1.6.3 Enthalpy Changes

Question 1

Enthalpy is

Answer 1

the total chemical energy inside a substance

Question 2

An enthalpy change is represented by the symbol

Answer 2

ΔH (Δ= change; H = enthalpy)

Question 3

A reaction is exothermic when the products have

Answer 3

less energy than the reactants

Question 4

Heat energy is given off by the reaction to

Answer 4

the surroundings

Question 5

The temperature of the environment

Answer 5

increases

Question 6

The energy of the system

Answer 6

decreases

Question 7

There is an enthalpy decrease during the reaction so ΔH is

Answer 7

negative

Question 8

Exothermic reactions are what possible

Answer 8

thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)

Question 9

However, if the rate is too slow, the reaction may not occur, in this case the reaction is

Answer 9

kinetically controlled

Question 10

The enthalpy change during an exothermic reaction

Answer 10

Question 11

A reaction is endothermic when the products have

Answer 11

more energy than the reactants

Question 12

Heat energy is absorbed by the reaction from

Answer 12

the surroundings

Question 13

The temperature of the environment

Answer 13

decreases

Question 14

The energy of the system

Answer 14

increases

Question 15

There is an enthalpy increase during the reaction so ΔH is

Answer 15

positive

Question 16

The enthalpy change during an endothermic reaction

Answer 16

Question 17

the system is

Answer 17

the substances that are reacting (i.e. the reaction itself)

Question 18

the system is the substances that are reacting (i.e. the reaction itself) and the surroundings is

Answer 18

everything else (e.g. the flask the reaction is taking place in)

Question 19

To be able to compare the changes in enthalpy between reactions the pressure is

Answer 19

100 kPa

Question 20

To be able to compare the changes in enthalpy between reactions the temperature is

Answer 20

298 K (25 oC)

Question 21

To be able to compare the changes in enthalpy between reactions, each substance involved in the reaction is in its

Answer 21

standard physical state (solid, liquid or gas)

Question 22

To show that a reaction has been carried out under standard conditions, the symbol

Answer 22

Ꝋ is used

Question 23

ΔHꝊ =

Answer 23

the standard enthalpy change

Question 24

Standard Enthalpy Change of reaction

Answer 24

the enthalpy change when the reactants in the stoichiometric equation react to give the products under standard conditions

Question 25

Standard Enthalpy Change of formation is

Answer 25

the enthalpy change when
one mole of a compound is formed from its elements under standard conditions

Question 26

Standard Enthalpy Change of combustion is

Answer 26

the enthalpy change when
one mole of a substance is burnt in excess oxygen under standard conditions

Question 27

Standard Enthalpy Change of neutralisation is

Answer 27

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

Question 28

Standard Enthalpy Change of reaction exo/endo

Answer 28

both

Question 29

Standard Enthalpy Change of formation exo/endo

Answer 29

both

Question 30

Standard Enthalpy Change of combustion exo/endo

Answer 30

exo

Question 31

Standard Enthalpy Change of neutralisation exo/endo

Answer 31

exo

Question 32

Answer 32

Question 33

Answer 33

Question 34

Answer 34

Question 35

The ΔHfꝊ of an element in its standard state is

Answer 35

zero, for example, ΔHfꝊ of O2(g) is 0 kJ mol-1