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

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

Answer 5

error: heat lost to surroundings
solution: adding a lid, insulating the system

Question 6

unit for ΔH

Answer 6

kJ/mol

make sure you work it out per mole!

Question 7

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

Answer 7

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

Question 8

breaking bonds

Answer 8

endothermic
positive ΔH
energy is needed from the surroundings to break the bonds

Question 9

making bonds

Answer 9

exothermic

negative ΔH

Question 10

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

Answer 10

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

Question 11

how activation energy effects reaction rate

Answer 11

Reactions with small activation energies are normally rapid, since energy needed to break bonds is readily available from surroundings.
Higher activation energies may present such a large energy barrier that reactions happen very slowly or not at all.

Question 12

what is average bond enthalpy

Answer 12

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

Question 13

limitations of average bond enthalpies

Answer 13

The bond is in a different environment

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

Question 14

state Hess’ law

Answer 14

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 15

How do you measure enthalpy change?

Answer 15

Sum of enthalpy of products - sum of enthalpy of reactants

Question 16

Enthalpy change of neutralisation

Answer 16

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 17

Enthalpy change of formation

Answer 17

the enthalpy change that takes place when one mole of a compound in its standard state is formed from its constituent elements in their standard states under standard conditions

Question 18

Enthalpy change of combustion

Answer 18

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 19

what is Hess’ law used for?

Answer 19

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

Question 20

What is bond enthalpy?

Answer 20

Energy required to break one mole of a covalent bond averaged over a range of environments