Module 3 Section 2 - Enthalpy

Question 1

What is the definition of enthalpy change?

Answer 1

Enthalpy change is the heat energy transferred in a reaction at constant pressure. The units of enthalpy change are kJ mol-1

Question 2

What are the standard conditions of a reaction?

Answer 2

100 kpa / 1 atm pressure
298k

Question 3

What is an exothermic reaction?

Answer 3

A reaction that gives out energy. The enthalpy change is negative

Question 4

Example of exothermic reaction?

Answer 4

Oxidation is usually exothermic . 2 examples include:

1) Combustion of a fuel like methane
2) the oxidation of carbohydrates such as glucose in respiration

Question 5

What is an endothermic reaction?

Answer 5

Endothermic reactions absorb energy. Their enthaloy change is positive.

Question 6

Examples of endothermic reactions?

Answer 6

1) the thermal decomposition of calcium carbonate
CaCO3 -> CaO + CO2

2) the main reactions of photosynthesis are also endothermic

Question 7

What does the enthalpy profile diagram look like for endo/exothermic reactions?

Answer 7

For endothermic - the products are higher than the reactants
For exothermic - the products are lower than the reactants

Question 8

What is the standard enthalpy change of reaction ?

Answer 8

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

Question 9

What is the standard enthalpy of formation?

Answer 9

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

Question 10

What is standard energy change of combustion?

Answer 10

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

Question 11

What is standard enthalpy change of neutralisation?

Answer 11

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

Question 12

What is bond dissociation enthalpy?

Answer 12

The amount of energy stored in a bond between atoms in a molecule

Question 13

What is average bond enthalpy?

Answer 13

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

Question 14

How to calculate enthalpy change?

Answer 14

1) calculate amount of heat given off using q=mct

2) divide this answer by 1000 to convert from j to kj

3) calculate the number of moles of fuel by using mol= mass/mr

4) divide the heat given off by the number of moles to get answer in kj mol -1

Question 15

What is Hess’s law?

Answer 15

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

Question 16

How do you calculate enthalpy change of formation?

Answer 16

1) have the elements beneath the equation.

2) A = -B + C

3) calculate B and C by plugging in numbers and make sure you balance.

Question 17

How do you calculate enthalpy change of combustion?

Answer 17

1) make sure combustion products are beneath reaction.

2) A = B - C