3.2 - Enthalpy Changes

Question 1

Define enthalpy (delta H)

Answer 1

The heat energy transferred in a reaction at constant pressure.

Units - kJ/mol

Question 2

What is the charge of an exothermic reaction?

Answer 2

Negative

Question 3

What is the charge of an endothermic reaction?

Answer 3

Positive

Question 4

Describe the enthalpy profile diagram of an endothermic reaction

Answer 4

• The reactants are more stable than the products
• The enthalpy change is positive
• High activation energy

Question 5

Describe the enthalpy profile diagram of an exothermic reaction

Answer 5

• The reactants are less stable than the products
• Enthalpy change is negative
• Low activation energy

Question 6

Give an example of an exothermic reaction

Answer 6

• The combustion of methane
CH4(g) + 2O2(g) = CO2 (g) + 2H2O(l)

•The oxidation of carbohydrates like glucose in respiration

Question 7

Give an example of an endothermic reaction

Answer 7

• The thermal decomposition of calcium carbonate:

CaCO3(s) = CaO(s) + CO2(g)

• The main reactions of photosynthesis (sunlight supplies the energy)

Question 8

Why do you need to specify the conditions of enthalpy changes?

Answer 8

Changes in enthalpy are effected by temperature and pressure

The standard conditions are 298 K and 100 kPa

Question 9

Why are endothermic reactions positive?

Answer 9

Because in endothermic reactions more energy is needed to break bonds than released when making bonds

Question 10

Why are exothermic reactions negative?

Answer 10

Because more energy is released making bonds than needed to break bonds

Question 11

Define bond enthalpy

Answer 11

The energy needed to break one mole of bonds in the gas phase, averaged over many different compounds

Question 12

What is the equation for enthalpy changes?

Answer 12

q=mc ΔT

Question 13

Describe the values in the equation for enthalpy changes

Answer 13

q - heat loss (joules)

m - mass of water in calorimeter or solution in the insulated container (grams)

c - specific heat capacity of water (4.18 J g-1 K-1)

ΔT - the change in temperature of the water or solution (Kelvin, this is the same as the change in °C)

Question 14

How so do calculate the enthalpy change of reaction?

Answer 14

Enthalpy change of reaction

= Total energy absorbed - Total energy released

Question 15

Define standard enthalpy of reaction

Answer 15

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

Question 16

Define standard enthalpy change of formation

Answer 16

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

E.g:

2C(s) + 3H2(g) + 1/2O2 (g) = C3H5OH(l)

Question 17

Define standard enthalpy change of combustion

Answer 17

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

Question 18

Define standard enthalpy change of neutralisation

Answer 18

The enthalpy change when an acid and an alkali react together, under standard conditions, to form 1 mole of water

Question 19

What is the general equation for complete combustion?

Answer 19

CH(g) + O2(g) = CO2(g) + H2O(l)