Energy and Equlibria

Question 1

Enthalpy

Answer 1

Chemical energy in substances
Measured in joules
(Delta h = energy (enthalpy) change

Question 2

Exothermic

Answer 2

-Enthalpy decreases, temp increases
-Heat energy released during reaction
-E.g Neutralisation : acid + metal
(Delta h = -ve )

-A reaction is exothermic if more energy is required to make than break bonds

{exo is negative } too many fangirls >_

Question 3

Endothermic

Answer 3

-Enthalpy increases. Temp decreases
-Energy taken in from surroundings
E.g thermal decomposition

-A reaction is endothermic if more energy is required to break than make bonds
(Delta h = +ive)

Question 4

Enthalpy change

Answer 4

Enthalpy change = Bonds broken - bonds made

During chemical reactions bonds in reactants must be broken. This takes in energy. Bonds are then formed in the products, which gives out energy.

Question 5

Energy level diagrams

Please draw/visualise

Answer 5

Exo: 
Reactants (top) 
Products (bottom) 
X-axis: reaction coordinates, y-axis: energy 
ARROWS MUST ALIGH
Endo:
Products
Reactants

Question 6

Calculating Enthalpy change from bond energy

Answer 6

Different bonds have different energy

1. Balance equation
2. Show all bonds
3. Add bonds together
4. Energy change = bonds broken - bonds made
   [KJ/mol]

Question 7

Calculating bond energy from an experiment

Answer 7

Q = m x c x delta T
(Joules, /1000 for KJ)
Energy released per gram = Q / mass of fuel
(KJ/g)

Q is delta h (energy released)
m is mass of liquid
C is 4.18 KJ/mol
T is change in temp

Question 8

Reversible reaction

Answer 8

A reaction in which the products can make the reactants and the reactants can make the products

(Denoted with ⇌ )

Question 9

Dynamic equilibrium

Answer 9

• When the forward/backward reactions happen at the same rate so no net change in amount of products and reactants
• There needs to be a closed system where nothing can enter/leave
• For any change imposed on the equilibrium, the position of the equilibrium will move to oppose the change

Question 10

Haber process
Nitrogen + hydrogen ammonia
delta h = -ive

Answer 10

Things affected:

• concentration
• temp
• pressure

E.g N2 + 3H2 2NH3
+ N2 concentration: system opposes change, forward reaction occurs more frequently, equilibrium shifts to the right, yield of ammonia increases

+ heat: oppose, backward, shifts to left, yield decrease

+ pressure: oppose, forward, shifts to right, yield increase
(Molecules take up the same space, look at big number)

Question 11

Measuring energy transfer: Calorimetry experiment

+ fair test

Answer 11

1. Cold water measured into a copper calorimeter
2. Starting temp of water recorded
3. Water heated using flame from burning fuel
4. Final temp of water recorded
5. Final mass of fuel burned: weigh spirit burner before/after
   Then: Q= m x 4.18 x T
   DIAGRAM
   Fair test:
   -constant mass/ starting temp of water
   -constant distance between flame + calorimeter

Question 12

Examples of reversible reactions

Answer 12

1. Blue copper (II) sulphate crystals ⇌ white anhydrous copper (II) sulphate

• when heating the former, the crystals turn into white powder and the water is driven off
• loss of water of crystallisation takes place

2. Ammonium chloride ⇌ ammonia + hydrogen chloride

• when heating the former, white crystals appear further up the test tube
• recombination takes place where it’s further up, when it’s cooler