Topic 8 Energetics

Question 1

define enthalpy change

Answer 1

heat change from a reaction under constant pressure

Question 2

define standard enthalpy change

Answer 2

enthalpy change of a reaction under 100kPa and 298K

Question 3

define standard enthalpy change of reaction

Answer 3

enthalpy change of materials reacting under standard conditions, with everything in standard state

Question 4

define standard enthalpy change of formation (3 marks)

Answer 4

enthalpy change when 1 mole of compound (1) is formed from its elements (1) under standard conditions (1atm, 298K) , everything in standard state (1)

Question 5

define standard enthalpy change of combustion

Answer 5

enthalpy change when 1 mole of compound is burned completely under oxygen in standard conditions and state

Question 6

define standard enthalpy change of neutralisation (2 marks)

Answer 6

enthalpy change of 1 mole of water is formed when an acid and alkali are mixed together

Question 7

What is the definition of bond enthalpy?

Answer 7

Energy required to break the one mole of the exact same bond in gaseous state

Question 8

What is the definition of mean bond enthalpy? 2 marks

Answer 8

Enthalpy change to break one mole of the same type of bond in different molecules (1) in gaseous state (1)

Question 9

What does Hess’s law state?

Answer 9

Enthalpy change of a reaction is independent to the route the reaction takes

Question 10

Q =

Answer 10

mcΔT

Question 11

what is the unit for enthalpy?

Answer 11

kJ mol ^-1

Question 12

What is the unit of enthalpy change and how to calculate?

Answer 12

+/-
mc ΔT / moles
kJ mol -1

Find Mc delta T for the solution that had change in temp
Find Mole for solid

Question 13

Use bond breaking and forming to explain why combustion of ethanol gives out energy

Answer 13

Energy taken in to break bonds (endo)
Energy released from forming bonds (exo)
More energy is used forming than breaking bonds

Question 14

ΔHr (using mean bond enthalpies)=

Answer 14

Σ Bonds broken - Σ bonds formed

Question 15

How is sodium hydrogencarbonate used in cooking?

Answer 15

Baking soda
Carbon dioxide released causes breads to rise

Question 16

Why are bond enthalpy values always positive?

Answer 16

Bond breaking requires energy, and bond breaking is endothermic

Question 17

What assumptions are made when doing Hess calculations?

Answer 17

• no heat is lost
• reactions went to completion

Question 18

What makes propanol very good fuels? 2 marks

Answer 18

• ignites easily
• releases thermal energy when it burns

Question 19

How to calculate bond enthalpies?

Answer 19

ΔH = Σ bond energies broken - Σ bond energies made

Question 20

What effect does using a copper beaker instead of glass would affect the enthalpy change in an exothermic reaction?

Answer 20

Glass is a better heat insulator
Reaction will be more negative since it’s more exothermic as more heat is conducted to water
So more accurate change in temp