Thermodynamics

Question 1

What is the standard enthalpy of formation

Answer 1

The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states

e.g. Na(s) + ½Cl2(g) -> NaCl(s)

Question 2

What is the standard enthalpy of combustion

Answer 2

The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactant and products in their standard states

e.g. C2H6(g) + 3½O2(g) -> 2CO2(g) + 3H2O(l)

Question 3

What is the standard enthalpy of atomisation

Answer 3

The enthalpy change when one of gaseous atoms is formed from an element in its standard state

e.g. Mg(s) -> Mg(g)

e.g. ½ Br2(l) -> Br(g)

Question 4

What is the mean bond enthalpy

Answer 4

The enthaply change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds

e.g. CH4(g) -> H(g) + CH3(g)

Question 5

What is the first ionisation enthalpy

Answer 5

The standard enthalpy change when one mole of electrons is removed from onr mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge

e.g. Na(g) -> Na+(g) + e-

e.g. Ca(g) -> Ca+(g) + e-

Question 6

What is the second ionisation enthalpy

Answer 6

The standard enthalpy change when one mole of electrons is removes from one mole of gaseous 1+ ions to give one mole of gaseous ions, each with a 2+ charge

e.g. Ca+(g) -> Ca2+(g) + e-

e.g. Mg+(g) -> Mg2+(g) + e-

Question 7

What is the First Electron affinity

Answer 7

The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge under standard conditions

e.g. Cl(g) + e- -> Cl-(g)

e.g. S(g) + e- -> S-(g)

Question 8

What is the second electron affinity

Answer 8

The standard enthalpy change when one mole of electrons is added to a mole of gasous ions each with a single negative charge, to form a mole of ions each with a 2- charge

e.g. S-(g) + e- -> S2-(g)

e.g. O- (g) + e- -> O2-(g)

Question 9

What is the Lattice formation enthalpy

Answer 9

The standard enthalpy change when one mole of solid ionic lattice is formed from its gaseous ions

e.g. Mg2+(g) + 2Br-(g) -> MgBr2(s)

Question 10

What is Lattic dissociation enthalpy

Answer 10

The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions

e.g. NaCl(s) -> Na+(g) + Cl-(g)

e.g. MgBr2(s) -> Mg2+(g) + 2Br-(g)

Question 11

What is the standard enthalpy of hydration

Answer 11

The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueou ions

e.g. Na+(g) + aq -> Na+(aq)

e.g. Cl-(g) + aq -> Cl-(aq)

Question 12

What is the standard enthalpy of solution

Answer 12

The standard enthalpy change when one mole of solute dissolves in enough solvent to form a solution in which the ions are far enough apart not to react with each other

e.g. NaCl(s) + aq -> Na+(aq) + Cl-(aq)

Question 13

Explain why the bond enthalpy of a Cl-Cl bond is greeater than that of a Br-Br bond

(2 marks)

Answer 13

• Cl is a smaller atom
• So attraction between nucleus and bonding pair is stronger

Question 14

Suggest why the electron affinity of chlorine is an exothermic change

(1 mark)

Answer 14

Net attraction between the chlorine nucleus and the extra electron

Question 15

Explain the meaning of the term perfect ionic model

(1 mark)

Answer 15

• No covalent character
• Ion is point charge

Question 16

Suggest why the calculated value of lattice dissociationof AgCl is greater than the theoretical value

(2 marks)

Answer 16

• AgCl has covalent character
• Forces in the lattice are stronger than pure ionic attractions

Question 17

In terms of the forces acting on the particles, suggest one reason why the first electron affinity of oxygen is an exothermic process

(1 mark)

Answer 17

There is an attractive force between the nucelus of an O and an external electron

Question 18

The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 degrees when it freezes

(1 mark)

Answer 18

The heat given out escapes

Question 19

Pure ice can look pale blue when illuminated by white light. Suggest why

(2 marks)

Answer 19

• Red light absorbed by light
• Blue light transmitted

Question 20

Explain why the electronegativity of fluorine is greater than the electronegativity of chlorine

(2 marks)

Answer 20

• Fluorine atom is smaller than chlorine
• Electrons attracted more strongly to the nucleus

Question 21

Explain why the hydration enthalpy of a fluoride ion is more negative than the hydration enthalpy of a chlorine ion

(2 marks)

Answer 21

• Fluoride smaller that chloride
• So attracts water more strongly

Question 22

Suggest why a value for the enthalpy of solution of MgO is not found in any data books

(1 mark)

Answer 22

MgO reacts with water/ forms Mg(OH)2