

Energy is taken in to break bonds → endothermic reaction

Question 2

What energy change is making bonds associated with?

Answer 2

Energy is released to make bonds → exothermic reaction

Question 3

What are some uses of thermochemistry?

Answer 3

Measuring and comparing the energy values of fuels
Calculating the energy requirements for industrial processes
Working out the theoretical amount of energy released/taken
in in a reaction
Predicting if a reaction will take place or not

Question 4

What is an endothermic reaction?

Answer 4

One with an overall positive enthalpy change (+ΔH) → energy in breaking bonds > energy out making bonds

Question 5

What is an exothermic reaction?

Answer 5

One with an overall negative enthalpy change (-ΔH) → energy in breaking bonds < energy out making bonds

Question 6

If a reversible reaction is endothermic one way, what type of reaction is the other way?

Answer 6

exothermic

Question 7

Give 2 examples of exothermic reactions

Answer 7

Combustion of fuels

Neutralisation

Question 8

Give an example of an endothermic reaction

Answer 8

Thermal decomposition

Question 9

Define enthalpy change; what symbol is used to

represent it?

Answer 9

Energy change of a system at a constant pressure represented by ΔH

Question 10

What are the standard conditions?

Answer 10

100kPa / 1atm pressure

298K / 250C temperature

Question 11

What does “in standard state” mean?

Answer 11

The state an element/compound exists at in standard conditions (100kPa, 298K)

Question 12

Define standard enthalpy of formation

Answer 12

The enthalpy change when one mole of a compound is formed from it constituent elements in standard conditions (100kPa, 298K), with reactants and products in their standard states.

Question 13

Give an example of an equation which represents

standard enthalpy of formation

Answer 13

H2 (g) + ½ O2(g) → H2O (l)

Question 14

Define standard enthalpy of combustion

Answer 14

The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states.

Question 15

Give an example of an equation which represents

standard enthalpy of combustion

Answer 15

C (s) + O2 (g) → CO2 (g)

Question 16

What is the difference between heat and

temperature?

Answer 16

Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.

Question 17

How can you calculate enthalpy change from experimental data?

Answer 17

Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K -1) and ΔT is the change in temperature

Question 18

What is a flame calorimeter; how does it differ to a

simple calorimeter?

Answer 18

Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.

Question 19

How would you measure the enthalpy change for a

reaction occurring in (aq)?

Answer 19

Use an expanded polystyrene cup as a calorimeter (good insulator → reduce heat loss).

Heat is generated in the solution; measure this
temperature change.

Take heat capacity of
solution to be 4.18 and density of solution = 1gcm-3

Question 20

What can you use to make experimental
determination of enthalpy change of reaction more
accurate?

Answer 20

Cooling curves

Question 21

What is Hess’s Law?

Answer 21

States that the enthalpy change for a reaction is the same regardless of the route taken

Question 22

What is the enthalpy of an element?

Answer 22

The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0

Question 23

Define bond dissociation enthalpy.

Answer 23

The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules.

Question 24

Define mean bond enthalpy.

Answer 24

Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond.

Question 25

Why may experimental methods for enthalpy

determination not be very accurate?

Answer 25

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

Question 26

Why will using bond enthalpies not be as accurate

as using standard enthalpy of combustion/formation?

Answer 26

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.