Topic 8: Energetics I

Question 1

State the standard conditions

Answer 1

standard conditions are 100 kPa and 298 K

Question 2

What is the enthalpy change?

Answer 2

Enthalpy change is the heat energy change measured at constant pressure.
Enthalpy change= enthalpy of products- enthalpy of reactants
H means enthalpy: change in H= H products- H reactants.

Question 3

Define standard enthalpy change of reaction

Answer 3

Standard enthalpy change of reaction:
The enthalpy change of a given reaction under standard conditions
eg: 2Na + Cl2 —> 2NaCl

Question 4

Define standard enthalpy change of formation

Answer 4

Standard enthalpy change of formation:
The enthalpy change when 1 mole of a substance is formed from its elements in their standard states.
The standard enthalpy of formation of an element in its standard state is 0 bc it hasn’t been formed from something else.

Question 5

Define standard enthalpy change of neutralisation

Answer 5

Standard enthalpy change of neutralisation:
The enthalpy change when 1 mole of water is produced from the reaction between an acid and a base under standard conditions

Question 6

Define standard enthalpy change of combustion

Answer 6

Standard enthalpy change of combustion:
The enthalpy change when 1 mole of a substance is completely burnt in 02 under standard conditions.

The products will always be the oxides of the elements present

Question 7

Define standard enthalpy change of atomisation

Answer 7

Standard enthalpy change of atomisation:
the enthalpy change when 1 mole of gaseous atoms is formed from the element under standard conditions eg: 1/2 Cl2 —-> Cl2 (g)

Question 8

Define enthalpy

Answer 8

The measure of a system’s total energy

Question 9

Describe an exothermic reaction

Answer 9

Heat energy is transferred from the system to the surroundings. Hay a negative enthalpy change because particles have less energy

Question 10

Describe an endothermic reaction

Answer 10

Heat energy is transferred from the surroundings to the system. The reaction takes in heat energy

Question 11

Explain energy changes in bond making and breaking

Answer 11

Bond making gives out energy.

Bond breaking takes in energy.

Question 12

Give the equation to find the enthalpy of a substance

Answer 12

energy transferred = mass x specific heat capacity × temperature change
(Q=mcΔT)

Enthalpy is then found by doing Q/moles
Find moles using m/Mr

Question 13

Describe how to find the enthalpy change of a redox reaction.

Answer 13

Measure out 25cm3 of 0.2M CuSO4 (aq) into a well insulated container and measure its temp. Calculate the mass of 0.1 moles of Zn powder, which is in excess. Add Zn powder to the copper sulphate. Stir and continuously record the temp every 20s.
Reaction: Cu2+(aq) + Zn(s) —> Cu(s) + Zn2+(aq)
Use results to find Q. The find enthalpy change by Q/moles. Find moles using mass/Mr

Question 14

Describe how to find the enthalpy change of citric acid and sodium hydrogencarbonate

Answer 14

Measure out 25cm3 of 1M Citric acid (aq) into a well insulated container and measure its temp.
Calculate the mass of 0.1 moles of sodium hydrogencarbonate powder, which is in excess.
Add NaHCO3 to the citric acid. Stir and continuously record the temp every 20s.
C6H8O7(aq) + 3NaHCO3(s) —> C3H5O7Na3(aq) + 3CO2 + 3H20
Use results to find Q. The find enthalpy change by Q/moles. Find moles using mass/Mr

Question 15

What do you need to remember when constructing enthalpy level diagrams for an endothermic reaction?

Answer 15

X axis should have H, y axis should say reaction progress
The formula for both reactants and products with the state symbols should be given. The delta H arrow should be pointing upwards from reactants to products. Products are higher than reactants.

Question 16

What do you need to remember when constructing enthalpy level diagrams for an exothermic reaction?

Answer 16

X axis should have H, y axis should say reaction progress
The formula for both reactants and products with the state symbols should be given. The delta H arrow should be pointing downwards from reactants to products. Products are lower than reactants.

Question 17

Describe an experiment to find the enthalpy change of combustion

Answer 17

Weigh an alcohol containing spirit burner. Add a known volume of water to a copper can, measure its temperature. Light the burner under the can.
Constantly stir the mixture with the thermometer. When the temp of the water has reached 20° above its initial, extinguish the flame and immediately reweigh the burner. Measure the final temperature of the water.
Q=mcAt. M and c refer to mass and SHC of WATER. Use results to find Q. Then find enthalpy change by Q/moles. Find moles using mass/Mr

Question 18

Evaluate limitations/errors that can be made in these experiments

Answer 18

For the combustion expt: Heat is lost to the surroundings instead of heating up the water. Use a draught excluder to reduce this. Ethanol evaporated so the ethanol apparently releases less energy per g. These are SYSTEMATIC errors

Random errors are errors finding the mass or the temperature. It creates a small % error

Question 19

What is meant by bond enthalpy?

Answer 19

Bond enthalpy is the enthalpy change when one mole of a bond in the gaseous state is broken.

Question 20

What is meant by mean bond enthalpy?

Answer 20

Mean bond enthalpy is the enthalpy change when one mole of a bond, averaged out over many molecules, is broken.
We use this because certain bond enthalpies vary within its environment.

Question 21

How do you calculate enthalpy change of a reaction using mean bond enthalpies? What are limitations of this method?

Answer 21

Change in H= sum of bonds broken - sum of bonds made
This method has limitations because bond enthalpies are measured in the gaseous state, and some reactants can be liquid or solid.