Thermodynamics

Question 1

Define enthalpy change

Answer 1

the heat energy transferred in a reaction at constant pressure.

Question 2

What are the standard conditions?

Answer 2

100kPa and 298K

Question 3

Do exothermic reactions have a negative or positive enthalpy change?

Answer 3

negative

Question 4

What is lattice formation enthalpy?

Answer 4

the enthalpy change when one mole of a solid compound is formed from its gaseous ions

Question 5

What is lattice dissociation enthalpy?

Answer 5

the enthalpy change when one mole of a solid ionic compound is completely dissociated into its gaseous ions

Question 6

What is enthalpy change of formation?

Answer 6

the enthalpy change when one mole of a compound is formed from its elements in their standard states
Ca(s) + Cl2(g) –> CaCl2(s)

Question 7

What is bond dissociation enthalpy?

Answer 7

the enthalpy change when all the bonds of the same type in one mole of gaseous molecules are broken
Cl2(g) –> 2Cl(g)

Question 8

What is the enthalpy change of atomisation of an element?

Answer 8

the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
1/2Cl2(g) –> Cl(g)

Question 9

What is the enthalpy change of atomisation of a compound?

Answer 9

the enthalpy change when one mole of a compound in its standard state is converted to gaseous ions
NaCl(s) –> Na(g) + Cl(g)

Question 10

What is the first ionisation energy?

Answer 10

the enthalpy change when one mole of gaseous 1+ ions is formed from one mole of gaseous ions
Mg(g) –> Mg+(g) + e-

Question 11

What is the second ionisation energy?

Answer 11

the enthalpy change when one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
Mg+(g) –> Mg2+(g) + e-

Question 12

What is the first electron affinity?

Answer 12

the enthalpy change when one mole of gaseous 1- ions is made from one mole of gaseous atoms
O(g) + e- –> O-(g)

Question 13

What is the second electron affinity?

Answer 13

the enthalpy change when one mole of gaseous 2- ions is made from one mole of gaseous 1- ions
O-(g) + e- –> O2-(g)

Question 14

What is the enthalpy of hydration?

Answer 14

the enthalpy change when one mole of aqueous ions is formed from gaseous ions
Na+(g) –> Na+(aq)

Question 15

What is enthalpy of solution?

Answer 15

t he enthalpy change when one mole of an ionic substance dissolves in enough solvent to form an infinitely dilute solution

Question 16

What does the purely ionic model of lattice assume?

Answer 16

all the ions are spherical and have their charge evenly distributed around them

Question 17

What is covalent character?

Answer 17

the negative and positive ions aren’t exactly spherical, positive ions polarise neighbouring negative ions to different extents. the more polarisation, he more covalent the bonding is

Question 18

What does it show if the experimental and theoretical lattice enthalpies for a compound are very different?

Answer 18

the compound has a lot of covalent character

Question 19

What happens when a solid ionic lattice dissolves in water?

Answer 19

the bonds between the ions break to give gaseous ions (lattice enthalpy of dissociation) and bonds between the ions and the water are made (enthalpy change of hydration)

Question 20

When calculating enthalpy change of solution, what is enthalpy change 1?

Answer 20

Lattice dissociation enthalpy

Question 21

When calculating enthalpy change of solution, what is enthalpy change 2?

Answer 21

enthalpy of hydration of both the positive and negative ions

Question 22

When calculating enthalpy change of solution, what is enthalpy change 3?

Answer 22

enthalpy change of solution

Question 23

What is entropy and what symbol represents it?

Answer 23

entropy tells you how much disorder there is, it is represented by the letter S

Question 24

What three things affect entropy?

Answer 24

physical state, dissolution and number of particles

Question 25

What state of matter has the highest entropy?

Answer 25

gas

Question 26

How can you increase the entropy of a solid?

Answer 26

dissolve a solid in solution

Question 27

How does number of particles affect entropy?

Answer 27

more particles=more entropy

Question 28

What is the calculation for working out entropy change?

Answer 28

entropy change=entropy of products-entropy of reactants

Question 29

What is free-energy change?

Answer 29

a measure used to predict whether a reaction is feasible

Question 30

What is a feasible reaction?

Answer 30

once started, it will carry on to completion without energy being supplied to it

Question 31

What is the formula for free-energy change?

Answer 31

free energy change(J/mol)=enthalpy change(J/mol)- temperature(K) x entropy change(J/K/mol)

Question 32

If the reaction is exothermic, how does this affect the entropy?

Answer 32

the entropy change will be positive

Question 33

If the reaction is endothermic, how does this affect the entropy?

Answer 33

the entropy change will be negative

Question 34

When will a reaction not be feasible?

Answer 34

when free-energy change is positive, when enthalpy and entropy are positive it won't be feasible below certain temperatures

Question 35

When free-energy change is zero, is the reaction feasible?

Answer 35

yes, it is just feasible