Chapter 9: Enthalpy (9.1-9.4)

Question 1

exothermic reactions

Answer 1

• products have less enthalpy than the reactants (negative ΔH)
• chemicals lose energy, the surroundings gain this energy (hence increase in temp)

Question 2

endothermic reactions

Answer 2

• products have more enthalpy than the reactants (positive ΔH)
• the surroundings lose energy (hence decrease in temp) which goes to the chemicals

Question 3

standard enthalpy changes are measured using the standard conditions.
what are those conditions?

Answer 3

1 atmosphere pressure / 100 kPa
room temperature of 25 degrees / 298K
solutions must have a concentration of 1mol/dm3
all substances in their standard states (physical state under standard conditions)

Question 4

equation for the heat energy change

Answer 4

q = m X c X ΔT

q: heat energy change in joules
m: mass of surroundings (the thing that you measure the temp change of) in grams
ΔT: temperature change

Question 5

unit for ΔH

Answer 5

kJ/mol

make sure you work it out per mole!

Question 6

common errors when determining enthalpy change of combustion (experimentally) and how to minimise them

Answer 6

errors: heat loss to surroundings, incomplete combustion of reactant in spirit burner, evaporation of reactant from wick, non-standard conditions being used (heated up during combustion)
solutions: adding a lid to water beaker, using draft shields around apparatus, insulating system

Question 7

breaking bonds

Answer 7

endothermic (positive ΔH)

energy needed from the surroundings to break the bonds

Question 8

making bonds

Answer 8

exothermic (negative ΔH)

Question 9

how to work out the enthalpy change of a reaction from average bond enthalpies - gaseous molecules of covalent substances

Answer 9

ΔrH = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)

Question 10

how activation energy effects reaction rate

Answer 10

small activation energy: rapid (energy needed to break bonds is readily available from surroundings)
Higher activation energies: slow (large energy barrier that reactions happen very slowly or not at all)

Question 11

what is average bond enthalpy

Answer 11

the mean amount of energy required to break 1 mole of a specified type of covalent bond in a gaseous molecule

Question 12

limitations of average bond enthalpies

Answer 12

The bond is in a different environment

Actual value for a bond in a certain molecule may be slightly higher/lower.

Question 13

state Hess’ law

Answer 13

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

Question 14

Enthalpy change of neutralisation

Answer 14

The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 15

Enthalpy change of formation

Answer 15

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 16

Enthalpy change of combustion

Answer 16

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

Question 17

what is Hess’ law used for?

Answer 17

to calculate enthalpy changes that are not easy to measure directly in experiments

Question 18

Working out enthalpy change (e.g delta h in diagrams)

Answer 18

H(products) - H(reactants)