Chemical Energetics 1

Question 1

What is the enthalpy of a system?

Answer 1

The measure of the energy content of the system.

Question 2

What is enthalpy change?

Answer 2

The change in energy content of a process in a system at constant pressure.
The difference in quantity of heat absorbed to break the bonds in the reactants and that released during the formation of new bonds in the products at constant pressure.

Question 3

Definition, temperature change and enthalpy change of exothermic and endothermic reactions:

Answer 3

Exothermic:
Definition - a reaction in which energy is released to the surroundings
Temperature change - increase in surrounding temperature
Enthalpy change - negative

Endothermic:
Definition - a reaction in which energy is absorbed from the surroundings
Temperature change - decrease in surrounding temperature
Enthalpy change - positive

Question 4

What are the 4 rules of enthalpy change?

Answer 4

1. All values must be accompanied by + or -.
2. Reversing a chemical equation reverses the sign of enthalpy change.
3. Multiplying a chemical equation by a factor would multiply enthalpy change by the same factor.
4. Always include the state symbols of all the chemical species given in a thermochemical equation.

Question 5

What are standard conditions?

Answer 5

Pressure: 1 bar

Stated temperature: usually 298K

Question 6

What is the standard state of a substance?

Answer 6

The most stable form of a substance at 1 bar and 298K.

Question 7

What is standard enthalpy change of reaction ΔHr?

Answer 7

The energy change in a chemical equation when the molar quantities of reactants stated in the chemical equation react under standard conditions.

Question 8

What is standard enthalpy change of formation ΔHf of a substance?

Answer 8

The energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard states under standard conditions.

Question 9

What is standard enthalpy change of formation ΔHf of an element?

Answer 9

Zero.

Question 10

What is standard enthalpy change of combustion ΔHc?

Answer 10

The energy released when 1 mole of the substance is completely burnt in excess oxygen under standard conditions.

Question 11

What is standard enthalpy change of neutralisation ΔHneut between an acid and base?

Answer 11

The energy change when the acid and base react to form 1 mole of water under standard conditions.

Question 12

What is standard enthalpy change of atomisation ΔHatom of an element?

Answer 12

The energy absorbed when 1 mole of gaseous atoms is formed from the element under standard conditions.

Question 13

What is standard enthalpy change of atomisation ΔHatom of a compound?

Answer 13

The energy absorbed when 1 mole of the compound is converted ti gaseous atoms under standard conditions.

Question 14

What is bond dissociation energy of a bond?

Answer 14

The energy required to break 1 mole of that particular X-Y bond in a particular compound in gaseous state.

Question 15

What is the bond energy of a bond?

Answer 15

The average energy required to break 1 mole of the X-Y bonds in the gaseous state.

Question 16

What is the relationship between bond energy and enthalpy change of atomisation of diatomic gases?

Answer 16

Bond energy of X-X bond = 2ΔHatom[X2(g)]

Question 17

What is the first ionisation energy of an element?

Answer 17

The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly positively charged gaseous ions.

Question 18

What is the second ionisation energy of an element?

Answer 18

The energy required to remove 1 mole of electrons from 1 mole of singly positively charged gaseous ions to form 1 mole of doubly positively charged gaseous ions.

Question 19

What is the first electron affinity of an element?

Answer 19

The energy change when 1 mole of gaseous atoms acquires 1 mole of electrons to form 1 mole of singly negatively charged gaseous ions.

Question 20

Why is the first electron affinity usually negative?

Answer 20

Energy released when the nucleus attracts the additional electron is usually larger than the energy taken in to overcome inter-electronic repulsion.

Question 21

What is the second electron affinity of an element?

Answer 21

The energy change when 1 mole of singly negatively charged gaseous ions acquires 1 mole of electrons to form 1 mole of doubly negatively charged gaseous ions.

Question 22

Why is the second electron affinity always positive?

Answer 22

Energy is required to overcome the repulsion between the two negatively charged species.

Question 23

What is lattice energy?

Answer 23

The energy released when 1 mol of the solid ionic compound is formed from its constituent gaseous ions under standard conditions.

Question 24

What is the standard enthalpy change of hydration?

Answer 24

The energy released when 1 mol of gaseous ion is hydrated under standard conditions.

Question 25

What does magnitude of enthalpy change of hydration depend on?

Answer 25

Charge density.

Question 26

What is the standard enthalpy change of solution?

Answer 26

The energy change when 1 mol of the substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions.

Question 27

What is specific heat capacity?

Answer 27

Quantity of heat required to raise the temperature of 1g of substance by 1℃ or 1K.

Question 28

What is heat capacity?

Answer 28

The quantity of heat required to raise the temperature of the substance by 1℃.

Question 29

What is the equation for heat change?

Answer 29

q=mcΔT or CΔT

Question 30

What is the equation for heat change?

Answer 30

ΔH=-q/n

Question 31

What are the assumptions made when solving for enthalpy change of a reaction from a graph?

Answer 31

Density of solution=density of water=1.0g cm^-3
Specific heat capacity of solution=Specific heat capacity of solution=4.18 J g^-1 ℃^-1
Heat loss to the surrounding air is corrected for via extrapolation to find the highest temperature reached from the graph.

Question 32

What is Hess’ Law?

Answer 32

The enthalpy change of a chemical reaction is the same regardless of whether the reaction takes place in one step or several steps, provided the initial and final states of the reactants and products are the same.

Question 33

What are the factors affecting lattice energy?

Answer 33

Effect of ionic charge (bigger charge, more negative lattice energy)
Effect of ionic size (smaller radius, more negative lattice energy)

Question 34

What is the acronym for the Born-Haber cycle?

Answer 34

F: Formation
A: Atomisation
I: Ionisation
L: Lattice energy

Question 35

What is experimental lattice energy?

Answer 35

Lattice energy obtained the experimental way, using experimentally obtained enthalpy changes in a Born-Haber cycle.

Question 36

What is theoretical lattice energy?

Answer 36

Lattice energy obtained in a theoretical way using an equation based on the purely ionic model of a lattice, assuming that all the ions are spherical and have their charge evenly distributed around them.

Question 37

How does degree of covalent character affect the difference between experimental and theoretical lattice energy?

Answer 37

Higher degree of covalent character causes discrepancy between the experimental and theoretical lattice energies (ie they are not in agreement).

Question 38

Steps for dissolving an ionic solid in water:

Answer 38

1. Separation of ions in the solid lattice into monoatomic gaseous ions
2. Hydration of gaseous ions