Energetics

Question 1

What is an exothermic reaction?

Answer 1

Release heat to surroundings.
Enthalpy of products less than enthalpy of reactants.

Question 2

What are some examples of exothermic reactions?

Answer 2

Combustion
Respiration

Question 3

What is an endothermic reaction?

Answer 3

Take in heat from surroundings.
Enthalpy of products greater than Enthalpy of reactants.

Question 4

What are some examples of endothermic reactions?

Answer 4

Decomposition
Photosynthesis

Question 5

What is the equation for the overall energy change, ∆H, of a reaction

Answer 5

∆H = bonds broken - bonds formed

Question 6

What is average bond enthalpy?

Answer 6

The energy needed to break one mole of a particular type of bond averaged over many compounds.

Question 7

What is the standard enthalpy of formation?

Answer 7

The enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Question 8

What is the enthalpy of formation of an element in its standard state?

Answer 8

Zero

Question 9

What is the standard enthalpy of combustion?

Answer 9

The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.

Question 10

What are enthalpies of combustion always?

Answer 10

Exothermic reactions.

Question 11

What products are always formed from the standard enthalpy of combustion of organic compounds?

Answer 11

Carbon dioxide
Water

Question 12

What is the conservation of energy?

Answer 12

Energy cannot be created or destroyed but it can change from one form to another.

Question 13

What is Hess’s Law?

Answer 13

The enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.

Question 14

What are the steps for using Hess’s Law in a standard enthalpy of combustion calculation?

Answer 14

1. Write equation for formation of compound.
2. Construct energy cycle using enthalpies of combustion.
3. Apply Hess’s Law to cycle.

Question 15

What is the procedure for calculating the enthalpy of combustion of a reaction?

Answer 15

1. Using measuring cylinder, measure 25cm^3 of water and transfer into boiling tube. Clamp at angle over spirit burner.
2. Measure temp of water using thermometer.
3. Weigh spirit burner containing alcohol.
4. Place burner under boiling tube and light fuel.
5. Heat water until temp rise by 20oC.
6. Stir and measure final temp of water.
7. Reweigh spirit burner.
8. Repeat steps 1-7.
9. Take average.

Question 16

What equation can be used to calculate the heat energy change released in a reaction?

Answer 16

q = m c ∆T

Question 17

What equation can be used to calculate the enthalpy change of an enthalpy of combustion reaction?

Answer 17

∆H = -q / n

Question 18

How can accuracy be improved when calculating the enthalpy of combustion of a reaction during a practical?

Answer 18

Insulting beaker of liquid.
Shielding flame from droughts.
Stir liquid so heat distributed evenly throughout it.

Question 19

Why would the experimental value for the enthalpy change be lower than the data book value when calculating the enthalpy of combustion of a reaction during a practical?

Answer 19

Energy lost due to surrounding so temp increase lower than under ideal conditions.
May be incomplete combustion.
May not be under standard conditions.

Question 20

What is the standard enthalpy of neutralisation?

Answer 20

Enthalpy change when one mole of water is produced by neutralisation reaction under standard conditions.

Question 21

What is the procedure for calculating the enthalpy of neutralisation of a reaction?

Answer 21

1. Use measuring cylinder to measure 50cm^3 of base and transfer to polyester cup.
2. Record temperature.
3. Measure 25cm^3 of acid and add to base.
4. Stir and record highest temp reached.
5. Use q = m c ΔT.
6. Use q and moles of water to calculate ΔH.