Thermodynamics

Question 1

give a definition for enthalpy change

Answer 1

The difference in energy taken in when bonds are broken, and given out when bonds are formed, during a reaction, at constant pressure.

Question 2

give a definition for enthalpy of formation

Answer 2

the enthalpy change when 1 mol of a substance is formed from its constituent elements, with all substances in their standard states.

Question 3

give a definition for the enthalpy of combustion

Answer 3

the enthalpy chnage when 1mol of a substance is completely burnt in oxygen, with all substances in their standard states

Question 4

give a definition for first ionisation energy

Answer 4

enthalpy change when each atom in 1mol of a gaseous substance loses on electron, to form 1mol of gaseous ions with a 1+ charge

Question 5

give a definition for seconed ionisation energy

Answer 5

enthalpy change when each ion in 1mol of gaseous ions with a 1+ charge lose 1 electron, forming 1mol of gaseous ions with a 2+ charge

Question 6

give the definition for first electron affinity

Answer 6

enthalpy change when each atom in 1mol of a gaseous substance, gain 1 electron, forming 1 mol of gaseous with a 1- charge
the sign is -ve

Question 7

give the definition for the seconed electron affinity

Answer 7

enthalpy change when each ion in 1mol of gaseous ions with a 1- charge, gain one electron to form 1mol of gaseous ions with a 2- charge
the sign is +ve (there is a repulsion between the -ve ion and the electron)

Question 8

give the definition for bond dissociation

Answer 8

enthalpy change when 1mol of covalent bonds are broken, in the gaseous state e.g. I2(g) –> 2I(g)

Question 9

give the definition for enthalpy of atomisation of an element

Answer 9

enthalpy change when 1mol of gaseous atoms are formed from an element in its standard state
e.g. 1/2 I2 (g) –> I (g)

Question 10

give the definition for enthalpy of atmoisation of a compound

Answer 10

enthalpy change when 1mol of gaseous atoms are formed from a compound in its standard state

Question 11

give a definition for lattice enthalpy of formation

Answer 11

enthalpy change when one mol of a solid ionic compound is formed from its constituent ions in the gas phase

Question 12

give a definition for lattice enthalpy of dissociation

Answer 12

enthalpy change when 1mol of a ionic compound is broken up into its constituent ions in the gas phase

Question 13

give a definition for hydration enthalpy

Answer 13

enthalpy change when 1 mol of an ionic solid is hydrated

Question 14

give a definition for the enthalpy of solution

Answer 14

enthalpy change when 1mol of an ionic solid dissolves in an amount of water large enough so that the dissolved ions are well separated and do not interact with eachother

Question 15

what is hess’s law

Answer 15

states that enthalpy change for any reaction is independent on the route taken

Question 16

what does the size of the arrow in a born habor cycle represent

Answer 16

the size of the enthalpy change

Question 17

Explain why the lattice dissociation enthalpy of NaBr is a bit less than that of NaCl

Answer 17

Lattice dissociation enthalpy is a measure of the forces of attractions between the positive and
negative ions. Bromide ions are bigger than chloride ions, and so the distance between the centres
of the positive and negative ions in the lattice is greater in NaBr. Increasing distance weakens the
forces of attraction between them, and hence the lattice enthalpy

Question 18

Explain why the lattice dissociation enthalpy of MgO is about 5 times greater than that of NaCl.

Answer 18

Magnesium ions are smaller than sodium ions, and oxide ions are smaller than chloride ions. That
means that the distance between the positive and negative ions is quite a lot less in MgO than in
NaCl, and so the forces of attraction will be greater in MgO.
But the main factor is the charge on the ions. In MgO, both positive and negative ions carry two
charges. In NaCl, they only carry one. The strength of the attractions is much greater in MgO than
in NaCl.

Question 19

The standard atomisation enthalpy of bromine is +112 kJ mol-1, and for sodium is +107 kJ mol-1
.
Write equations for the reactions that these figures relate to

Answer 19

1/2 Br2 (g) –> Br(g)

Na(s)–> Na(g)

Question 20

write an equation for the lattice enthalpy of sodium chloride

Answer 20

Na+ (g) + Cl- (g) –> NaCl (s)

Question 21

why are the theoretical values usually lower than the experimental values

Answer 21

because of polarisation

Question 22

why do most ionic compounds dissolve in water

Answer 22

water is a polar molecule because of the difference in electronegativity of the oxygen and hydrogen in the molecule. the hydrogen has a +ve partial charge, and the oxygen has a -ve partial charge

Question 23

give three stages that dissolving an ionic compound can be broken down into

Answer 23

• breaking the solid into gaseous ions
• hydrating the +ve ions
• hydrating the -ve ions

Question 24

give an equation for calculating the enthalpy of solution

Answer 24

delta sol = delta LE + delta hydration

Question 25

what does gibbs free energy determine

Answer 25

whether or not a reaction will occur

Question 26

give an equation for gibbs free energy change

Answer 26

deltaG = deltaH - TdeltaS

Question 27

what value must gibbs free energy be for the reaction to be feasible

Answer 27

less than or equal to 0

Question 28

rearrange the equation to calculate the temperature

Answer 28

T= (deltaH-deltaG)-deltaS