Section 4 - Pgs 59-63

Question 1

Does breaking bonds require energy to be supplied or is energy released?

Answer 1

Energy has to be supplied to break bonds. (ENDOTHERMIC)

Question 2

Does forming bonds require energy to be supplied or is energy released?

Answer 2

Energy is released when new bonds are formed. (EXOTHERMIC)

Question 3

What is an exothermic reaction?

Answer 3

One which gives out energy to the surroundings, usually in the form of heat.

Question 4

How can an exothermic reaction be detected?

Answer 4

By a rise in temperature (of the surroundings).

Question 5

What causes an exothermic reaction to be exothermic?

Answer 5

The energy released in bond formation is greater than the energy required to break the original bonds.

Question 6

What is an endothermic reaction?

Answer 6

One which takes in energy from the surroundings, usually in the form of heat.

Question 7

How can an endothermic reaction be detected?

Answer 7

By a fall in temperature (of the surroundings).

Question 8

What causes an endothermic reaction to be endothermic?

Answer 8

The energy required to break the original bonds is greater than the energy released in bond formation.

Question 9

What is enthalpy change?

Answer 9

The overall change in energy in a reaction (per mole).

Question 10

What is the symbol for enthalpy change?

Answer 10

ΔH

Question 11

What is the unit for enthalpy change (ΔH)?

Answer 11

kJ/mol (Kilojoules per mole of the reactant)

Question 12

When is enthalpy change (ΔH) positive and why?

Answer 12

When the reaction is endothermic - because the reaction takes IN energy.

Question 13

When is enthalpy change (ΔH) negative and why?

Answer 13

When the reaction is exothermic - because the reaction is giving energy OUT.

Question 14

What is an energy level diagram?

Answer 14

A diagram used to show how the energy in a reaction changes as the reaction progresses.

Question 15

What is on the x and y axis of an energy level diagram?

Answer 15

x axis - Progress of reaction

y axis - Energy

Question 16

What does the height from the reactants to the top of the curve represent on an energy level diagram?

Answer 16

The activation energy.

Question 17

What is activation energy?

Answer 17

The minimum energy required to break the original bonds in the reactants (i.e. How much energy in needed for the reaction to happen).

Question 18

In an exothermic energy level diagram, are the products higher or lower than the reactants and why?

Answer 18

Lower - because energy is given out from the reaction, reducing how much energy is in the reaction.

(Diagram pg 60)

Question 19

In an endothermic energy level diagram, are the products higher or lower than the reactants and why?

Answer 19

Higher - because energy is taken in by the reaction, increasing how much energy is in the reaction.

(Diagram pg 60)

Question 20

What does the height from the reactants to the products represent on an energy level diagram?

Answer 20

The enthalpy change.

Question 21

What can lower the activation energy of a reaction?

Answer 21

A catalyst.

Question 22

How does a catalyst work?

Answer 22

It provides an alternative pathway for the reaction with a lower activation energy.

Question 23

How is a catalyst represented on an energy level diagram?

Answer 23

A lower curve which reaches the products line earlier.

Question 24

Does a catalyst change the overall energy change for a reaction?

Answer 24

No, it remains the same.

Question 25

What is a catalyst?

Answer 25

A substance that can speed up a reaction (without being used up itself) by providing an alternative route to the product which has a lower activation energy.

Question 26

What is a bond energy?

Answer 26

• The amount of energy that is required to break a certain bond or is released by its formation.
• It is specific to each bond (e.g. H-H has a specific bond energy.

Question 27

Are bond energies for a specific bond always the same?

Answer 27

No, they can vary depending on the compound they are in. (In the exam you are provided with an average though).

Question 28

What are bond energies used for?

Answer 28

Calculating enthalpy change of a reaction.

Question 29

What is the equation for calculating enthalpy change from bond energies?

Answer 29

Enthalpy Change = Total Energy to Break Bonds - Total Energy Released in Making Bonds

Question 30

Calculate the enthalpy change for:
H2 + Cl2 -> 2HCl
Bond energies:
H-H: +436kJ/mol
Cl-Cl: +242kJ/mol
H-Cl: +431kJ/mol

Answer 30

1) Breaking 1 mole of H-H and 1 mole of Cl-Cl bonds requires:
436 + 242 = 678kJ

2) Forming 2 moles of H-Cl bonds releases:
2 x 431 = 862kJ

3) ΔH = 678 - 862 = -184kJ/mol
4) The ΔH is negative so the reaction much be exothermic.

Question 31

What is calorimetry?

Answer 31

The determining of energy change in a reaction by measuring heat exchanges with with surroundings.

Question 32

How is calorimetry carried out in dissolving, displacement and neutralisation reactions?

Answer 32

The reagents should be in solution.

1) The temperature of the reagents is measured (making sure it is the same).
2) The reagents are mixed in a polystyrene cup.
3) Heat loss can be reduced by putting the cup in a beaker of cotton wool and putting a lid on the cup.
3) The temperature of the products is measured after the reaction (when the temperature reaches its highest or lowest).
4) Temperature change is calculated and, from this, enthalpy change is calculated.

Question 33

In calorimetry, how do you know when the reaction has finished?

Answer 33

You measure the highest (exothermic) or lowest (endothermic) temperature it reaches.

Question 34

How is calorimetry carried out in combustion reactions?

Answer 34

1) A set amount of water is put into a copper can and its temperature is recorded.
2) The spirit burner and lid is weighed.
3) The spirit burner is put under the copper can and lit.
4) Draughts should be reduced using a screen to maximise energy transfer.
5) The water is heated, stirring, until the temperature reaches an approximate set temperature (e.g. 50*C).
6) The flame is put out using the lid and the precise temperature of the water is measured.
7) The spirit burner and lid is weighed again.
8) Enthalpy change is calculated.

Question 35

What is the equation for energy change (used in enthalpy calculations)?

Answer 35

Energy Change = Mass of Solution x Specific Heat Capacity (4.18J/g/*C) x Change in Temperature

Q = m x c x ΔT

Question 36

When calculating energy change, what mass is used (as ‘m’)?

Answer 36

• If a solid is dissolved in another or if some water is heated by a fuel (i.e. when there is only one liquid involved), the mass of the solution is used.
• If two solutions are mixed, it is the sum of their two masses.
  (i. e. Just use the heat of the thing that is being heated up)

Question 37

How can you calculate the molar enthalpy change of a reaction?

Answer 37

1) Find Q (energy change) using the mass of the solution (where 1cm3 = 1g), the specific heat capacity (4.18J/g/*C) and the change in temperature of the solution. (Q = m x c x ΔT)
2) This gives you the overall energy change for the reaction.
3) Work out the moles of the fuel used or of one of the reactants (if it is a dissolving, displacement or neutralisation reaction) depending on what the question asks.
4) For solids: Moles = Mass / Mr
For solutions: Moles = (Volume / 1000) x Concentration
5) Find ΔH by dividing the energy change by the moles. (ΔH = Q / moles)
6) Remember to change ΔH to a negative value if the reaction is exothermic!

Question 38

What is the value for specific heat capacity of water?

Answer 38

4.18*C

Question 39

What is specific heat capacity?

Answer 39

The amount of energy required to heat 1g of a certain solution up by 1*C.