Post on 19-Jan-2016
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