Chemical Energetics

Question 1

What is the study of chemical energetics?

Answer 1

The study of chemical energetics involves the energy changes that accompany chemical reactions.

Question 2

Define the enthalpy change of a reaction, ΔH

Answer 2

The ΔH is the amount of energy absorbed or released in a chemical reaction. ΔH represents the difference in energy level between products and reactants.

Question 3

What are the units for ΔH

Answer 3

kJ mol-1

Question 4

Define the standard enthalpy change of reaction ΔH

Answer 4

The standard enthalpy change of reaction ΔH° is the amount of energy absorbed or released in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions of 298K and 1 bar.

Question 5

Define exothermic reation

Answer 5

Exothermic reaction is a reaction where heat is released to the surroundings. Bonds are formed.

Question 6

What happens to the temperature of the surroundings and the reaction container during an exothermic reaction

Answer 6

The temperature of the surroundings increases and the reaction container becomes hot during an exothermic reaction.

Question 7

What is the ΔH for an exothermic reaction?

Answer 7

The enthalpy change, ΔH, for an exothermic reaction is negative. This indicates that the total energy of the products is lower than the total energy of the reactants. The energy lost is given out in the form of heat.

Question 8

6 examples of exothermic reactions

Answer 8

Combustion reactions, neutralisastion reactions, haber process, respiration, freezing and condensation

Question 9

Define combustion reactions

Answer 9

It is the reaction between a compound/element and oxygen under heat which releases energy to the surroundings.

Question 10

Chemical equation for the combustion of carbon

Answer 10

1C(s) + O2 (g) –> CO2 (g)
ΔHc = -400kJ mol-1

Question 11

What does the equation for the combustion of carbon indicates?

Answer 11

400kJ of heat is released when 1mol of C reacts with 1mol of O2 to produce 1mol of CO2.

Question 12

Define the standard enthalpy change of combustion

Answer 12

The standard enthalpy change of combustion, ΔH°c or a substance is the energy released when one mole of the substance is completely burnt in oxygen at standard conditions of 298K and 1bar. ΔH°c is always exothermic.

Question 13

Define neutralisation reaction

Answer 13

It is the reaction between an acid and an alkali which releases heat to the surroundings.

Question 14

Chemical equation between aqueous potassium hydroxide and nitric acid

Answer 14

KOH (aq) + HNO3 (aq) –> KNO3 (aq) + 1H2O (l)
ΔHn = -57.3 kJ mol-1

Question 15

Define standard enthalpy change of neutralisation

Answer 15

Standard enthalpy change of neutralisation, ΔHn° is the energy released when an acid and base react to form one mole of water at standard conditions of 298K and 1bar

Question 16

What happens during the Haber process

Answer 16

Ammonia is manufactured by reacting nitrogen and hydrogen gas during the Haber process.

Question 17

Chemical equation of the formation of ammonia via the Haber process

Answer 17

N2 (g) +3H2 (g) –> 2NH3 (g)
ΔH = -92 kJ mol-1

Question 18

Define the process of respiration

Answer 18

Respiration is the process whereby glucose is oxidised to form carbon dioxide and water. Energy is also used in the process.

Question 19

Chemical equation for the respiration of glucose

Answer 19

C6H12O6 (s) + 6O2 (g) –> 6CO2 (g) + 6H2O (l) + Energy
ΔH = -ve

Question 20

What happens during freezing and condensation

Answer 20

Freezing and condensation are processes where heat is released to the surroundings. When particles come close together during freezing and condensation, heat is released to the surroundings.

Question 21

Define endothermic reaction

Answer 21

Endothermic reaction is a reaction where heat is absorbed from the surroundings.

Question 22

What happens to the temperature of the surroundings and the reaction container during an endothermic reaction?

Answer 22

During an endothermic reaction, the temperature of the surroundings decreases and the reaction container becomes cold

Question 23

What is the enthalpy change ΔH for an endothermic reaction?

Answer 23

The enthalpy change, ΔH, for an endothermic reaction is positive. This indicates that the total energy of the products is higher than the total energy of the reactants

Question 24

5 examples of endothermic reactions

Answer 24

Thermal decomposition of carbonates, dissolution of potassium or ammonium salts, photosynthesis, melting and boiling

Question 25

Why is thermal decomposition of carbonate an endothermic reaction?

Answer 25

Heat is required to decompose carbonates to form their respective oxides and carbon dioxide gas.

Question 26

Chemical equation for the thermal decomposition of copper (II) carbonate

Answer 26

CuCO3 (s) --> CuO (s) + CO2 (g)

Question 27

Why is the dissolution of potassium or ammonium salts an endothermic reaction

Answer 27

Dissolving crystals of potassium or ammonium salts in water absorbs heat from the surroundings.

Question 28

Chemical equation of dissolving potassium chloride in water

Answer 28

KCl (s) + aq --> KCl (aq)

Question 29

Define photosynthesis

Answer 29

Photosynthesis is the process whereby glucose is synthesises from carbon dioxide and water. Energy from sunlight is absorbed in the process.

Question 30

Chemical equation of photosynthesis

Answer 30

6CO2 (g) + 6H2O (l) + Energy --> C6H12O6 (s) + 6O2 (g)

Question 31

Why does melting and boiling of substances occur

Answer 31

Particles absorb energy from the surroundings during melting or boiling to overcome the forces of attraction between them so that they can become further apart from one another.

Question 32

Define activation energy

Answer 32

Activation energy, Ea, is the minimum amount of energy that molecular collisions must possess in order for a chemical reaction to occur.

Question 33

Define transition state

Answer 33

Transition state is the point at which reactants are about to change to products. It occurs at the maximum points, i.e. point of highest energy

Question 34

All chemical reactions involve:

Answer 34

1. Breaking of bonds in reactant particles 2. Formation of new bonds in produce particles

Question 35

Bond breaking in reactants absorbs energy. Hence, bond breaking is an ____________________ process where the enthalpy change is positive e.g. breaking up of a methane molecule

Answer 35

endothermic

Question 36

In an exothermic reaction, the energy given out to form bonds in products particles is __________________ the energy absorbed to break bonds in reactants particles

Answer 36

Larger than

Question 37

In an endothermic reaction, the energy liberated to form bonds in products particles is _______________ the energy absorbed to break bonds in reactants particles

Answer 37

Smaller than

Question 38

Is HCl (g) --> H (g) + Cl (g) an endothermic or exothermic reaction

Answer 38

Endothermic

Question 39

Is N (g) + 2O (g) --> NO2 (g) an endothermic or exothermic reaction

Answer 39

Exothermic

Question 40

Is C2H6 (g) --> 2C (g) + 6H (g) an endothermic or exothermic reaction

Answer 40

Exothermic

Question 41

Is C2H5OH (g) --> C2H5OH (l) an endothermic or exothermic reaction

Answer 41

Exothermic

Question 42

Why is the reaction for formation of water an exothermic reaction

Answer 42

The energy given out to form covalent bonds between H and O in water molecules is larger then the energy absorbed to break the H-H and O-O bonds in H2 and O2 respectively.

Question 43

Define bond energy

Answer 43

Bond energy of X-Y bond is the average energy absorbed to break one mole of covalent bond in the gas phase into constituent gaseous atoms under standard conditions Equation: X - Y (g) --> X (g) + Y (g)

Question 44

How is bond energy related to covalent bond strength

Answer 44

bond energy ∝ covalent bond strength (directly proportional to)

Question 45

How to determine the enthalpy change of reaction?

Answer 45

Calorimetric method

Question 46

Experimental procedure of the calorimetric method

Answer 46

1. Using a measuring cylinder, place 25cm^3 of X in a polystyrene cup and note the temperature. 2. Using another measuring cylinder, measure 25cm^3 of Y. 3. To X in the polystyrene cup, add Y. Stir carefully with the thermometer and record the maximum temperature reached.

Question 47

How to calculate the temperature change, ΔH?

Answer 47

ΔH = |Maximum/minimum temperature - Initial temperature| ΔH = |T max/min - T initial|

Question 48

Method to calculate enthalpy changes, ΔH

Answer 48

1. Calculate quantity of heat absorbed or released by solution, Q using: Q = m c Δ T or Q = C Δ T m soln = total volume of solution (assume density of solution = 1g cm^-3) 2. Calculate quantity of heat absorbed by reaction, Q' using Q' = -Q (assuming no heat loss to surroundings, i.e. 100% efficiency) 3. Calculate enthalpy change asked for in the question. ΔH = Q' / n, '+' for endothermic reaction, '-' for exothermic reaction n = amount of limiting reagent

Question 49

3 formulas that is used when calculating enthalpy change

Answer 49

Q = m c Δ T Δ H = Q' / n n = m/Mr where: Δ H: enthalpy change of reaction Q': quantity of heat absorbed or released by reaction n: amount of limiting reagent m: mass of solution/volume of solution Mr: molar mass of compound c: specific heat capacity of solution Δ T: change in temperature

Question 50

Define specific heat capacity, c, of a substance

Answer 50

Specific heat capacity, c, of a substance is the quantity of heat required to raise the temperature of 1g of the substance by 1 K, units usually in J g^-1 K^-1

Question 51

Define heat capacity, C, of a substance

Answer 51

Heat capacity, C, of a substance is the quantity of heat required to raise the temperature of m g of the substance by 1K, units usually in J K^-1

Question 52

How to derive Q = C ΔT from C = m c

Answer 52

C = m c Q = m c Δ T Q = C Δ T

Question 53

3 assumptions made in calculating the enthalpy change using the above experiment

Answer 53

1. Negligible heat loss to the surroundings due to insulation, hence, all the chemical energy involved in the reaction is transformed into heat energy which is used to change the temperature of the solution 2. The density of the solution is approximated to that of water (1.00g cm^-3). Q = m c ΔT becomes Q = V c Δ T where V is the total volume of the solution in cm^3 3. The specific heat capacity of the solution approximates that of water (4.18 J g^-1 K^-1)