C3.2 Energetics

Question 1

Exothermic reaction

Answer 1

Temperature of reaction mixture increases during a chemical reaction
Release energy into the surroundings
Examples: Combustion, neutralisation

(Combustion: fuel + oxygen -> carbon dioxide + water + energy)

Question 2

Endothermic reaction

Answer 2

Temperature of the reaction mixture decreases during a chemical reaction
Take energy in from the surroundings
Examples: reaction between citric acid and sodium hydrogencarbonate
Photosynthesis

Question 3

What are reaction profiles

Answer 3

Chart that shows the energy involved in a chemical reaction

Question 4

energy change

Answer 4

energy transferred from the surroundings to break bonds in the reactants minus the energy transferred to the surroundings when bonds form in the products

Basic equation: Energy for Bonds broken - energy for bonds made

Question 5

What is activation energy

Answer 5

Minimum energy needed for a reaction to start
Often provided by a flame or by heating, and it breaks bonds in the reactants

Question 6

Reaction profile for endothermic reaction

Answer 6

In endothermic reaction amount of energy transferred to break bonds > energy transferred to form new bonds
Positive energy change
Surroundings cool down
Upward arrow on the energy profile

Example graph:
Energy | ∩ products
| ||__________
|reactants|↑ energy change
|__________|
|______________________
Progress of reaction

Question 7

reaction profiles for exothermic reaction

Answer 7

In exothermic reaction amount of energy transferred to break bonds < energy transferred to form new bonds
negative energy change
Surroundings warm up
Downward arrow on the energy profile

Example graph:
Energy | reactants ∩
| _________||
|energy ↓| products
|change |_________
|______________________
Progress of reaction

Question 8

Drawing reaction profiles

Answer 8

1) draw both axis
2) Draw 2 horizontal lines representing the chemical energy stores in the reactants and products.
Reactant line lower than product line for endothermic reaction
Reactant line higher than product line for exothermic reaction
3) Draw a curve to represent the activation energy and add labels to the diagram

Question 9

What are bond energies

Answer 9

Energy needed to break 1 mol of a particular bond
measured in kJ/mol

Question 10

How do you calculate energy changes in reactions

Answer 10

Example question: nitrogen + hydrogen -> Ammonia
H
[ | ]
1) N≡N + 3[H-H] -> 2 H-N-H

Bonds broken: N≡N , H-H x 3 : energy: 945 + 3(436) = 2253kJ/mol
Bonds made: N-H x 6 : energy: 6(391) = 2346kJ/mol
2) Change in energy = bonds broken - bonds made = 2253 - 2346 = -93kJ/mol
The negative sign in the answer shows that the reaction is exothermic

Question 11

Advantage of displayed formula (O=O for O2)

Answer 11

Shows the bonds present in a molecule