2.1 Thermochemistry

Question 1

What does matter possess energy in the form of?

Answer 1

kinetic energy
potential energy

Question 2

What is the kinetic energy of matter?

Answer 2

The energy of motion at a molecular level

Question 3

What is the potential energy of matter?

Answer 3

Arises from the positions of the atoms relative to one another.

Question 4

What involves changes in potential energy?

Answer 4

bond making
bond breaking

Question 5

Define exothermic reaction.

Answer 5

One that releases energy to the surroundings.

There is a temperature rise and the enthalpy (delta H) is negative

Question 6

Give examples of exothermic reactions.

Answer 6

Acids with metals
In hand warmers (oxidation of iron)
Thermite reaction (aluminium and iron (III) oxide)

Question 7

Define endothermic reactions.

Answer 7

One that takes in energy from the surroundings

There is a temperature drop and enthalpy change (delta H) is positive

Question 8

Give examples of endothermic reactions.

Answer 8

Melting ice
In cold packs (dissolving ammonium chloride in water)
Thermal decomposition of Group 2 carbonates

Question 9

What is enthalpy

Answer 9

The heat content of a system at constant pressure

Question 10

How do you calculate enthalpy change?

Answer 10

Enthalpy of products - Enthalpy of reactants

Question 11

How can enthalpy changes be represented?

Answer 11

As enthalpy profile diagrams.

Question 12

Draw an enthalpy profile for an exothermic reaction

Answer 12

Question 13

Draw an enthalpy profile for an endothermic reaction

Answer 13

Question 14

What does the principle of conservation of energy state?

Answer 14

The principle of conservation of energy states that energy cannot be created or destroyed, only changed from one form to another

Question 15

What are the standard conditions for standard enthalpy change?

Answer 15

1. All substances in their standard states
2. Temperature of 298K
3. A pressure of 1 atm.

Question 16

What is the symbol for standard enthalpy change?

Answer 16

ΔH°

Question 17

Define standard enthalpy change of formation.

Answer 17

The enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions

Question 18

Define the standard enthalpy change of combustion?

Answer 18

This is the enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.

Question 19

What does Hess’s law state?

Answer 19

Hess’s law states that the total enthalpy change for a reaction is independent of the route taken from the reactants to the products.

Question 20

Draw a rough diagram of the Hess’s cycle.

Answer 20

Question 21

Define bond enthalpy

Answer 21

Bond enthalpy is the enthalpy required to break a covalent X-Y bond into X atoms and Y atoms all in the gas phase.

Question 22

What are average bond enthalpies?

Answer 22

The average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species.

Question 23

How do you make calculations using bond enthalpies?

Answer 23

1. Draw out each molecule to show the bonds
2. Calculate the energy required to break all the bonds in the reactants
3. Calculate the energy released in forming all the bonds in the products
4. Use the equation:
   Bonds broken - bonds formed = enthalpy

Question 24

State the equation for measuring enthalpy changes.

Answer 24

q = mc(delta T)

where,
m is the mass of the solution
c is the specific heat capacity of the solution
T is the temperature change

Question 25

How do you calculate molar enthalpy change?

Answer 25

1. Calculate the amount of heat transferred in the experiment 2. Calculate the moles of the reactant 3. Calculate molar enthalpy change: -q/n

Question 26

How do you obtain an approximate value for the enthalpy of combustion of a fuel such as an alcohol?

Answer 26

1. Allow a suitable gap between the base of the metal container and the top of the spirit burner 2. Accurately measure the amount of water being added to the metal container. 3. Use an accurate thermometer to measure the initial temp of water. When a steady value has been obtained, record the temperature 4. Weigh the spirit burner containing alcohol and record the initial mass 5. After lighting the wick, adjust the gap between the metal container and the spirit burner (if necessary) 6. Allow alcohol to heat the water to a suitable temperature (an increase of about 20 degrees) 7. Extinguish the flame and record final max temp 8. Allow spirit burner to cool thoroughly before reweighing and recording final temperature 9. Use the equation: q = mc(delta T) to calculate enthalpy change of combustion.

Question 27

Why will the enthalpy value obtained in the experiment be lower than the book value?

Answer 27

Some energy transferred to the surroundings (lost) The alcohol is not completely combusted.

Question 28

How do you calculate enthalpy change through Hess's law (experiment)?

Answer 28

1. Measure an appropriate volume of acid using a burette or pipette and place it in a polystrene cup (acid must be in excess) 2. Use a thermometer to measure initial temperature of the acid. Record temp when steady value obtained. 3. Accurately weigh the solid, in powder form in a suitable container. 4. Add all solid to the cup and stir mixture well and start a stop-clock. 5. Keep stirring with the thermometer and record temp regularly (every 30s). Stop recording temp when it has fallen for 5 min. 6. Re-weigh the weighing container to ensure correct mass of solid added is recorded. 7. Plot a graph of temperature against time to calculate the max temperature the mixture might have reached. 8. Calculate the amt of heat transferred using (mc delta T) 9. Calculate enthalpy change for the reaction 10. Repeat procedure with other solid. 11. Use Hess's law to calculate the required enthalpy change.