M3-Enthalpy Changes

Question 1

Axis on energy profile diagram

Answer 1

X- reaction pathway
Y- enthalpy

Question 2

Exothermic enthalpy profile

Answer 2

Product line below Reactants
Delta H = negative
Surrounding temperature increases

Question 3

Endothermic enthalpy profile

Answer 3

Reactants line below products line
Delta H = positive
Surrounding temperature decreases

Question 4

Activation energy

Answer 4

The minimum energy required to start a reaction by breaking bonds

Question 5

Standard conditions

Answer 5

298k (25°c)
100kPa (1 atmosphere)
1moldm-3 for concentrations of solutions

Question 6

Standard state

Answer 6

A state a substance is under standard conditions

Question 7

Delta r H (Reaction)

Answer 7

The enthalpy change associated with a stated equation
(In there standard stated under standard conditions)

Question 8

Delta f H (formation)

Answer 8

The enthalpy change when one mole of a compound is formed from its elements
(In there standard stated under standard conditions)

Question 9

Delta c H (combustion)

Answer 9

The enthalpy change when one mole of a substance is completely combusted with oxygen (In there standard stated under standard conditions)

Question 10

Delta neut H (neutralisation)

Answer 10

The enthalpy change when an aqueous acid is neutralised by an aqueous base forming one mole of water (In there standard stated under standard conditions)

Question 11

Calculating enthalpy changes from experimental data

Answer 11

Q=-mcDetlaT
Q- heat energy is joules
m- mass of liquid being heated
c- 4.18
Detla T- change in temperature
Calculate moles of reactant
Delta H= Q(Kj)/n(mol)

Question 12

Experimental values less than those determined theoretically

Answer 12

Heat loss
Incomplete combustion
Non-standard conditions

Question 13

Where does heat loss occur and how can it be minimised

Answer 13

Occur- Through the open top of the container
Through the material of the container
Minimised- Add a lid
Use an insulating material

Question 14

Water- mass conversion

Answer 14

1cm3 =1g

Question 15

Enthalpy change of neutralisation

Answer 15

Calculate n of acid and alkali, if they match, use either to find n of water
If they are different, use the limiting factor

Question 16

What value approximately should the enthalpy change of neutralisation be

Answer 16

-50Kjmol-1

Question 17

Average bond enthalpy

Answer 17

The average enthalpy when breaking 1 mole of a bond in the molecules of a gaseous species

Question 18

Ms answers to why a reaction is endothermic (in terms of bond making and breaking)

Answer 18

More energy required to break bonds than released when bonds are made

Question 19

Ms answer to why a reaction is exothermic (in terms of bond making and breaking)

Answer 19

More energy is released when bonds are made in the products than was required to break the bonds in the reactants

Question 20

Rule for average bond enthalpy

Answer 20

ALWAYS +

Question 21

Calculate delta H using average bond enthalpies

Answer 21

Delta H= total of bonds broken - total of bonds made
(Reactants) (Products)

Question 22

Hess’ law

Answer 22

The enthalpy change of a reaction is independent of the rout taken, providing the initial and final conditions are the same

Question 23

Hess’ law acronym for different data

Answer 23

When combustion data given- CD (combustion down)
When formation data given- FU (formation up)

Question 24

Remember for Hess’ law calculations

Answer 24

Correct direction of arrows- Remember CD FU
Use molar ratios, make sure to x enthalpies by mole ratio as they are the Delta H per mole