T8: Energetics I

Question 1

1/2

State possible practicals that may be carried out

In relation to the topic

Answer 1

Calorimetry experiments:
* Displacement
* Neutralisation

Objective: Investigating the enthalpy of combustion of a homologous series of
alcohols.

Question 2

Give a definition and state the units

What is enthalpy change?

Answer 2

Heat energy change in a reaction measured at constant
pressure

* The units of enthalpy change are kJ mol-1
* Written as “delta H”

Question 3

What are the usual standard conditions?

Answer 3

• Pressure - 100kPa
• Temperature - 298K
• Concentration - 1mol/dm-3

* 298K = 25°C

Question 4

Give a decription

What is an endothermic reaction

Answer 4

• Reactions which absorb heat energy
• ΔH is positive
• The temperature often falls

e.g. Thermal decomposition of calcium carbonate

Question 5

Give a description

What is an exothermic reaction?

Answer 5

• Reactions which gives out heat energy.
• ΔH is negative
• The temperature often goes up

e.g. Combustion of a fuel like methane

Question 6

What is the difference between a reaction profile and an enthalpy level diagram?

Answer 6

• Reaction profiles: show enthalpy changes during a reaction
• Enthalpy level diagrams: show the overall change of a reaction

Activation energy is not shown in enthalpy level diagrams but it is shown in
reaction profile diagrams.

Question 7

Describe an enthalpy level diagram for an exothermic reaction

Include references to stability

Answer 7

In an exothermic reaction, the reactants **release **energy to the sorroundings, so the products have less enthalpy than the reactants

The less enthalpy a substance has, the more stable it is

Question 8

Describe an enthalpy level diagram for an endothermic reaction

Include references to stability

Answer 8

In an endothermic reaction, the reactants take in energy from the sorroundings, so the products have more enthalpy than the reactants

The more enthalpy a subtance, the more unstable it is

Question 9

What is the** standard enthalpy change of reaction**?

Answer 9

The enthalpy change when the reaction occurs in molar quantities shown in the chemical equation (balanced), under standard conditions, wth all species in their standard states

Question 10

What is the standard enthalpy change of formation?

Answer 10

Th enetghalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions

Question 11

What is the** standard enthalpy change of combustion**?

Answer 11

The enthalpy change when 1 mole of a substance is completely burned in oxygen, under standard conditions

Question 12

What is the** standard enthalpy change of neutralisation**?

Answer 12

Th enthalpy change when an acid and an** alkali** react together, under standard conditions, to form 1 mole of water

Question 13

What do you need to work out the enthalpy change for a reaction?

Answer 13

• the number of moles of the stuff that’s reacting
• the change in temperature of the reaction

Heat change is the same as enthalpy change if pressure is constant

Heat energy is the energy required to change the temp. of a substance

Question 14

What is the equation for energy transferred?

Answer 14

energy transferred = mass x specific heat capacity × temperature change
(Q=mcΔT)

q = heat energy (J)
m = mass (g)
c = SHC (JgK)
ΔT = change in temp (K)

Question 15

What is the specific heat capacity of water?

Answer 15

4.186 J/g°C

SHC: the amount of energy to raise the temp. of 1g of a substance by 1°C

Question 16

2 points

What is the problem with measuring the temp. change of some reaction mixtures directly?

Answer 16

• Some heat will be lost to the sorroundings (or gained if the reaction is endothermic)
• The temp. change measured will be less than the actual temp. change of the reaction

Question 17

How can you account for this problem?

In reference to the reaction mixture practical

Answer 17

You should carry out the reaction in an insulated container e.g. a polystyrene beaker, so that you don’t lose or gain much heat through the sides

You shoul also use a lid to heat loss/gain through the top

Question 18

There are 8 points

Give the generalised method used to find the enthalpy chnage of an endothermic reaction between XY reactants