1.04 Energetics

Question 1

What energy change is breaking bonds associated with ?

Answer 1

Endothermic reactions - as energy is taken in to break the bonds.

Question 2

What energy change is forming bonds associated with ?

Answer 2

Exothermic reaction - energy is released to form bonds.

Question 3

What are some uses of thermochemistry ?

Answer 3

•Measuring and comparing the energy values of fuels
•Calculating the energy requirements of industrial processes
•Working out the theoretical amount of energy taken / released by a reaction
•Predicting if a reaction will take place or not.

Question 4

What is an endothermic reaction ?

Answer 4

•A reaction where the overall enthalpy change is positive (+dH)
•Energy in breaking bonds > energy out of making bonds.

Question 5

What is an exothermic reaction ?

Answer 5

•One where the overall enthalpy change is negative (dH-)
•Energy in breaking bonds > energy out of making bonds.

Question 6

If a reversible reaction is exothermic one way, what is it the other way ?

Answer 6

Endothermic.

Question 7

Give two examples of exothermic reactions.

Answer 7

•Combustion of fuels
•Neutralisation.

Question 8

Give an example of an endothermic reaction.

Answer 8

Thermal decomposition.

Question 9

Define enthalpy change, what symbol is used to represent it ?

Answer 9

•It is the energy change of a system at a constant pressure
•It is represented by dH.

Question 10

What are the standard conditions ?

Answer 10

•100kPa / 1atm
•298K or 25•C

Question 11

What does ‘in standard state’ mean ?

Answer 11

The state an element/compound exists at in standard conditions (298k, 100kPa).

Question 12

Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction.

Question 13

Define standard enthalpy of formation.

Answer 13

The enthalpy change when one mole of a compound is formed by its constituent elements in standard conditions (100kPa, 298K), with reactants and products all in their standard states.

Question 14

Give an example of an equation which represents the enthalpy of formation.

Answer 14

H2 (g) + 1/2O2 (g) -> H2O (l)

Question 15

Define standard enthalpy of combustion.

Answer 15

The enthalpy change when one mole of a substance is burnt completely in oxygen all in standard conditions (100kPa, 298K), with reactants and products all in their standard states.

Question 16

Give an example of an equations which represents the enthalpy of combustion.

Answer 16

C (s) + O2 (g) -> CO2 (g)

Question 17

What is the difference between heat and temperature ?

Answer 17

•Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance
•Temperature is related to the mean kinetic energy of all particles in a system, so it is independent of the amount of substance.

Question 18

How do you calculate heat change from experimental data ?

Answer 18

•Use the equation Q=mcΔT
•m = mass of substance that is being heated (typically water)
•c = specific heat capacity of the heated substance (water)
•dT is the change of temperature.

Question 19

Describe a simple calorimeter.

Answer 19

•A spirit burner containing the substance to be tested should be placed below a tripod
•Atop of a mesh on a tripod, a beaker containing water should be placed with a thermometer
•You can use the change in temperature of water and mass of substance in spirit burner to calculate enthalpy change.

Question 20

How can calorimeters be made more accurate ?

Answer 20

•Insulate the water beaker as much as possible from all angles but below, eg add a lid and mineral mesh.

Question 21

Flame

Answer 21

Mark as 5

Question 22

How would you measure enthalpy change for a reaction occurring in (aq) ?

Answer 22

•Use a polystyrene cup as a beaker (good insulator minimises heat loss)
•Heat is generated in the solution during the reaction; calculate the temperature change
•Take heat capacity to be waters and density to be 1g/cm3.

Question 23

What can you use to make experimental determination of enthalpy change of reactions more accurate ?

Answer 23

Cooling curves

Question 24

What is Hess’ Law ?

Answer 24

Hess’ Law states that the enthalpy change for a reaction is the same regardless of the route taken.

Question 25

What is the enthalpy of an element ?

Answer 25

The enthalpy of all elements in their standard states is 0.

Question 26

Define bond dissociation enthalpy.

Answer 26

•The enthalpy change required to break a covalent bond
•All species must be in the gaseous state
•The value differs for the bond in each compound.

Question 27

Define mean bond enthalpy.

Answer 27

The average value (across different chemical compounds) for the bond dissociation enthalpy of a given covalent bond.

Question 28

Why may experimental methods for enthalpy determination not be accurate ?

Answer 28

•Heat can be lost to surroundings
•May not be in standard conditions
•Reaction may not go to completion.

Question 29

Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion / formation ?

Answer 29

•Bond enthalpies are an average for the same bond across different molecules
•Standard enthalpy of formation / combustion apply only to that molecule, therefore are more accurate.