Chapter 9: Enthalpy

Question 1

What is enthalpy?

NOTE: Enthalpy is sometimes thought of as the energy stored within bonds. Enthalpy cannot be measured, but enthalpy changes can.

Answer 1

Enthalpy H is a measure of the heat energy in a chemical system. The chemical system referes to the atoms, molecules or ions making up the chemicals.

Question 2

What is the enthalpy change? Give an equation

Answer 2

The enthalpy change is the difference in enthalpies between the reactants and products. It can be written as:
ΔH = H_(products) – H_(reactants).

ΔH can be positive or negative, depending on whether the products contain more or less energy than the reactants.

Question 3

Describe and explain the law of conservation of energy. Refer to enthalpy change.

Answer 3

The law of conservation of energy states that energy cannot be created or destroyed.

When a chemical reaction involving an anthalpy change takes place, heat energy is transferred between the system and the surroundings.

• The system is the chemicals – the reactants and products.
• The surroundings are the apparatus, the laboratory and everything that is not the chemical system.

Question 4

1. Compare an exothermic and an endothermic change.

Answer 4

1. An exothermic change takes place from the system to the surroundings. An endothermic chnage takes place from the surroundings to the system.

Question 5

Describe an exothermic reaction. Include a general enthalpy profile diagram for an exothermic reaction.

Answer 5

• In an exothermic reaction, the chemical system releases heat energy to the surroundings.
• Any energy loss by the chemical system is balanced by the same energy gain by the surroundings.
• ΔH is negative.
• The temperature of the surroundings increase as they gain energy.

Question 6

Describe an endothermic reaction. Include the general enthalpy profile diagram for an endothermic reaction.

Answer 6

• In an endothermic reaction, heat energy is transferred from the surroundings to the chemical system.
• Any energy gain by the chemical system is balanced by the same energy loss by the surroundings.
• ΔH is positive.
• The temperature of the surroundings decrease as they lose energy.

Question 7

Describe and explain the concept of activation energy. Include a generic exothermic and endothermic enthalpy profile diagram.

Answer 7

During chemical reactions, the bonds in the reactants ned to be broken by an input of energy. New bonds in the products can then form to complete the reaction. The energy input is known as the activation energy E_a. Activation energy is the minimum energy required for a reation to take place.

In general, 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 may present such a large energy barrier that a reaction may not even take place.

Question 8

1. What are the units of enthalpy change?
2. How is an enthalpy change under standard conditions denoted?

Answer 8

1. Enthalpy change is usually measured is kJ mol^–1, with mol^–1 referring to the amount in mol given by the balancing numbers of the chemicals in a stated equation for the reaction.
2. Image below.

Question 9

What are the standard conditions for an enthalpy change?

Answer 9

Question 10

Define the standard enthalpy change of reaction.

Answer 10

The standard enthalpy change of reaction Δ_rH^⊖ is 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 states.

Question 11

Define the enthalpy change of formation.

Answer 11

The standard enthalpy change of formation Δ_fH^⊖ is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 12

Define the standard enthalpy change of combustion.

Answer 12

The standard enthalpy change of combustion Δ_cH^<em>⊖</em> is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 13

Define the enthalpy change of neutralisation.

Answer 13

• The standard enthalpy change of neutralisation Δ_neutH^{<em>⊖ </em>}is the energy change that accompanies the reaction of an acid by a base to form one mole of H₂O(l), under standard conditions, with all reactants and products in their standard states.
  
  • Neutralisation involves the reaction of H⁺(aq) with OH^–(aq) to form one mole of H₂O(l). The value of Δ_neutH^<em>⊖</em> is the same for all neutralisation reactions.

Question 14

What is the Kelvin scale of temperature?

Answer 14

The Kelvin scale of temperature starts at absolute zero, 0 K and is equivalent to –273 ˚C.

Question 15

The energy change of the surroundings is calculated from three quantities. Describe these quantities.

Answer 15

1. The mass of the surroundings m. The mass is measured simply by weighing. You have to identify the materials that are changing temperature. Mass is usually measured in grams (g) to match the scale often used in experiments.
2. The specific heat capacity of the surroundings c. Different materials require different quantities of energy to produce the same temperature change. The specific heat capcaity c is the energy required to raise the temperature of 1 g of a substance by 1 K. Every substance has a specific heat capacity. In most experiments, you will be measuring the temperature change of water or aqueous solutions for which c = 4.18 J g^-1 K^-1.
3. The temperature change of the surroundings ΔT. The temperature change ΔT is determined from the thermometer readings: ΔT = T(final) – T(initial).

Question 16

How is energy change calculated?

Answer 16

Question 17

Describe how the enthalpy change of combustion for methanol can be experimentally dertermined.

Answer 17

Question 18

Why is the experimental value for enthalpy change of combustion often very different from data book values?

Answer 18

• Heat loss to the surroundings other than the water. This includes the beaker but mainly the air surrounding the flame.
• Incomplete combustion of methanol.
  There may be some incomplete combustion, with carbon monoxide and carbon being produced instead of carbon dioxide. You would see carbon as a black layer of soot on the beaker.
• Evaporaion of methanol from the wick.
  The burner must be weighed as soon as possible after extinguishikn the flame. Otherwise some methanol may avaporate from the wick. Spirin burners usually have a cover to reduce this error.
• Non-standard conditions.
  The data book value is a standard value. The conditions for this experiment are unlikely to be identical to the standard conditions.

Question 19

How can heat loss and incomplete combustion be minimised in the enthalpy change of combustion experiment?

Answer 19

Use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion.

Question 20

Explain how Δ_rH can be determined experimentally.

Answer 20

Many reactions take place between solutions, or between a solid and a solution. The enthalpy change of these reactions can be determined using plastic cups made of polystyrene foam. These are cheap, waterproof and light weigh, and offer some insulation against heat loss to the surroundings.

When carrying out reactions between aqueous solutions, the solution itself is the immediate surroundings. The chemical particles within the solutions may react when they collide, and any energy transfer is between the chemical particles and water molecules in the solution. A thermometer in the solution will record any temperature change, allowing the heat energy change to be calculated using mcΔT.

Question 21

How is the enthalpy change of neutralisation determined?

Answer 21

The enthalpy change of neutralisation is determined in the same way as determining the enthalpy change of reaction. The only difference is that two solutions react, rather than a solution and a solid.

Question 22

1. What is average bond enthalpy?
2. What are the limitations of average bond enthalpies?

Answer 22

1. Average bond enthalpy is the energy required to break one mole of a specified type of bond in a gaseous molecule. Energy is always required to break bonds thus bond enthalpies are always endothermic and have a positive enthalpy value.
2. The actual bond enthalpy can vary depending on the chemical environment of the bond. An average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments.

Question 23

Describe the energy of bond breaking and bond making. Refer to ΔH.

Answer 23

Question 24

Give the enthalpy profile diagrams for bond breaking and bond making.

Answer 24

Question 25

What enthalpy changes can be calculated from average bond enthalpies?

Answer 25

The enthalpy change of reaction Δ_rH can be found by calculating the bond enthalpies of the bonds in the reactants and the products.

Question 26

1. What is Hess’ law?
2. Illustrate Hess’ law in an enthalpy cycle.

Answer 26

1. Hess’ law states that, if a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route.
   2.

Question 27

What two rules can be used for calculations using enthalpy changes of formation and combustion.

Answer 27