C10 Enthalpy Changes

Question 1

Enthalpy change

Answer 1

The difference between the enthalpy of the reactants and the enthalpy of the products

Change in H= enthalpy of products- enthalpy of reactants

Question 2

Activation energy

Answer 2

The minimum energy needed for a reaction to take place

Question 3

Bond enthalpy

Answer 3

The enthalpy change required to break and separate 1 mol of bonds in the molecules of a gas so that the resulting gaseous (neutral) particles exert no forces upon each other.

Question 4

Specific heat capacity

Answer 4

The heat required to increase the temperature of 1.0 g of the substance by 1 degree Celsius (1K)

Question 5

Standard enthalpy change of formation

Answer 5

The enthalpy change when one mol of a substance is formed from its elements, in their natural state, under standard conditions

Question 6

Standard enthalpy change of combustion

Answer 6

The enthalpy change when one mol of a substance is burned completely, in excess of oxygen, under standard conditions

Question 7

Standard enthalpy change of neutralisation

Answer 7

The enthalpy change when one mol of water is formed in a reaction between an acid and a base

Question 8

Standard enthalpy change of reaction

Answer 8

The enthalpy change when the amount in moles of the substances in the equation as written react under standard conditions

Question 9

Hess’ law

Answer 9

States that, if a reaction can take place by more than one route, the enthalpy change for the reaction is the same irrespective of the route of taken, provided that the initial and final conditions are the same

Question 10

Exothermic

Answer 10

Energy given out
Eg. Hot packs 
Delta H is negative 
Lost energy 
Reactants higher than products

Question 11

Endothermic

Answer 11

Heat taken in
Delta H is positive
Eg. Cold pack, photosynthesis
Reactants lower than products

Question 12

Bond enthalpy equation

Answer 12

Delta H= sum of bond enthalpies of reactants- sum of bond enthalpies of products

Question 13

Relevant bond enthalpies

Answer 13

C-H 413kJ mol
C- -O 805 kJ mol
O - - O 498 kJ mol
O-H 464 kJ mol

Question 14

Specific heat capacity equation

Answer 14

q= mct
Delta H= mc delta T/ 1000n
If t increases exothermic delta H negative
If t decreases endothermic delta H positive

Question 15

Reliability of thermochemical experiments

Answer 15

Lack of precision of the measuring equipment such as the thermometer
Heat loss to the surroundings
The slowness of the reaction

Question 16

Bomb calorimeter

Answer 16

Used to find out the the energy change more effectively

Contains 2 insulating layers to help results be more reliable and valid

Question 17

Standard conditions

Answer 17

25°C or 298K

100kPa

Question 18

Standard of enthalpy of formation conditions

Answer 18

Show reactants in their natural state
Produce 1 mol of the substance, even if this means that there are fractions in the equation
Include state symbols

Question 19

Standard enthalpy of combustion conditions

Answer 19

Show 1 mol of the substance reacting completely with excess oxygen, even if this means having fractions in the equation
Include state symbols

Question 20

Standard enthalpy change of neutralisation conditions

Answer 20

Show on1 mol of water being formed from the named base or acid, even if this means having fractions in the equation
Include state symbols

Question 21

Constructing enthalpy cycles

Answer 21

1. Start with the enthalpy change that has to be calculated (Delta H1). Write an equation for the reaction. This is the top line of the cycle
2. Construct an enthalpy cycle with 2 alternative routes.
3. H1 + H3 = H2
   Then rearrange