U1S6- Thermodynamics

Question 1

Define enthalpy change.

Answer 1

The heat energy transferred in a reaction at constant pressure.

Question 2

Define the term exothermic reaction.

Answer 2

Heat energy is given out. Negative delta H value.

Question 3

Define the term endothermic reaction.

Answer 3

Heat energy is absorbed. Delta H is positive.

Question 4

Define enthalpy change of formation.

Answer 4

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 5

Define enthalpy change of atomisation of an element.

Answer 5

The enthalpy change when 1 mole of gaseous atoms is formed from an element in its standard state.

Question 6

Define bond dissociation enthalpy.

Answer 6

The enthalpy change when all the bonds of the same type in 1 mole of gaseous molecules is broken.

Question 7

Define the enthalpy change of atomisation of a compound.

Answer 7

The enthalpy change when 1 mole of a compound in its standard state is converted into gaseous atoms.

Question 8

Define first ionisation energy.

Answer 8

The enthalpy change when 1 mole of gaseous 1+ ions are formed from 1 mole of gaseous atoms.

Question 9

Define second ionisation energy.

Answer 9

The enthalpy change when 1 mole of gaseous 2+ ions are formed from 1 mole of gaseous 1+ ions.

Question 10

Define first electron affinity.

Answer 10

The enthalpy change when 1 mole of gaseous 1- ions are formed from 1 mole of gaseous atoms.

Question 11

Define second electron affinity.

Answer 11

The enthalpy change when 1 mole of gaseous 2- ions are formed from 1 mole of gaseous 1- ions.

Question 12

Define enthalpy change of hydration.

Answer 12

The enthalpy change when 1 mole of aqueous ions are formed from 1 mole of gaseous ions.

Question 13

Define enthalpy of solution.

Answer 13

The enthalpy change when 1 mole of solute is dissolved in enough solvent that no further enthalpy change occurs on further dilution.

Question 14

Define lattice enthalpy of formation.

Answer 14

The enthalpy change when 1 mole of a solid ionic compound is formed from its gaseous ions under standard conditions.

Question 15

Define lattice enthalpy of dissociation.

Answer 15

The enthalpy change when 1 mole of a solid ionic compound is completely dissociated into its gaseous ions under standard conditions.

Question 16

State Hess’s law.

Answer 16

The total enthalpy change of a reaction is always the dame, no matter which route is taken.

Question 17

What is the theoretical lattice enthalpy based on?

Answer 17

Purely ionic model.

Question 18

What 2 things does the purely ionic model assume?

Answer 18

That all the ions are spherical.

Their charges are evenly distributed around them.

Question 19

Define covalent character.

Answer 19

The bonds are strongly polarised and the difference between the experimental and theoretical values are bigger.

Question 20

What 2 things happen when a solid ionic lattice dissolves in water?

Answer 20

. Bonds between ions break to give free ions-endothermic.

. Bonds between the ions and water are formed-exothermic.

Question 21

Why can the water molecules bond to the ions? (4 stages)

Answer 21

. O more electronegative than H.
. So draws electrons to itself which creates a dipole.
. + H atoms form bonds with - ions.
. - O atoms form bonds with + ions.

Question 22

When do substances generally only dissolve?

Answer 22

. Energy released is greater than or equal to energy taken in.
. Therefore, exothermic.

Question 23

Draw a Born Haber cycle and work out enthalpy change of solution for NaCl.
Enthalpy of hydration of Na+(g)=-406
Enthalpy of hydration of Cl-(g)=-364
Lattice dissociation enthalpy=+787

Answer 23

+17 kJmol^-1

Question 24

Define entropy (3 things)

Answer 24

A measure of the number of ways that the particles can be arranged.
A measure of the number of ways energy can be shared out.
The more disordered the particles are, the higher the entropy.

Question 25

State 2 things that affect entropy?

Answer 25

. Physical state.

. More particles.

Question 26

When are particles more energetically stable?

Answer 26

When there is more disorder, so they tend towards disorder.

Question 27

When are some reactions more feasible?

Answer 27

An increase in entropy in a reaction means that that reaction is feasible.

Question 28

Define standard entropy.

Answer 28

The entropy of 1 mole of that substance under standard conditions.