Enthalpy

Question 1

define: enthalpy (H)

Answer 1

enthalpy is the heat content that is stored in a chemical reaction.

Question 2

define: exothermic

Answer 2

exothermic refers to a reaction where the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings. (ΔH=negative)

Question 3

define: endothermic

Answer 3

endothermic refers to a reactions where the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings. (ΔH=positive)

Question 4

an enthalpy profile is…

Answer 4

a diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products.
(make sure you can draw these!)

Question 5

activation energy is..

Answer 5

the minimum energy required to start a reaction (by the breaking of bonds)

Question 6

what are standard conditions?

Answer 6

pressure: 100kPa (1 atmosphere)
temperature: 298k (25°C)
(and 1.0 mol dm-3 solutions, where solutions are involved)

Question 7

standard state is…

Answer 7

the physical state of a substance under the standard conditions.

Question 8

define: the standard enthalpy of reaction

Answer 8

the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation, under standard conditions.

Question 9

define: the standard enthalpy of neutralisation

Answer 9

the enthalpy change when 1 mole of water is formed from a reaction between an acid and an alkali, under standard conditions.

Question 10

define: the standard enthalpy change of combustion

Answer 10

The enthalpy change when one mole of a substance reacts completely with oxygen under standard conditions

Question 11

define: the standard enthalpy change of formation

Answer 11

The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

Question 12

define: Specific Heat Capacity, (c)

Answer 12

the amount of energy required to raise the temperature of 1g of a substance by 1°C

Question 13

equation in which enthalpy change can be calculated:

Answer 13

Q=mcΔT

Question 14

define: bond enthalpy

Answer 14

the enthalpy change that takes place when breaking, by homolytic fission, one mole of a given type of bond in gaseous molecules

Question 15

what is average bond enthalpy?

Answer 15

the average enthalpy change when breaking, by homolytic fission, one mole of a given type of bond in gaseous molecules

Question 16

is bond breaking exothermic or endothermic?

Answer 16

endothermic (it requires energy)

Question 17

is bond making exothermic or endothermic?

Answer 17

exothermic (it releases energy)

Question 18

what does Hess’ Law state?

Answer 18

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 19

an enthalpy cycle is…

Answer 19

a diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other know enthalpy changes using Hess’ Law.