Chemical Energetics: Thermochemistry & Thermodynamics

Question 1

standard enthalpy change of reaction ΔH°r definition

Answer 1

the enthalpy change when molar quantities of reactants as specified by the chemical equation react to form products at standard conditions, 1 bar and 298 K.

Question 2

standard enthalpy change of formation ΔH°f definition

Answer 2

the enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 bar

Question 3

standard enthalpy change of combustion ΔH°c definition

Answer 3

the heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298K and 1 bar

Question 4

standard enthalpy change neutralisation ΔH°neut definition

Answer 4

the heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K and 1 bar

Question 5

formula of heat change of solution

Answer 5

q = m c ΔT
q = C ΔT

Question 6

ΔH of an endothermic reaction and an exothermic reaction equals…

Answer 6

ΔH = +q / n
ΔH = -q / n

Question 7

Hess’ law of constant heat summation

Answer 7

The enthalpy change (ΔH) of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken

Question 8

bond energy definition

Answer 8

bond energy is the energy required to break 1 mole of a covalent bond in the gaseous state

Question 9

ΔHr = ?

Answer 9

ΔHr = BE bonds broken - BE bonds formed

Question 10

standard enthalpy change of atomisation ΔH°atom definition

Answer 10

the energy required when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar (element)

the energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar (compound)

Question 11

Lattice energy definition

Answer 11

lattice energy is the heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions.

Question 12

ionisation affinity definition

Answer 12

the first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of gaseous atoms to form 1 mole of singly charged gaseous atoms

Question 13

why is the first EA always a negative

Answer 13

effective nuclear charge of an atom leads to an attraction of the incoming electron. Stronger attraction, more energy given off, EA more negative

Question 14

why is the second EA positive

Answer 14

energy is required to overcome the electrostatic repulsion between the incoming electron and anion

Question 15

what does a difference between the theoretical and experimental LE show?

Answer 15

theoretical (assumes that it is completely ionic) and experimental (uses born-haber cycle) LE shows that there is covalent character in the ionic compound. This is most apparent when a cation with a high charge density distorts an anion with a large electron cloud

Question 16

standard enthalpy change of hydration ΔH°hyd definition

Answer 16

the heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar.

Question 17

standard enthalpy change of solution ΔH°sol definition

Answer 17

the enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar

Question 18

what does ΔH°sol > or < 0 mean?

Answer 18

ΔH°sol > 0: salt is insoluble
ΔH°sol <0: salt is soluble

Question 19

entropy definition

Answer 19

entropy (S) is a measure of the randomness or disorder in a system, reflected in the number of ways that the energy of a system can be distributed through the motion of its particles

Question 20

when is ΔS > or < 0?

Answer 20

ΔS > 0 if a reaction results in more ways to disperse or distribute the energy

ΔS < 0 if a reaction results in less ways to disperse or distribute the energy

Question 21

what are some factors that affects ΔS

Answer 21

change in temperature
change in phase
change in the number of gas particles
mixing of particles
dissolution of an ionic solid

Question 22

change in temperature

Answer 22

as temperature increases, the average kinetic energy of the particles and the range of energies increases. There are more ways to disperse the energy among the particles. Hence, entropy increases.

Question 23

change in phase

Answer 23

the particles in the solid state vibrate about their fixed positions. The energy is thus the least dispersed and the solid has the lowest entropy. when the solid melts, the particles move more freely in the liquid state and become more disordered. Hence there is an abrupt increase in entropy as there are more ways to distribute the particles and their energy in the liquid state.

during vaporisation, in gaseous state, particles are able to move even more freely. Hence there is a large increase in entropy as there are more ways to distribute the particles and their energy in the gaseous state.

Question 24

change in the number of particles

Answer 24

when there is an increase in the number of gas particles, the particles in gas are the most disordered and the number of ways that the particles and the energy can be distributed increase greatly.

Question 25

mixing of particles
expansion of gases

Answer 25

as the volume available for each gas increase (or distribution of the particles for expansion of gas), there are more ways to distribute their particles and hence their energy. entropy increased.

Question 26

dissolution of an ionic solid

Answer 26

2 cases
entropy increases because the ions in the solid are free to move in solution

entropy decreases because water molecules that were originally free to move become restricted in motion as they arrange themselves around the ions

depends on which is more significant

Question 27

formula for gibbs free energy change

Answer 27

ΔG = ΔH - TΔS

Question 28

what does ΔG < or > 0 mean

Answer 28

ΔG > 0: reaction is not spontaneous
ΔG = 0: reaction is at equilibrium, there is no net reaction in the forward or backward direction
ΔG < 0: reaction is spontaneous

(u want a reaction to be spontaneous)

Question 29

what are the limitations of ΔG°

Answer 29

ΔG° can only be used to predict spontaneity under standard conditions

while spontaneity can be determined, it does not take into account the kinetics of the reaction, which is the rate at which the reaction takes place