Chemical Energetics

Question 1

Briefly describe an exothermic process

Answer 1

Temperature increases and bonds are formed

Question 2

Briefly describe an endothermic process

Answer 2

Temperature decreases and bonds are broken

Question 3

What are standard conditions?

Answer 3

298K and 1atm (101kPa)

Question 4

Standard enthalpy changes

Answer 4

A standard enthalpy change is the energy change that takes place per mole of reactant, under standard conditions

Question 5

Standard enthalpy change of reaction

Answer 5

It is the amount of energy released/absorbed when reactants are converted to products, according to the balanced chemical equation, under standard conditions

Question 6

Standard enthalpy change of formation

Answer 6

It is the energy change that occurs when 1 mole of a compound is formed from its constituent elements under standard conditions

Question 7

Standard enthalpy change of combustion

Answer 7

It is the energy released when one mole of a substance undergoes complete combustion under standard conditions, with all substances in their standard states
(always exo)

Question 8

Standard enthalpy change of neutralisation

Answer 8

It is the amount of energy released when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

Question 9

Explain why the standard enthalpy change of neutralisation of any combination of a strong acid and a strong alkali is almost the same?

Answer 9

Strong acids and alkalis are fully dissociated in aqueous solutions hence the reaction between any 2 strong acids and alkalis to the same ionic equation. For weak acids/alkalis, more energy is required for dissociating the substance before they can react

Question 10

Standard enthalpy change of atomisation

Answer 10

It is the energy change when 1 mole of gaseous atoms are formed from the element found in standard state, under standard conditions

Question 11

Standard enthalpy change of solution

Answer 11

It is the enthalpy change when 1 mole of an ionic compound is dissolved in a large amount of water to form an infinitely dilute solution, under standard conditions

Question 12

Standard enthalpy change of hydration

Answer 12

It is the enthalpy change when 1 mole of gaseous ions is dissolved in a large amount of water, under standard conditions
(always exo)

Question 13

Lattice energy

Answer 13

It is the energy released when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions
(always exo)

Question 14

Electron affinity

Answer 14

First e.a is the energy released when 1 mole of gaseous atoms each gain an electron to form 1 mole of gaseous ions having -1 charge

Question 15

Why is the first e.a negative and the second positive?

Answer 15

2nd, 3rd etc e.a values are endothermic due to repulsive forces between anion and entering electron

Question 16

Bond energies

Answer 16

It is the amount of energy required to break 1 mole of covalent bonds to produce individual atoms

Question 17

Formula for delta H from bond energies

Answer 17

Summation of bond energies reactants - summation of bond energies of products

Question 18

Formula for standard enthalpy change of atomisation from B.E

Answer 18

1/2 B.E

Question 19

State Hess’ Law

Answer 19

Hess’ Law states that the enthalpy change of a reaction is independent of the path taken given that the initial and final conditions are the same

Question 20

Formula for energy

Answer 20

E= mc(delta T)

Question 21

What is directly and indirectly proportional to force?

Answer 21

Directly proportional to charges and indirectly proportional to ionic radii

Question 22

Formula for standard enthalpy change of solution

Answer 22

delta H solution= -lattice energy + delta H hydration

Question 23

Define entropy

Answer 23

Entropy of a system is the number of possible arrangements of the particles and their energy in the system

Question 24

Predict the change in entropy during a chemical reaction

Answer 24

There is a gain in entropy in any process which the number of gaseous particles in the system increases

Question 25

Predict the change in entropy with temperature changes

Answer 25

When the temperature of a substance increases there is a gain in entropy as the particles gain kinetic energy and the amount of disorder in a system increases

Question 26

Equation for entropy during change in temperature

Answer 26

Entropy = q(amount of heat transferred to system)÷ T (absolute temp in k)

Question 27

Equation for entropy for a chemical reaction

Answer 27

S = sum of entropy of products - sum of entropy of reactants

Question 28

Equation for Gibbs

Answer 28

Gibbs = delta H - T delta S

Question 29

When is a reaction spontaneous?

Answer 29

A reaction is said to be spontaneous if delta G is less than 0

Question 30

Both delta H and S are positive

Answer 30

Reaction is spontaneous at high temperatures Tdelta S > delta H
Delta G<0

Question 31

Both delta H and S are negative

Answer 31

Reaction is spontaneous at low temperatures. Tdelta S < delta H
Delta G<0

Question 32

Delta H is negative and delta S is positive

Answer 32

Delta G is always negative reaction is spontaneous at all temperatures

Question 33

Delta H is positive and delta S is negative

Answer 33

Delta G is always > 0. Reaction is never spontaneous