CH8: Enthalpy Changes

Question 1

What is enthalpy change ?

Answer 1

The heat energy change in a reaction at constant pressure
Units : kJ/mol

Question 2

What are the standard conditions ?

Answer 2

100 kPa - pressure
298K - temp

Question 3

Definition of standard enthalpy change of reaction ?

Answer 3

The enthalpy change when the reaction occurs in the molar quantities shown in the chemical equation , under standard conditions

Question 4

Standard enthalpy change of formation?

Answer 4

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

Question 5

Standard enthalpy change of combustion?

Answer 5

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

Question 6

Standard enthalpy change of neutralisation?

Answer 6

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

Question 7

What is calorimetry?

Answer 7

Measurement of enthalpy changes in chemical reactions
Calorimeter can be made from:
- polystyrene cup /lid

Question 8

Eg of how to calculate enthalpy change for reaction between citric acid and sodium bicarbonate ?

Answer 8

1. Add set volume of citric acid to polystyrene cup
2. Add lid/ Measure temp every 30 secsuntil temp is stabilised
3. Add set volume of sodium bicarbonate and stir
4. Measure temp of reaction every 30secs until reached min/max temp and starts to return to initial temp
5. Draw temperature against time graph / extrapolate line where it start to return to normal temp/ find temp change

Question 9

Enthalpy calculation equations?

Answer 9

H = q/moles

Q = mct

Question 10

What does Hess’s law state?

Answer 10

The total enthalpy change of a reaction is always the same, no matter which route is taken

Question 11

When bond are made and broken, is it endothermic or exothermic ?

Answer 11

BROKEN : endothermic (energy is needed )
MADE : exothermic (energy is released)

Question 12

What is bond enthalpy ?

Answer 12

Amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Question 13

What is an average bond energy? And why is it needed?

Answer 13

Energy needed to break one mole of bonds in a gas phase, averaged over many different compounds

• bond energies are effected by other atoms in the molecule (environment) so average is taken , when that bond is in different environments

Question 14

What does the energy profile diagram look like for endothermic/exothermic reactions?

Answer 14

EXOTHERMIC : heat energy is given off to surrounding , so energy of system DECREASES

Reactants are ABOVE products = enthalpy change is NEGATIVE

ENDOTHERMIC: heat energy absorbed from surroundings , so energy of system INCREASES

Reactants BELOW products= enthalpy change is POSITIVE

Question 15

Give 3 reasons other than human error that lead to fpdiffernce in experimental value and value on internet for enthalpy change ?

Answer 15

• energy loss
• evaporation of substance
• incomplete combustion

Question 16

Why are bond enthalpy values always positive?

Answer 16

Bond breaking needs energy