34: Energy Changes in Chemical Reactions

Question 1

What is the unit of energy?

Answer 1

Joule (J)

Question 2

What is the specific heat capacity of a substance?

Answer 2

The specific heat capacity (c) of a substance is the amount of heat required to raise the temperature of 1g of the substance by 1 K.

Question 3

What is the unit of heat capacity?

Answer 3

J K^-1

Question 4

What is the unit of specific heat capacity?

Answer 4

J g^-1 K^-1

Question 5

What is the amount of heat required to raise the temperature of a given substance, in terms of its heat capacity?

Answer 5

Amount of heat required = C × ΔT (heat capacity × change in temperature)

Question 6

What is the amount of heat required to raise the temperature of a given substance, in terms of its specific heat capacity?

Answer 6

Amount of heat required = m × c × ΔT (mass × specific heat capacity × change in temperature)

Question 7

What are system and surroundings?

Answer 7

The system refers to the reaction mixture** while the surroundings refers to everything outside of the system.

Question 8

In what ways can energy transfer be done from the system to the surroundings?

Answer 8

Energy can be transferred as heat flowing from the system to the surroundings (or vice versa), or work done by the system on the surroundings (or vice versa).

Question 9

What is the internal energy of a system?

Answer 9

The internal energy of a system is the sum of kinetic and potential energies of the particles that form the system.

Question 10

For what values of ΔE does the system gain energy from the surroundings?

Answer 10

ΔE > 0

Question 11

For what values of ΔE does the system lose energy from the surroundings?

Answer 11

ΔE < 0

Question 12

What is the definition of enthalpy change?

Answer 12

An enthalpy change (ΔH), is the heat released or taken in during any change in a system, provided that the system is kept at constant pressure.

Question 13

Why can ΔH usually give a good indication of the change in internal energy of a system during a reaction?

Answer 13

The difference between the enthalpy change (ΔH) and the internal energy change (ΔE) is small in most cases.

Question 14

List two examples of exothermic reactions.

Answer 14

Reaction of calcium oxide with water, and combustion of petrol.

Question 15

List three examples of endothermic reactions.

Answer 15

Reaction of hydrogencarbonates and dilute acids, decomposition of limestone, photosynthesis.

Question 16

What is the definition of exothermic reaction?

Answer 16

In an exothermic reaction, the system releases heat to the surroundings and the surroundings get hotter.

Question 17

What is the definition of endothermic reaction?

Answer 17

In an endothermic reaction, the system absorbs heat from the surroundings and the surroundings get colder.

Question 18

What is the enthalpy change for an exothermic reaction?

Answer 18

It is a negative quantity.

Question 19

What is the enthalpy change for an endothermic reaction?

Answer 19

It is a positive quantity.

Question 20

Compare the total energy released in the bond forming process and that in the bond breaking process for exothermic reactions.

Answer 20

The total energy released in the bond forming process is greater than the total energy required in the bond breaking process.

Question 21

Compare the total energy released in the bond forming process and that in the bond breaking process for endothermic reactions.

Answer 21

The total energy released in the bond forming process is smaller than the total energy required in the bond breaking process.

Question 22

What is a thermochemical equation?

Answer 22

A thermochemical equation is a balanced chemical equation that also includes the value of ΔH.

Question 23

What is the unit of enthalpy change in thermochemical equations?

Answer 23

kJ mol^-1

Question 24

It is given that in the equation H₂ (g) + 1/2O₂ (g) -> H₂O (l), ΔH = -286 kJ mol^-1.
Find the enthalpy change of the reaction 2H₂ (g) + O₂ (g) -> 2H₂O (l)

Answer 24

The enthalpy change is doubled since the coefficients of the equation are doubled.
ΔH = -572 kJ mol^-1

Question 25

What is standard enthalpy change?

Answer 25

Standard enthalpy changes are enthalpy changes when all reactants react together under **standard conditions** to give products.

Question 26

What are standard conditions for calculating standard enthalpy changes?

Answer 26

1. At a temperature of 25ºC (298 K) 2. At a pressure of **1 atm** 3. All substances in their **standard states** / the **most stable physical state** at 298 K and 1 atm 4. All solutions at a concentration of **1 mol dm^-3**.

Question 27

What is the standard enthalpy change of formation?

Answer 27

The standard enthalpy change of formation of a substance (ΔH_f^⦵) is the enthalpy change when **one mole** of the substance is formed **from its elements** in their standard states under **standard conditions**.

Question 28

What is the standard state of carbon?

Answer 28

graphite

Question 29

What is the enthalpy change of formation of elements in their standard states?

Answer 29

0 kJ mol^-1, as there is **no change** involved in their formation.

Question 30

Give two examples of substances with a positive enthalpy change of formation.

Answer 30

Nitrogen monoxide and benzene

Question 31

What is the standard enthalpy change of combustion?

Answer 31

The standard enthalpy change of formation of a substance (ΔH_c^⦵) is the enthalpy change when **one mole** of the substance undergoes **complete combustion** in oxygen under **standard conditions**.

Question 32

What is the standard enthalpy change of neutralisation?

Answer 32

The standard enthalpy change of neutralisation of a substance (ΔH_n^⦵) is the enthalpy change when an acid reacts with an alkali to produce **one mole** of water under **standard conditions**.

Question 33

What is the equation showing the standard enthalpy change of formation for ammonium carbonate?

Answer 33

N₂ (g) + 4H₂ (g) + C (graphite) + 1 1/2O₂ (g) -> (NH₄)₂CO₃ (s)

Question 34

What is the equation showing the standard enthalpy change of neutralisation for ammonia and sulphuric acid?

Answer 34

NH₃ (aq) + 1/2H₂SO₄ -> 1/2(NH₄)SO₄ (aq)

Question 35

What is the equation showing the standard enthalpy change of neutralisation for zinc hydroxide and phosphoric acid?

Answer 35

1/2Zn(OH)₂ (aq) + 1/3H₃PO₄ -> 1/6Zn₃(PO₄)₂ (aq)