Enthalpy

Question 1

Define enthalpy

Answer 1

the measure of the heat content of a substance. measured in kilojoules per mole

Question 2

can enthalpy be measured

Answer 2

no, however, a change in enthalpy can be

Question 3

what are the standard conditions

Answer 3

pressure: 100kPa
temp: 298K

Question 4

define activation energy

Answer 4

the minimum energy required to initiate a reaction

Question 5

what’s an enthalpy profile diagram and how can it show if a reaction is exothermic/endothermic?

Answer 5

its an enthalpy-reaction progress diagram. if the enthalpy of the products is higher than the reactants then energy has been absorbed making it an endothermic reaction. if its lower then energy was released so its an exothermic reaction

Question 6

define enthalpy change of formation

Answer 6

enthalpy change when one mole of a substance is made from its constituent elements in there standard states under standard conditions

Question 7

define enthalpy change of combustion

Answer 7

enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 8

define enthalpy change of neutralisation

Answer 8

enthalpy change when one mole of water is formed from an acid and alkali under standard conditions

Question 9

define specific heat capacity

Answer 9

the energy required to raise 1kg of a substance by 1 degree Celsius (or by one kelvin would be the same value)

Question 10

possible problems with calorimetry experiments

Answer 10

heat loss to the surroundings,
container absorbing some of the heat
the mass and SHC of water are used not the solution

Question 11

what is Hess’s law

Answer 11

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

Question 12

how can the change in enthalpy (ΔH) be calculated for a reaction if given the enthalpy change of formation

Answer 12

ΔH= enthalpy change of formation of the products - enthalpy change of formation of the reactants. this is due to Hess’s law

Question 13

example of Hess cycle using ΔcH

Answer 13

Reactants ΔH –> products
ΔcH ΔcH
Oxides

ΔcH of reactants to the oxides is the same as the ΔH of the reaction + ΔcH of the products to the oxides

Question 14

why does oxygen not have an enthalpy change of combustion

Answer 14

oxygen can not be added to oxygen so it has no combustion reaction

Question 15

define average bond enthalpy

Answer 15

the average enthalpy change when breaking one mole of a given type of bond in the molecules in a gaseous species

Question 16

how can average bond enthalpy be used to calculate the enthalpy change of a reaction

Answer 16

add the average bond enthalpy for every bond in the reactants. take away the average bond enthalpy of every bond in the products

Question 17

define ‘lattice enthalpy’

Answer 17

enthalpy change when 1 mole of an ionic lattice is formed from its constituent gaseous ions at an infinite separation so the ions don’t interact

Question 18

define ‘enthalpy of atomisation’

Answer 18

the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state

1/2 Cl2 –> Cl

Question 19

define ‘bond dissociation enthalpy’

Answer 19

the enthalpy change when one mole of a covalent bond is broken into two gaseous atoms

Question 20

define ‘first ionisation enthalpy’

Answer 20

the enthalpy change when one moles of electrons is removed from 1 mole of gaseous atoms forming one moles of gaseous 1+ ions

Question 21

define ‘electron affinity’

Answer 21

the enthalpy change when one mole of gaseous atoms gain one mole of electrons to form one mole of gaseous 1- ions

Question 22

define ‘enthalpy change of hydration’

Answer 22

the enthalpy change when one mole of gaseous ions become aqueous ions
(always exothermic)

Question 23

define ‘enthalpy change of solution’

Answer 23

the enthalpy change when one mole of an ionic solid dissolves in a large enough amount of water to ensure that the dissolved ions are well separated and do not interact