Energetics

Question 1

Exothermic Processes

Answer 1

Energy is released from the chemical to the surroundings during the process
The temperature of the surroundings increases
ΔH is negative

Question 2

Endothermic Processes

Answer 2

Energy is taken in by the chemical from the surroundings during the process
The temperature of the surroundings decreases
ΔH is positive

Question 3

Enthalpy Changes definiton

Answer 3

heat energy change at constant pressure (kJ mol-1)

Question 4

Standard Conditions

Answer 4

• 100 kPa pressure
• 298 K temperature
• 1.0 mol dm^3 concentration
• When an enthalpy change is measured under standard conditions it is written as ΔHθ.

Question 5

Enthalpy level diagrams

Answer 5

Breaking bonds requires energy and is an endothermic process

Making bonds releases energy and is an exothermic process

*need initial input of energy- Ea

Question 6

Ea definiton

Answer 6

Activation Energy, Ea, is the minimum needed energy to start a reaction

Question 7

Mean bond enthalpy definition

Answer 7

heat energy required to break one mole of a covalent bond into gaseous atoms, averaged over a range of different compounds

E.g. O2(g) → 2O(g)

• always endothermic
• more positive bond enthalpy, larger the amount of energy needed to break bond= stronger bond

Question 8

Standard enthalpy of formation

Answer 8

The enthalpy change when one mole of substance is formed from its constituent elements under standard conditions with all reactants and products being in their standard states.

e.g. H2(g) + ½O2(g) → H2O(l)

-ΔHf of all elements in standard state = 0

-why would reaction not show ΔHf - forms two moles not one

Hess cycle = ↑

Question 9

Standard enthalpy of combustion

Answer 9

The enthalpy change when one mole of a substance is completely burnt in excess oxygen under standard conditions, all reactants and products being in their standard states

e.g. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

• Substances that cannot be combusted like water, carbon dioxide and most other oxides, have zero enthalpy of combustion
• always exo, always negative
• difficult to measure = incomplete combustion

Hess cycle = ↓

Question 10

Enthalpy of Reaction

Answer 10

any reaction that does not match any of the above

Question 11

Hess’ Law definition

Answer 11

the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products.

Question 12

Bond enthalpy Calculation

Answer 12

BERP

reactants - products

Question 13

Oxygen bond

Answer 13

O2 only substance with O=O bond

Question 14

Why is databook value different

Answer 14

databook derived from different compounds

Question 15

calorimetry equations

Answer 15

Q=mcΔT (Q/1000)
ΔH=Q/n

Question 16

Combustion calorimetry

Answer 16

Temp of water ↑ = exo, -ΔH
Temp water ↓ - endo, +ΔH

use mass of water

Question 17

Calorimetry with two solutions reacting

Answer 17

• use total solution vol when calculating mass
• use moles of limiting factor - lower mol

Question 18

Calorimetry- adding solid to solution

Answer 18

• only use mass of water
• don’t use excess moles

Question 19

Assumptions made in calorimetry

Answer 19

energy transferred from reacting chemicals is equal to energy released by reaction

Question 20

Combustion Calorimetry Sources of error

Answer 20

• Heat loss to the surroundings
• Incomplete combustion of the fuel
• Heat energy transferred to the metal calorimeter
• Some fuel evaporates

Improvements to Minimise Sources of Error
* Add a lid – reduces heat loss
* Insulate sides of calorimeter – reduces heat loss
* Reduce distance between flame and beaker – reduces heat loss
* Put sleeve around flame to protect it from draughts

Question 21

Solution Calorimetry Sources of error

Answer 21

• Heat loss to the surroundings

Improvements to Minimise Sources of Error
* Add a lid – reduces heat loss
* Insulate calorimeter – reduces heat loss

HEAT LOSS BIGGER ISSUE THAN UNCERTAINTY

Question 22

Enthalpy Change Using a Cooling Curve

Answer 22

1)
-measure water with measuring cylinder (100cm3)
-into polystyrene cup
-add known mass of MgCl

2)
-record initial temp until constant
-min 5 solid added
- record temp at regular intervals
-cooling till trend established

3)
Q=mcΔT
ΔH=Q/n