Thermal Energy from Chemical

Reactions

Thermochemical Equations

• An equation with the amount of energy produced or absorbed– C8H18(l) + 12½O2(g) 8CO2(g) + 9H2O (g) ΔH = –

5054kJ mol –1

• If you burnt twice as much octane, twice the amount of energy is produced– 2C8H18(l) + 25O2(g) 16CO2(g) + 18H2O(g) ΔH =

– 10108kJ mol –1

Thermochemical Equations

• The coefficients of the reactants indicates the number of moles

• The equation states that 2 moles of C8H18 reacting with 25 moles of O2 produces 10108 kJ

• States must be specified since change of state can produce or needs energy

Thermochemical Equations

• The combustion of Octane can also produce liquid water– C8H18(l) + 12½O2(g) 8CO2(g) + 9H2O (l) ΔH = –

5450kJ mol –1

• This is because the evaporation of water absorbs energy

• H2O (l) H2O (g) ΔH = + 44kJ mol–1

Thermochemical Equations

• A Reaction that occurs in reverse has the same size ΔH but reversed sign

• H2O (l) H2O (g) ΔH = + 44kJ mol–1

• H2O (g) H2O (l) ΔH = – 44kJ mol–1

Calculations Involving Thermal Equations

• Involves Stoichiometry to determine how many moles are reacting

• multiplying the ΔH by the number of moles and dividing by the coefficient from equation

Calculations Involving Thermal Equations

• Example, how much energy would 11g of C3H8 generated if burnt in O2

– C3H8(l) + 10O2(g) 3CO2(g) + 4H2O (l) ΔH = – 2220kJ mol –1

kJxenergy

moln

55522201

25.0

25.044

11 )HC( 83

Connection Between Energy and Temperature Change

• Objects heat up at different rates

• This is expressed by the Specific Heat Content

• Values are given in a table of values

Specific Heat Content

• The amount of energy needed to raise the temperature of 1g of a substance by 1°C

• The higher the specific heat, the more effectively the substance will store heat

• Has the unit Jg–1°C –1

Energy needed to heat =

Specific Heat

X mass (g) XTemperature

Rise (°C)

Specific Heat Content

• For Water this would become

• 4.184 X mass of water X temperature rise

• 1 g = 1mL for water density = 1 g mL-1

Enthalpy

• The energy in a chemical bond• During a chemical reaction where chemical

bonds are broken then formed there will be a change in enthalpy

• If energy (products) < energy (reactants)– Energy change is negative– Energy will be released– Exothermic Reaction

Measuring Heat Released During a Reaction (Enthalpy Change)

• Measured by a Calorimeter• When a reaction takes place, the heat

change causes a rise or fall in temperature in the contents of the calorimeter.

• Before use calorimeter must be calibrated• Find out how much energy is needed to

change the temperature by 1°C (Calibration Factor)

Bomb Calorimeter

Thermometer

Electric Heater for Calibration

StirrerElectric Heater to ignite sample

WaterInsulated Container Sample in Crucible

PressurisedVessel

Oxygen under pressure

Solution Calorimeter

Thermometer

Electric Heater for Calibration

Stirrer

Insulated Container

Glass Bulb Containing Second Reactant

Solution of one Reactant

Calibration Factor

• Energy = Voltage X Current X Time

• E = VIt– Voltage is measured in Volts– Current is measured in Amps– Time is measured in seconds

Calculate Energy Change During Reaction

Energy Change =

Calibration Factor

X TemperatureChange

Calculate ΔH

• This is the change of energy for 1 mole

• If reaction produced heat ΔH = negative

• If reaction absorbed heat ΔH = positive

If change in heat = 30kJ for 0.25mol

ΔH = 30 X 0.25 = 120 kJ mol-1

Heat of Combustion of a Substance

• The energy released when a specified amount (1g, 1L or 1 mol) of the substance burns completely in Oxygen.

• Substances that are mixtures of chemical are measured in terms of grams or litres

Which Fuel is Best

• Factors to be considered

• Energy released per unit of mass

• Availability

• Cost

• Ease of transport

• Hazards associated with waste products