Key Terms

Question 1

ΔH

Answer 1

‘Delta H’ symbol used to represent enthalpy change

Question 2

Enthalpy

Answer 2

A measure of the energy stored in (or heat content of) a system. Given the symbol H.

Question 3

Enthalpy level diagram

Answer 3

A diagram showing:

• amount of energy contained in both the reactants and the products for a reaction
• therefore the difference between them
• also called an enthalpy profile diagram

Question 4

Specific heat capacity

Answer 4

Amount of energy required:

• to raise the temperature 1 gram of a substance
• by one degree Kelvin (or Celcius)

Question 5

Heat

Answer 5

Energy transferred from one body to another due to a temperature difference

Question 6

Exothermic reaction

Answer 6

• a chemical reaction
• during which heat energy is given out to the surroundings
• ΔH for the reaction is negative

Question 7

Calorimetry

Answer 7

A technique used for measuring the amount of heat energy absorbed or given out by a reaction

Question 8

First law of thermodynamics

Answer 8

A law stating:

• increase in the internal energy of a system
• to the equal amount of energy added by heating the system
• minus the amount of energy lost as a result of the work done by the system
• it is sometimes stated in the form ‘energy cannot be created or destroyed; it can only change form’.

Question 9

Enthalpy change (for a reaction)

Answer 9

• heat energy exchange between the reacting system and its surroundings at constant pressure
• given the symbol ΔH.

Question 10

Endothermic reaction

Answer 10

• chemical reaction during which heat energy is taken in from the surroundings
• ΔH for the reaction is positive

Question 11

Hess’s Law

Answer 11

• a rule stating that the overall enthalpy change for a reaction
• is independent of the route the reaction takes

Question 12

Bond dissociation enthalpy

Answer 12

The energy required to break a particular bond

Question 13

Enthalpy cycle

Answer 13

• a diagram showing a reaction taking place by 2 or more alternative routes
• all of which involve the same enthalpy change, according to Hess’s law
• also called a Hess cycle

Question 14

Hess cycle

Answer 14

• a diagram showing a reaction taking place by 2 or more alternative routes
• all of which involve the same enthalpy change, according to Hess’s law
• also called an enthalpy cycle

Question 15

Bomb calorimeter

Answer 15

• a type of calorimeter that virtually eliminates heat loss

- and so provides very accurate for enthalpies of combustion

Question 16

Standard enthalpy of formation

Answer 16

• enthalpy change when 1 mole of a substance is formed from its elements
• under standard conditions (298K and 100kPa)
• with all the substances being in their standard states

Question 17

Thermodynamics

Answer 17

• a branch of study concerned with the transfer of heat and work
• conversion between heat and other forms of energy
• in its modern form it is based on 4 laws, the zeroth, first, second and third laws of thermodynamics

Question 18

Standard enthalpy of atomisation

Answer 18

• enthalpy change when 1 mole of gaseous atoms of a substance is formed from the substance in its standard state
• under standard conditions (298K and 100kPa)

Question 19

Standard state

Answer 19

State a substance is under standard conditions (298K and 100kPa)

Question 20

Standard enthalpy of neutralisation

Answer 20

• enthalpy change when 1 mole of water is produced by the reaction of acids and a base
• under standard conditions (298K and 100kPa)
• with all the substances being in their standard states

Question 21

Standard enthalpy of combustion

Answer 21

• enthalpy change for a reaction under standard conditions (298K and 100kPa)
• with all substances being in their standard states

Question 22

Standard enthalpy of combustion

Answer 22

• enthalpy change when 1 mole of a substance undergoes complete combustion
• under standard conditions (298K and 100kPa), with all

Question 23

Mean bond enthalpy

Answer 23

• average (mean) bond dissociation enthalpy for a particular bond in a range of different compounds
• precisely, it is the average enthalpy change for breaking one mole of a particular bond
• in a range of different compounds in the gas phase.