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 —> exothermic 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 in a reaction

Predicting if a reaction will take place or not

Question 4

What is an endothermic reaction?

Answer 4

One with an overall positive enthalpy change —> energy in breaking bonds> energy out making bonds

Question 5

What is an exothermic reaction?

Answer 5

One with an overall negative enthalpy change —> energy in breaking bonds < energy out making bonds

Question 6

If a reversible reaction is endothermic one way, what type of the reaction is the other way?

Answer 6

Exothermic

Question 7

Give 2 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

Heat energy change of a system at a constant pressure

Question 10

What are the standard conditions?

Answer 10

100kPa/ 1atm pressure

289K / 25 degree Celsius temperture

Question 11

What does in standard state mean?

Answer 11

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

Question 12

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

Answer 12

Slide 26

Question 13

Define standard enthalpy of formation

Answer 13

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

Question 14

Give an example of an equation which represents standard enthalpy of formation

Answer 14

There are many 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 in standard conditions (100kPa,298K) with reactants and products in their standard states

Question 16

Give an example of an equation which represents standard 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 ammount 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 18

How can you calculate enthalpy change from experimental data?

Answer 18

Use the equation Q=MCΔT

M is mass of substance being heated
C is specific heat capacity of that substance being heated
ΔT change in temperature

Question 19

Draw a simple calorimeter

Answer 19

Slide 40

Question 20

How could this calorimeter be made more accurate?

Answer 20

Add draught screens at the sides, add a lid on the top of the beaker, add mineral wool around the beaker —> all to insulate and reduce heat lost to the surroundings

Question 21

What is a flame calorimeter

How does it differ to a simple calorimeter?

Answer 21

Reduces heat lost to the surrounding to give more accurate results:has spiral chimney made of copper, enclosed flame and burnt in pure oxygen not air

Question 22

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

Answer 22

Use an expanded polystyrene cup as a calorimeter (good insulator therefore reduces heat loss)
Heat is generated in the solution; measure this temperature change. Take heat capacity I’d solution to be 4.18 and density of solution = 1gcm^-3

Question 23

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

Answer 23

Cooling curves

Question 24

What is hess’s law?

Answer 24

Enthalpy change in a reaction is independent of the route taken.

Question 25

What is the enthalpy of an element?

Answer 25

The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0

Question 26

Define bond dissociation enthalpy

Answer 26

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 27

Define mean bond enthalpy

Answer 27

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

Question 28

Why may experimental methods for enthalpy determination not be very accurate?

Answer 28

Heat is lost to the surroundings Not 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 a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply to that molecule therefore it is more accurate