5.1 Energy changes5.1 Energy changes

Exothermic reaction, e.g. respiration

• Energy ‘exits’ reaction – heats surroundings

• Thermometer readings rises

Endothermic reaction, e.g. photosynthesis

• Energy ‘enters’ reaction – cools surroundings • Thermometer readings fall

5.2 Energy and reversible 5.2 Energy and reversible reactionsreactions

Hydrated Anhydrous copper sulphate copper sulphate + water

Exothermic reaction

Endothermic reaction

5.3 Haber process (again!)5.3 Haber process (again!)

N2 + 3H2 2NH3

Exothermic reaction

inc temperature, dec products

dec temperature, inc products

Endothermic

reaction

inc temperature, inc products

dec temperature, dec products

5.3 Haber process (again!)5.3 Haber process (again!)

N2 + 3H2 2NH3

Smaller vol. of gas produced

inc pressure, inc products

dec pressure, dec products

Larger vol. ofgas produced

inc pressure, dec products

dec pressure, inc products

5.3 Haber process (again!)5.3 Haber process (again!)

N2 + 3H2 2NH3

Pressure:- The higher the better- High pressure is dangerous!- Compromise by using 200-350 atm

Temperature:- Forward reaction is exothermic, so low temperature is preferred- But this makes reaction slow- Compromise by using 450OC

Catalyst:- Iron- Speeds up both sides of reaction

C3

Energy from fuelsAcids

& Alkalis

Calorimeter

Think HSW!

Bomb calorimeter

4.2J raises temp of 1 g of water by

1 degree

Food high in carbs and fats have lots of energy!! more than

body needs obesity

A + B CIf 0.1 mole of reactants. Total mass of A and B is 100g.Temp start is 19.6, temp max is 26.1Work out diff….6.5

(Don’t need to learn this, you would get this)So for 0.1 moles = 2730J

For 1 mole 2730 x 10 = 27300J (27.3kJ)…..exothermic reaction ( as temp rise) = -27.3kJ/mol

Energy change = mass x 4.2 x temp change

Energy changesAcids

& Alkalis

Reaction = bond breaking ( endo) and bond making ( exo)

EXOTHERMIC

Energy required to break bonds in less than energy released when new bonds

are formed

ENDOTHERMIC

Energy required to break bonds in greater than

energy released when new bonds are formed

CATALYST…. Lowers activation

energy

∆H = - ve ∆H = + ve

Bond energiesAcids

& Alkalis

CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Identify the bonds…..stick diagrams!

Bond Bond energy kJ/mol

H-H 436

Cl-Cl 242

H-Cl 431

O-H 464

C-C 347

C-O 335

O=O 498

∆H = bond breaking - bond making

Add up on the bonds in the reactants.This is bond energy needed to break the bonds

Add up on the bonds in the products.This is bond energy needed to make new bonds.

REMEMBER… making new bonds is an exothermic reaction…so it is always a –ve number

Example 1

BOND ENERGY (kJ/mol)

H-H 436 N-H 391N≡N 945

a) Complete the balanced symbol equation.

b) Draw the structural formula.

c) Calculate the energy in for the reactant bonds being broken.

d) Calculate the energy out for the product bonds being made.

e) Work out the overall change in energy

Hydrogen + Nitrogen Ammonia

Example 2

BOND ENERGY (kJ/mol)

O=O 498 H-H 436 O-H 464

a) Complete the balanced symbol equation.

b) Draw the structural formula.

c) Calculate the energy in for the reactant bonds being broken.

d) Calculate the energy out for the product bonds being made.

e) Work out the overall change in energy

Hydrogen + Oxygen Water