Topic 8 Energetics I

Question 1

Why is first electron affinity exothermic?

Answer 1

The attractive force of the nuclear charge outweighs any shielding effect.

Question 2

Standard enthalpy change of reaction

Answer 2

The enthalpy change measured at 100kPa & (usually) 298K when the number of moles of substances in the equation as written react.

Question 3

Heat energy

Answer 3

Directly proportional to the absolute temperature of the substance.

Question 4

What is chemical energy made up of?

Answer 4

Kinetic energy & potential energy (a measure of how strongly particles attract & repel each other).

Question 5

Enthalpy

Answer 5

Measure of the total energy of a system. This can’t be determined directly, but the enthalpy change for a physical or chemical process can be determined.

Question 6

Enthalpy change of a process

Answer 6

The heat energy transferred between the system and the surroundings at a constant pressure.

Question 7

Key difference between an enthalpy level & an enthalpy profile diagram.

Answer 7

Activation energy is only shown on an enthalpy profile diagram.

Question 8

How can we draw an enthalpy level diagram?

Answer 8

Enthalpy, H on the vertical y-axis. Progress of reaction on the x-axis (this is not always necessary). Give the value for delta H, incl. the sign. Give the formula & state symbols of reactants and products on each level. Draw an arrow from reactants to products.

Question 9

Delta H

Answer 9

H products - H reactants

Question 10

For an exothermic enthalpy level diagram

Answer 10

Down arrow, delta H is negative.

Question 11

For an endothermic enthalpy level diagram

Answer 11

Up arrow, delta H is positive.

Question 12

Standard enthalpy change of combustion

Answer 12

The enthalpy change at 100kPa & (usually) 298K when one mole of a substance is completely burnt in oxygen. (Ensure only one mole of the substance being burnt is represented in the equation.)

Question 13

Experimental determination of enthalpy change of combustion of a liquid

Answer 13

Weigh a spirit burner containing the liquid.
Add a known volume of water to a copper can.
Measure the temperature of the water.
Light the burner.
Constantly stir the mixture.
When the temperature of the water has reached ~ 20°C above its initial temperature, extinguish the flame and immediately reweigh the burner.
Measure the final temperature.

Question 14

When calculating enthalpy change of combustion remember…

Answer 14

Mass of water.
Divide by the moles of the liquid burnt.
Include a sign.

Question 15

Sources of error in the experimental determination of enthalpy change of combustion of a liquid

Answer 15

• Some of the heat energy produced in the burning is transferred to the air not the water.
• Incomplete combustion, which would cause soot to form on the bottom of the can.
• Some of the heat energy is transferred to the copper can not the water.
• The conditions are not standard, e.g., producing water vapour not liquid water.

Question 16

Standard enthalpy change of neutralisation

Answer 16

Th enthalpy change measured at 100kPa & (usually) 298K when one mole of water is produced by the neutralisation of an acid by an alkali.

Question 17

Experimental determination of enthalpy change of neutralisation

Answer 17

Use a pipette with a safety filler to place 25 cm^3 of 1 mol dm^-3 acid in a polystyrene cup.
Measure the temperature of the acid.
Use a pipette with a safety filter to place 25 cm^3 of alkali into a beaker.
Measure the temperature of the alkali.
Add the acid to the alkali and measure the maximum temperature reached.

Question 18

Sources of error/assumptions made in the experiments

Answer 18

Some heat energy will be transferred to the thermometer and the polystyrene cup. Temperature is only given to 2 s.f., so give answers to 2 s.f.. Measurement uncertainties.

Question 19

Calculation of enthalpy change of neutralisation

Answer 19

The mass is the sum of the 2 volumes, assuming density is 1 g cm^-3.
Divide by the moles of limiting reagent, which equals the amount of water formed.

Question 20

Calculating enthalpy change of reaction using mean bond enthalpies

Answer 20

Delta rH = Sum of bonds broken - sum of bonds made
If delta H reaction is given instead, rearrange to find the missing bond enthalpy required.

Question 21

Limitations of calculating enthalpy change of reaction using mean bond enthalpies

Answer 21

Bond enthalpies are measured in the gaseous state.

Question 22

Bond enthalpy

Answer 22

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

Question 23

Mean bond enthalpy

Answer 23

The enthalpy change when one mole of a bond, averaged out over many different molecules, is broken.

Question 24

What… varies with its environment?

Answer 24

Bond enthalpy

Question 25

Standard enthalpy change of formation

Answer 25

The enthalpy change measured at 100kPa & (usually) 298K when one mole of a substance is formed from its elements in their standard states. (for this definition, the standard state of an element is the form in which it exists at the specified temperature).

Question 26

Hess’s law

Answer 26

States that the enthalpy change of a reaction is independent of the path taken in converting reactants to products, provided the initial & final conditions are the same in each case.

Question 27

Why is Hess’s law useful?

Answer 27

It allows us to find enthalpy changes that cannot be determined experimentally. It is based off the 1st Law of Thermodynamics (energy cannot be created or destroyed).

Question 28

Why does the temperature of the solution decreased after the endpoint of a neutralisation reaction is reached?

Answer 28

There is no further release of energy, so there is just a colder solution being added to the reaction mixture.

Question 29

Apart from increasing uncertainty, why is repeated use of a smaller measuring cylinder disadvantageous?

Answer 29

It takes longer and leads to greater transfer losses.

Question 30

Energy changes are not described as lower or higher, but…

Answer 30

More exo-/endothermic.

Question 31

The databook value for an enthalpy change may differ from the calculated value because…

Answer 31

• The mean bond enthalpies are not specific to the compounds involved.
• Mean bond enthalpies are given for the gaseous state.
• The equation may be for more/less moles of product than the standard enthalpy change.

Question 32

Enthalpy change of solution for a hydrated compound

Answer 32

Endothermic, but for the anhydrous equivalent compound, the enthalpy change is less endothermic because the anhydrous compound releases energy when it reacts with water as less energy is required to separate fewer water molecules from the ions in the crystal structure.

Question 33

Why might a catalyst be warmed?

Answer 33

To provide the activation energy.

Question 34

When should there be extrapolation when determining the temperature change of a reaction?

Answer 34

At the point at which the second reactant was added.