energetics 1 and 2

Question 1

Enthalpy

Answer 1

measure of the energy content of the system

Question 2

Enthalpy change

Answer 2

difference between the 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

units of enthalpy change

Answer 3

kj mol-1

Question 4

activation energy

Answer 4

minimum amount of energy that the reactant particles must possess before they can collide successfully to form products

Question 5

enthalpy change of reaction

Answer 5

energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions (ie. 1 bar and 298K)

Question 6

enthalpy change of formation

Answer 6

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 (ie. 1 bar and 298K)

Question 7

enthalpy change of combustion

Answer 7

energy released (cause always exo) when 1 mole of the substance is completely burnt in excess oxygen under standard conditions (ie. 1 bar and 298K)

Question 8

enthalpy change of neutralisation

Answer 8

between an acid and a base is the energy change when the acid and the base react to form 1 mole of water under standard conditions (ie. 1 bar and 298K)

Question 9

value for enthalpy change of neutralisation for strong acid and base

Answer 9

-57.3 kj mol-1

Question 10

explanation for less exo neutralisation for weak acid strong base

Answer 10

1. weak acid exists mainly as molecules hence further dissociation is required to produce more H+ ions
2. dissociation involves the breaking of the O-H bond (for the case of CH3COOH) which is an endothermic process and the hydration of the CH3COO- and H+ ions which is an exothermic process
3. overall, the dissociation process is endothermic
4. thus as energy is consumed for further dissociation, the energy released by the reaction will be reduced and it will be less exothermic

Question 11

enthalpy change of atomisation

Answer 11

energy absorbed when 1 mole of gaseous atoms is formed from the element under standard conditions (ie. 1 bar and 298K)

Question 12

enthalpy change of atomisation of a compound

Answer 12

energy absorbed (always endo cos breaking bonds) when 1 mole of the compound is converted to gaseous atoms under standard conditions (ie. 1 bar and 298K)

Question 13

enthalpy change of hydration

Answer 13

of an ion is the energy released (always exo) when 1 mole of gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions (ie. 1 bar and 298K)

• proportional to charge over radius (charge density)

Question 14

enthalpy change of solution

Answer 14

of a substance is the energy change when 1 mole of the substance is completely dissolved in a solvent to form an infinitely dilute solution (does not produce any further enthalpy change when more solvent is added) under standard conditions (ie. 1 bar and 298K)

Question 15

bond dissociation energy

Answer 15

BDE of a X-Y bond is the energy required to break 1 mole of that particular X-Y bond in a particular compound in gaseous state

always positive

Question 16

bond energy

Answer 16

of a X-Y bond is the average energy required to break 1 mole of the X-Y bond in the gaseous state

Question 17

ionisation energy 1st and 2nd

Answer 17

The first ionisation energy (1st IE) of an element is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly positively charge gaseous ions

The second ionisation energy (2nd IE) of an element is 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 charge gaseous ions

Question 18

electron affinity

Answer 18

→ The first electron affinity (1st EA) of an element is the energy CHANGE when 1 mole of gaseous atoms acquires 1 mole of electrons to form 1 mole of singly negatively charged gaseous ions

→ The second electron affinity (2nd EA) of an element is the energy ABSORBED 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 19

lattice energy

Answer 19

of an ionic compound is the energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions under standard conditions

Question 20

specific heat capacity/heat capacity

Answer 20

quantity of heat required to raise the temperature of 1g of the substance by 1˚C

quantity of heat required to raise the temperature of the substance by 1˚C

Question 21

assumptions for the calculations

Answer 21

1. negligible heat loss to the surroundings due to insulation
2. specific heat capacity of solution is approx that of water (4.18J g-1 K-1)
3. density of solution is approx that of water (1.00g cm-3)

Question 22

Hess Law

Answer 22

enthalpy change of reaction is determined by the initial and final states of the system and is independent of the pathways taken

Question 23

explain theoretical and experimental LE deviation

Answer 23

in agreement
- predominantly ionic
- fits in the purely ionic model which consists of spherical ions with evenly distributed charge

differ by quite a bit
- not purely ionic
- has covalent character due to the substantial polarisation of the anion by the cation
- if the value is more negative, suggests that bonding is stronger than that suggested by the purely ionic model

Question 24

dissolving steps

Answer 24

1. Separation of the ionic crystal lattice into its gaseous ions
   - breaking down of the solid ionic crystal lattice
   - endothermic
2. Hydration of the gaseous ions
   - formation of ion dipole moments between the gaseous ions and water molecules to form hydrated ions in aqueous solution
   - exothermic

for dissolution of smth in water
= -LE + enthalpy change of hydration for each ion

Question 25

Solubility

Answer 25

• more soluble when enthalpy change of solution is more negative (less when positive)
• when it is negative, the hydration energy released is sufficient to compensate for the energy required to break the solid ionic crystal lattice
  and vice versa

Question 26

Spontaneous and non spontaneous

Answer 26

Spontaneous
- once started will continue without external assistance

(not part of definition but always trying to achieve a lower energy state and reach a state of greater entropy)

Non spontaneous
- will not occur unless external assistance is continuously applied

Question 27

Entropy

Answer 27

of a system is a measure of the disorder of matter and energy in the system. The more ways matter in the system can be arranged, the more ways energy in the system can be dispersed, the more disordered the system is and the larger is its entropy

Question 28

entropy change

Answer 28

measure of change of disorder of a system