TOPIC 8 Chemical Energetics

Question 1

What is the first law of thermodynamics?

Answer 1

During a chemical, energy cannot be created or destroyed. However, one form of energy can be converted into another

Question 2

What are the 2 components that make up chemical energy? What are they?

Answer 2

Kinetic energy - measure of the motion of particles in a substance
Potential energy - measure of how strongly these particles interact

Question 3

What is heat energy?

Answer 3

The portion of potential energy and kinetic energy of a substance that is responsible for the temperature of the substance
It is directly proportional to its absolute temperature

Question 4

What is enthalpy?

Answer 4

A measure of the total energy of a system

Question 5

What is enthalpy change?

Answer 5

The heat energy that is transferred between the system and the surroundings at constant pressure

Question 6

What are the standard conditions that all enthalpy changes should be quoted at?

Answer 6

Pressure = 100kPa
Temperature = usually 298K

Question 7

What is the enthalpy change measured at standard conditions called?

Answer 7

Standard enthalpy change

Question 8

In standard enthalpy change of reaction, what does ‘per mole’ refer to?

Answer 8

One mole of equation and not one mole of any reactant or product

Question 9

Draw the enthalpy level diagram for an exothermic and endothermic reaction.

Answer 9

Exothermic - final enthalpy is less than initial enthalpy

Endothermic - final enthalpy is more than initial enthalpy

Question 10

What are some (4) points to remember when constructing enthalpy level diagrams?

Answer 10

Only need to label the vertical axis
The formulae of both the reactants and products should be given
The value for 🔼H should be given, including the correct sign
It is not essential to show the activation energy

Question 11

Draw an enthalpy level diagram for the following reaction:

C(s) + O2(g) —> CO2(g)
🔼H = -394kJ mol-1

Answer 11

Reactants higher than products

Question 12

Draw an enthalpy level diagram for the following reaction:

C(s) + CO2(g) —> 2CO(g)
🔼H = +172kJ mol-1

Answer 12

Reactants less than products

Question 13

Draw an enthalpy level diagram for the following reaction:

CH4(g) + 2O2(g) —> CO2(g) + 2H2O(l)
🔼H = -890kJ mol-1

Answer 13

Reactants higher than products

Question 14

What is the experiment method for finding the enthalpy change of combustion?

Answer 14

(1) a spirit burner containing the liquid under test is weighed
(2) a known volume of water is added to a copper can
(3) the temperature of the water is measured
(4) the burner is lit
(5) the mixture is constantly stirred
(6) when the temperature of the water is approximately 20 C above initial temperature , the flame is extinguished and the burner immediately reweighed
(7) the final temperature is measured

Question 15

What are the 3 steps for calculating enthalpy change of combustion and neutralisation and the formulas that are part of these steps?

Answer 15

Step 1: calculate the heat energy , Q , transferred to the water in kJ

Q = mc🔼T

Step 2: calculate the amount , n , of liquid burned

n = m/M

Step 3: calculate enthalpy change of combustion

🔼H = -Q/n

Question 16

What are the typical sources of error when calculating enthalpy change of combustion? (5)

Answer 16

Some of the heat energy produced is transferred to the air and not the water
Some of the liquid is not burned completely
Some of the heat energy produced is transferred to the copper can and not the water
The conditions are not standard (water vapour not liquid water is produced)
Not all of the heat energy transferred from the water to the surrounding is compensated for

Question 17

What is the experiment method for finding the enthalpy change of neutralisation?

Answer 17

(1) using a pipette fitted with a safety filter , place 25.0 cm3 of 1.00 mol dm-3 acid into an expanded polystyrene cup
(2) measure the temperature of the acid
(3) using a pipette fitted with s safety filter , place 25.0 cm3 of the alkali into a beaker
(4) measure the temperature of the alkali
(5) add the alkali to the acid, stir with the thermometer and measure the maximum temperature reached

Question 18

What is the typical bond enthalpy change for a diatomic molecule, XY?

Answer 18

XY (g) —> X (g) + Y (g)

Question 19

How would you calculate the bond enthalpy of diatomic molecules?

Answer 19

Consider each bond separately

Question 20

What is the process for calculating the enthalpy change of reaction using mean bond enthalpies?

Answer 20

Step 1: calculate the sum of the mean bond enthalpies of the bonds broken
Step 2: calculate the sum of the mean bond enthalpies of the bonds made
Step 3: calculate the enthalpy change by subtracting the sum of the bonds made from the sum of the bonds broken