Energetics 3.1.4

Question 1

What is Hess’ law?

Answer 1

The enthalpy change for a chemical reaction is independent of the route taken

Question 2

What directions do the arrows point in for enthalpy of combustion?

Answer 2

Down

Question 3

What directions do the arrows point in for enthalpy of formation ?

Answer 3

Up

Question 4

What is the enthalpy change of a reaction definition

Answer 4

The heat change in a reaction at constant pressure

Question 5

What is an endothermic reaction

Answer 5

Reactions that absorb energy from the surroundings

Question 6

What does the energy profile of endothermic reactions look like

Answer 6

Products are higher in energy than the reactants
Delta H is positive

Question 7

What is an exothermic reaction

Answer 7

Reactions that release energy to the surroundings

Question 8

What does the energy profile of an exothermic reaction look like

Answer 8

Products are lower in energy than reactants .
Delta H is negative

Question 9

What type of process / reaction involves breaking a bond

Answer 9

To break a bond energy needs to be absorbed
Bonds are broken in the reactants and this is an endothermic process so delta H is positive

Question 10

What process / reaction is needed to make bonds

Answer 10

When bonds are formed energy is released
Bonds are made when products are being produced and this is an exothermic process so delta H is negative

Question 11

what is the equation needed to work out enthalpy change in mean bond enthalpy?

Answer 11

Enthalpy change = Total energy to break bonds - Total energy released forming bonds

Question 12

What is the standard enthalpy change of formation ?

Answer 12

Enthalpy change when 1 mole of the compound is formed from its elements under standard conditions , all reactants and products being in their standard states

Question 13

What is the standard enthalpy change of combustion ?

Answer 13

The enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions , all reactants and products being in their standard states .

Question 14

What are the standard conditions ?

Answer 14

100kPa (pressure)
298k ( room temp)
Solutions at 1mol dm^-3

Question 15

What is the equation to measure the enthalpy change for a reaction experimentally ?

Answer 15

q=mc deltaT

Question 16

Why do we use a polystyrene cup?

Answer 16

For insulation and support

Question 17

Why do we stir the mixture

Answer 17

ensures that all of the solution is at the same temperature

Question 18

What is the mean bond enthalpy

Answer 18

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

Question 19

What are some errors that might occur when measuring enthalpy change

Answer 19

Incomplete combustion
Heat loss

Question 20

How to improve the method and analysis of the result in finding enthalpy change experimentally (6)
No reference to precision of measuring equipment

Answer 20

•Use polystyrene cup or lid to reduce heat loss
• Record temperature values at regular intervals to plot the temperature result against time on a graph
• Extrapolate the cooling back to the point of addition to establish a temperature change

Question 21

EQ : State one condition necessary for enthalpies of formation to be quoted as standard values at a specified temperature of 298k (1)

Answer 21

100KPa