M3 Enthalpy

Question 1

Define enthalpy

Answer 1

• Enthalpy H is a measure of the heat energy in a chemical system. The chemical system refers to the atoms, molecules or ions making up the chemicals.
• Enthalpy is thought of as the energy stored within bonds. It cannot be measured but enthalpy changes can.

Question 2

What is enthalpy change?

Answer 2

• Reactants and products are likely to have different enthalpies. The difference in enthalpies is the enthalpy change (ΔH):
  ΔH = H(products) - H(reactants)

Question 3

What is the law of conservation of energy?

Answer 3

• Energy cannot be created or destroyed.
• When a chemical reaction involving an enthalpy change occurs, heat energy is transferred between the system and the surroundings.
  • the system is the chemicals (reactants and products)
  • the surroundings is the apparatus, the laboratory and everything not in the chemical system

Question 4

What is an exothermic energy change/reaction?

Answer 4

• Energy transferred from the system to the surroundings.
• ΔH is negative.
• Chemical system looses energy, surroundings gain energy (increase in temperature)
• Any loss of energy by the chemical system is balanced by the same energy gain by the surroundings.

Question 5

What is an endothermic energy change/reaction?

Answer 5

• Energy is transferred from the surroundings to the system.
• The chemical system gains energy (in the form of heat) and the surroundings loose energy.
• ΔH is positive.
• Any energy gain by the chemical reaction is balanced by the same energy loss by the surroundings.
• Temperature of the surroundings decrease as they loose energy.

Question 6

What is activation energy?

Answer 6

• The energy input required to break bonds, that acts as an energy barrier to the reaction is known as activation energy (Ea).
• It is the minimum energy required for a reaction to take place.
• During chemical reactions the bonds in the reactants need to be broken by an input of energy (activation energy).

Question 7

How does activation energy affect rate of reaction?

Answer 7

• Reactions with small activation energies take place very rapidly, because the energy needed to break bonds is readily available from the surroundings.
• Very large activation energies present such a large energy barrier that a reaction may take place extremely slowly or even not at all.

Question 8

What is the standard enthalpy change?

Answer 8

• The enthalpy change under standard conditions (ΔH°)
• The enthalpy change for a reaction can vary depending on conditions used. Chemists use standard conditions for physical measurements such as enthalpy changes.

Question 9

What is standard pressure?

Answer 9

100kPa

Question 10

What is standard temperature?

Answer 10

298K (25°C)

Question 11

What is standard solution?

Answer 11

1 mol dm-3

Question 12

What is standard state?

Answer 12

The physical state of a substance under standard conditions

Question 13

What is the definition of standard enthalpy change of a reaction (ΔrH°)?

Answer 13

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard state.

Question 14

What is the definition of standard enthalpy change of formation (ΔfH°)?

Answer 14

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions of 100kPa and 298K.

(Product formed always is 1 mole, and there is no enthalpy change so elements have an enthalpy change of formation of 0kJ mol-1)

Question 15

What is the standard enthalpy change of combustion (ΔcH°)?

Answer 15

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions of 100kPa and 298k.

(The compound reacting with oxygen always needs to be 1 mole, enthalpy change is always negative)

Question 16

What is the standard enthalpy change of neutralisation (ΔneutH°)?

Answer 16

The energy change that accompanies the reaction of an acid by a base to form one mole of water under standard conditions of 100kPa and 298k.

(One mole of water formed, the value for enthalpy change of neutralisation is the same for all neutralisation reactions)

Question 17

What is specific heat capacity?

Answer 17

The energy required to raise the temperature of 1g of a substance by 1K.
For water this is 4.18 J g-1 K-1

Question 18

How to calculate energy change?

Answer 18

q = mc Δt

q = energy change (J)
m = mass of the materials changing temperature (g)
c = specific heat capacity (for water 4.18)

Δt = change in temperature (K or °C)

Question 19

Why can experimental enthalpy change values be less exothermic than expected?

Answer 19

• Heat loss to the surroundings other than water (to the beaker and air).
• Incomplete combustion of methanol, producing carbon monoxide and carbon instead of carbon dioxide.
• Evaporation of alcohol from the wick. The burner must be weighed as soon as possible after extinguishing flame/placing a lid to reduce this.
• Non-standard conditions.

All but non standard conditions would lead to a value that is less exothermic than expected.

Question 20

What is the reaction to measure the enthalpy change of combustion?

Answer 20

1. Measure out 150cm3 of water, pour it into a beaker and record the initial temperature of the water to the nearest 0.5°C.
2. Add methanol to the spirit burner and weigh the spirit burner.
3. Place the spirit burner under the beaker and light the burner and burn the methanol whilst stirring the water with the thermometer.
4. After three minutes extinguish the flame and record the maximum temperature reached by the water.
5. Re-weigh the spirit burner containing the methanol.

Question 21

What is the reaction to determine the enthalpy change of reaction?

Answer 21

• Reactions that take place between two solutions, or between a solid and a solution can have their enthalpy change of reaction measured by hiding polystyrene cups.
• These are cheap, waterproof and light weight, and offer some insulation against heat loss to the surroundings.
• A thermometer in the solution will measure any temperature change.

Question 22

Define average bond enthalpy

Answer 22

The energy required to break one mole of a specified type of bond in a gaseous molecule.
- Energy is always required to break bonds.
- Bond enthalpies are always endothermic (positive enthalpy change).

Question 23

What are the limitations of average bond enthalpies?

Answer 23

• The actual bond enthalpy can vary depending on the chemical environment of the bond, and the conditions it is under.
• An average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.

Question 24

Is bond breaking endothermic or exothermic?

Answer 24

Endothermic (ΔH is positive)

Question 25

Is bond making endothermic or exothermic?

Answer 25

Exothermic (ΔH is negative)

Question 26

How to calculate enthalpy change of a reaction (ΔrH) from average bond enthalpies?

Answer 26

ΔrH = bond enthalpies in reactants - bond enthalpies in products

Question 27

What is Hess’ Law?

Answer 27

If a reaction can take place by two routes, and the starting and finishing conditions are the same, total enthalpy change is the same for each route.

Question 28

How to calculate energy changes of formation (ΔfH)?

Answer 28

Total ΔfH products - total ΔfH reactants

Question 29

How to calculate energy changes of combustion?

Answer 29

total ΔcH reactants - total ΔcH products