Energetics

Question 1

What is the unit of energy?

Answer 1

Joules

Question 2

What is an exothermic reaction and how do we identify them?

Answer 2

These are reactions that give out heat energy to the surroundings. We recognise them because
the temperature of the surroundings gets hotter.

Question 3

What is an endothermic reaction and how do we recognise them?

Answer 3

These are reactions that take in heat energy from the surroundings. We recognise them because
the temperature of the surroundings gets cooler.

Question 4

Explain the energetics of the copper sulphate reaction.

Answer 4

Hydrated copper sulphate is blue because it has water in its crystals. If we heat this the water
evaporates and the copper sulphate turns white. This is endothermic as the reaction requires heat.
If we add water to the white copper sulphate then it gets hotter and the copper sulphate turns blue.
This is an exothermic reaction.

Question 5

What are the units for energy changes?

Answer 5

Kjmol-1

Question 6

Define the term enthalpy change.

Answer 6

This is the heat change at constant pressure

Question 7

What is the sign for enthalpy change?

Answer 7

∆H

Question 8

Draw the enthalpy diagram for an exothermic and endothermic reaction? Explain these diagrams

Answer 8

Endothermic Reaction – more heat is taken in to break bonds than is released when making bonds.

Exothermic reaction – more energy is released when making bonds than is needed to break bonds.

Question 9

Define the term enthalpy of formation - ∆fHo

Answer 9

This is the enthalpy change when one mole of substance is formed from its constituent elements under standard conditions, all reactants and products being in their standard states

Question 10

Define the term standard molar enthalpy of combustion - ∆cHo.

Answer 10

This is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, all reactants and products being in standard states.

Question 11

What is the difference between heat and temperature?

Answer 11

The temperature is a measure of how hot a substance is and it is measured by a thermometer. How hot a substance is a measure of the kinetic energy of all the particles present. Therefore, a warm bath has more energy than a hot cup of water because there are more particles present.

Question 12

What does the term specific heat capacity mean?

Answer 12

This is the amount of heat needed to raise 1g of a substance by 1K. Its units are joules per gram per kelvin or Jg-1K-1.

Question 13

How do you calculate the energy transferred/enthalpy change during a chemical reaction?

Answer 13

Energy transferred/enthalpy change = mass x specific heat capacity x temperature rise

Q = m x c ∆T

Question 14

How do you convert energy transferred/enthalpy change to Kjmol-1?

Answer 14

Divide by the number of moles used in the reaction.

Question 15

What is a calorimeter and what is it made of?

Answer 15

It is a piece of equipment used to hold water and measure temperature changes. It is made of copper as this metal is a good heat conductor.

Question 16

How would you do an experiment to find the enthalpy change of combustion for an alcohol?

Answer 16

· Find the mass of a spirit burner and lid.

· Pour 100 cm3 of water into a copper calorimeter and take the temperature.

· Put the spirit burner under the calorimeter and set fire to the alcohol.

· Stir the water and extinguish the flame on the spirit burner when the temperature of the water has risen about 15oC. Continue to stir the water and record the highest temperature.

· Re-weigh the spirit burner and lid.

· Find the temperature rise of the water and the mass of alcohol burnt.

· Use q = mc∆T to find the energy transferred to the water and then divide by the number of moles burnt to find the enthalpy of combustion in kjmol-1

Question 17

What is the greatest source of error in calorimeter experiments?

Answer 17

Heat loss to the surroundings, incomplete combustion of the alcohol ( bit of yellow in the flame), heat retained by the equipment so it does not reach the water.

Question 18

How can the errors be overcome?

Answer 18

How can the errors be overcome?

Use a flame calorimeter where oxygen is supplied to the flame, use draft shielding around the spirit burner.

Question 19

If we want to measure the enthalpy change in a solution we use polystyrene cups – why?

Answer 19

Polystyrene cups prevent heat loss to the surroundings from exothermic reactions. Polystyrene cups prevent heat gain from the surroundings from endothermic reactions.

Question 20

Define Hess’s law.

Answer 20

The energy change accompanying a chemical reaction is independent of the route taken as long as the starting and final conditions are the same.

Question 21

How do you find an accurate temperature change for an reaction?

Answer 21

· Take the temperature of water, record it and start a stop watch.

· Take the temperature for a further three minutes recording the temperature every minute.

· On the fourth minute add the solid and stir continually. Do not take the temperature at this time.

· Now take the temperature on the fifth minute and every minute for 15 minutes.

· Plot a graph of the temperatures at the different times and then extrapolate the line back to the fourth minute.

· Find the temperature rise by drawing a vertical line from the line of best fit for the starting temperatures up to the line extended to the fourth minute.

Question 22

How can you find the enthalpy change for one mole of anhydrous copper sulphate being converted into one mole of hydrate copper sulphate?

Answer 22

· Measure out 0.025 moles of anhydrous copper sulphate.

· Take the temperature of 30cm3 of water and start the stop watch. Now take the temperature every minute for 3 minutes.

· On the fourth minute add the anhydrous copper sulphate that has been weighed out and stir continuously. Take the temperature on minute 5 and then every minute for a total of 15 minutes. Plot the graph and extrapolate the lines of best fit to find the temperature rise.

· Repeat for 0.025 moles of hydrated copper sulphate using 27.75cm3 of water instead of 30cm3 as there is already water in the hydrated copper sulphate and this has to be controlled to make this a fair test.

· Find the energy transferred to water for both and then use this energy cycle to find the required enthalpy change.

Question 23

We can use Hess’s law and enthalpy of formation values to find the enthalpy change of a reaction – how?

Answer 23

· Write the equation you are calculating the energy for across the top of the cycle.

· Write the elements at the bottom of the cycle.

· Put arrows going upwards from the elements to the reactants and products.

Question 24

We can use Hess’s law and enthalpy of combustion to find enthalpy changes – how?

Answer 24

· Write the equation you are calculating the energy for across the top of the cycle.

· Write the combustion products at the bottom of the cycle.

· Put arrows going downwards from the reactants and products to the combustion products.

Question 25

What is the enthalpy value for elements under standard conditions?

Answer 25

Zero

Question 26

Define the bond dissociation energy.

Answer 26

This is the enthalpy change required to break a covalent bond with all species in the gaseous state.

Question 27

Define mean bond energy.

Answer 27

The Mean bond enthalpy is the enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules.

Question 28

What is the energy change for bond breaking?

Answer 28

Endothermic +

Question 29

What is the energy for bond making?

Answer 29

Exothermic -

Question 30

How do you calculate the overall energy change using bond energies?

Answer 30

· Calculate the energy required to break bonds – this has a + sign as it is endothermic.

· Calculate the energy released when bonds are made – this has a – sign as it is exothermic.

· Add the energy released to the energy required.