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

Give 2 examples of exothermic reactions

Answer 6

Combustion of fuels
Neutralisation

Question 7

Give an example of an endothermic reaction

Answer 7

Thermal decomposition

Question 8

Define enthalpy change; what symbol is used to represent it?

Answer 8

Energy change of a system at a constant pressure

represented by ΔH

Question 9

What does “in standard state” mean?

Answer 9

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

Question 10

Define standard enthalpy of formation

Answer 10

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 11

Give an example of an equation which represents standard enthalpy of formation

Answer 11

H2 (g) + ½ O2 (g) → H2O (l)

Question 12

Define standard enthalpy of combustion

Answer 12

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 13

Give an example of an equation which represents standard enthalpy of combustion

Answer 13

C (s) + O2 (g) → CO2 (g)

Question 14

What is the difference between heat and temperature?

Answer 14

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 15

How can you calculate enthalpy change from experimental data?

Answer 15

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 16

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

Answer 16

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 17

How would you measure the enthalpy change for a reaction occurring in (aq)?

Answer 17

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 18

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

Answer 18

Cooling curves

Question 19

What is Hess’s Law?

Answer 19

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

Question 20

What is the enthalpy of an element?

Answer 20

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

Question 21

Define bond dissociation enthalpy.

Answer 21

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 22

Define mean bond enthalpy.

Answer 22

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

Question 23

Why may experimental methods for enthalpy determination not be very accurate?

Answer 23

Heat is lost to the surroundings

Not in standard conditions

Reaction may not go to completion

Question 24

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?

Answer 24

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.