1. Energetics

Question 1

How can a reaction be endothermic? How can a reaction be exothermic?

Answer 1

(a) If more energy is put into breaking bonds than given out when new bonds are formed
(b) If more energy is given out when new bonds are formed than used to break bonds

Question 2

Name some examples of exothermic reactions.

Answer 2

• Burning fuels
• Neutralisation

Question 3

Name some examples of endothermic reactions.

Answer 3

• Thermal decomposition
• Heating blue copper sulfate (CuSO4.5H20): The bonds attaching water molecules to the copper sulfate must be broken to produce white anhydrous copper sulfate

Question 4

If a reaction is exothermic in one direction, what will it be in the other direction?

Answer 4

Endothermic

Question 5

What is heat energy measured in?

Answer 5

KJ/mol

Question 6

What is a fuel’s energy density?

Answer 6

Energ y given out per gram. Iit is ver important in considering its efficiency

Question 7

Why is hyrogen difficult to use as a fuel?

Answer 7

Although it has a very high energy density when burnt, it is difficult to store and take up a lot of space because it is a gas

Question 8

What is elthalpy change at standard state and how is it shown?

Answer 8

ΔH°₂₉₈- standard conditions are 298K and 100kPa of pressure
Temperature and pressure can both affect elthalpy change so it is important to state the conditions under which an experiment takes place

Question 9

How can pressure affect elthalpy change?

Answer 9

Some energy is needed to overcome the atmospheric pressure

Question 10

Why should state symbols always be used when considering enthalpy changes?

Answer 10

It requires energy to turn a liquid into a gas

Question 11

What is the standard enthalpy of formation (ΔH°_f)?

Answer 11

The enthalpy change when one mole of a sustance is formed from its elements under standard conditions and will all products and reactants in their standard states

Question 12

What is the standard enthalpy of combustion (ΔH°_c)?

Answer 12

The enthalpy chnge when oe le of a substance is burned completely in oxygen under standard conditions and with all products and reactants in their standard states

Question 13

What is temperature?

Answer 13

Tthe average kinetic energy of particles in a system. This is measured with a thermometer

Question 14

What is heat?

Answer 14

The total energy of all particles. This depends on how many particles there are, unlike temperature. Heat always flows from high –> low temperature

Question 15

What is the equation used to measure heat energy change?

Answer 15

Q=mcΔt

m = g
c = J/g/K
t = K

Question 16

What is the equation used to measure enthalpy change?

Answer 16

ΔH = Q/mols

Question 17

How could we improve a simple calorimetry experiment?

Answer 17

• Add a draught screen
• Add a lid
• Add an insulator

Question 18

Look over different types of calorimeters in your textbook.

Answer 18

Done!

Question 19

Why can polystyrene cups often be used in calorimetry experiments?

Answer 19

They have a low SHC and are good insulators

Question 20

Read notes and see diagram on ‘allowing for heat loss’.

Answer 20

Done!

Question 21

What does Hess’ Law state?

Answer 21

The elthalpy change for a chemical reaction is the same regardles of the route taken from reactants to products. We can show this in a thermochemical cycle.

Do practice with Hess’ Law!

Question 22

What is the equation to find out the enthalpy of formation?

Answer 22

ΔH_f = ΔH_p- ΔH_r

Question 23

What is the equation to find out the elthalpy of combustion?

Answer 23

ΔH_c = ΔH_r - ΔH_p