1.4 Energetics

Question 1

What energy change is breaking bonds associated with

Answer 1

Energy is taken in to break bonds —> endothermic reaction

Question 2

What energy change is making bonds associated with

Answer 2

Energy is released to make bonds —> endothermic reaction

Question 3

What are some uses of thermochemistry

Answer 3

Measuring and comparing the energy values of fuels
Calculating the energy requirements for industrial processes
Working out the theoretical amount of energy released / taken in during a reaction

Question 4

What is an endothermic reaction

Answer 4

One with an overall positive enthalpy change (+^H) —> energy in breaking bonds > energy out making bonds

Question 5

What is an exothermic reaction

Answer 5

One with an overall negative enthalpy change (-^H) —> energy in breaking bonds < energy out making bonds

Question 6

If a reversible reaction is endothermic one way, which is it the other way

Answer 6

Exothermic

Question 7

Give 2 examples of exothermic reactions

Answer 7

Burning fuels
Neutralisation reactions

Question 8

Give an example of an endothermic reaction

Answer 8

Thermal decomposition

Question 9

Define enthalpy change
What signal is used to represent it

Answer 9

Energy change of a system at a constant pressure
Represented by delta H

Question 10

What are the standard conditions

Answer 10

100k Pa
298K

Question 11

What does ‘in standard state’ mean

Answer 11

The state that an element is in at standard conditions

Question 12

Define standard enthalpy of formation

Answer 12

The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions with all elements in their standard states

Question 13

Give an example of an equation which shows the standard enthalpy of formation

Answer 13

There are many e.g. H2 (g) + 1/2 O2 (g) —> H2O (l)

Question 14

Define standard enthalpy of combustion

Answer 14

The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions with reactants and products in their standard states

Question 15

Give an example of an equation which represents the standard enthalpy of combustion

Answer 15

C (s) + O2 (g) —> CO2 (g)

Question 16

What is the difference between heat and temperature

Answer 16

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

Question 17

How can you calculate enthalpy change from experimental data

Answer 17

Use the equation Q = mc^T
Where m is the mass of substance (usually water) , c is the specific heat capacity (waters SHC = 4.18 g/J/K) and ^T is the change in temperature

Question 18

What is a flame calorimeter and does it differ to a simple calorimeter

Answer 18

Reduces heat loss to the surroundings to give more accurate results : has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen not air

Question 19

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

Answer 19

Use an expanded polystyrene cup as a calorimeter (good insulator —> reduce heat loss)
Heat is generated in the solution; measure this temperature change
Take heat capacity of solution to be 4.18 and density of solution to be 1 g/cm3

Question 20

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

Answer 20

Cooling curves

Question 21

What is Hess’s Law

Answer 21

States that the enthalpy change of a reaction is the same regardless of the route taken

Question 22

What is the enthalpy of an element

Answer 22

The enthalpy of all elements when in their standard states is defined as 0

Question 23

Define bond dissociation enthalpy

Answer 23

The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules

Question 24

Define mean bond enthalpy

Answer 24

Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond

Question 25

Why may experimental values for enthalpy determination not be very accurate

Answer 25

Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion

Question 26

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

Answer 26

Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate