Defintions

Question 1

Enthalpy

Answer 1

Enthalpy,H, is the heat content that is stored in a chemical system.

Question 2

Exothermic

Answer 2

Exothermic refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.

Question 3

Endothermic

Answer 3

Endothermic refers to a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants,resulting in heat being taken in from the surroundings.

Question 4

Enthalpy profile diagram

Answer 4

An enthalpy profile diagram is a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products.

Question 5

Activation energy

Answer 5

Activation energy is the minimum energy required to start a reaction by the breaking of bonds.

Question 6

Enthalpy profile diagram

Answer 6

An enthalpy profile diagram is a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products.

Question 7

Standard conditions

Answer 7

Standard conditions are a pressure of 100KPa( 1 atmosphere), a stated temperature, usually 298K (25*) and a concentration of 1.0 mol dm^-3 (for reactions with aqueous solutions).

Question 8

Standard state

Answer 8

Standard state is the physical state of a substance under the standard conditions of 100KPa(1 atmosphere) and 298K(25*).

Question 9

Standard enthalpy change of reaction , ^Hr*

Answer 9

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard state.

Question 10

Standard enthalpy change of combustion, ^Hc*

Answer 10

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.

Question 11

Standard enthalpy change of formation, ^Hf*

Answer 11

The standard enthalpy change of formation ^Hf* of a compound is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.

Question 12

Specific heat capacity

Answer 12

Specific heat capacity,c, is the energy required to raise the temperature of 1g of a substance by 1*C.

Question 13

Bond enthalpy

Answer 13

Bond enthalpy is the enthalpy change that takes place when breaking y homolytic fission 1 mol of a given bond in the molecules of a gaseous species.

Question 14

Average bond enthalpy

Answer 14

Average bond enthalpy is the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species.

Question 15

Hess’ law

Answer 15

Hess’law states that, if a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.

Question 16

Enthalpy cycle

Answer 16

An enthalpy cycle is a diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess’ law.