Thermal Energy from Chemical Reactions

Thermochemical Equations An equation with the amount of energy produced or absorbed –C 8 H 18(l) + 12½O 2(g)  8CO 2(g) + 9H 2 O (g) ΔH = – 5054kJ mol –1 If you burnt twice as much octane, twice the amount of energy is produced –2C 8 H 18(l) + 25O 2(g)  16CO 2(g) + 18H 2 O (g) ΔH = – 10108kJ mol –1

Thermochemical Equations The coefficients of the reactants indicates the number of moles The equation states that 2 moles of C 8 H 18 reacting with 25 moles of O 2 produces 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 –C 8 H 18(l) + 12½O 2(g)  8CO 2(g) + 9H 2 O (l) ΔH = – 5450kJ mol –1 This is because the evaporation of water absorbs energy H 2 O (l)  H 2 O (g) ΔH = + 44kJ mol –1

Thermochemical Equations A Reaction that occurs in reverse has the same size ΔH but reversed sign H 2 O (l)  H 2 O (g) ΔH = + 44kJ mol –1 H 2 O (g)  H 2 O (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 C 3 H 8 generated if burnt in O 2 –C 3 H 8(l) + 10O 2(g)  3CO 2(g) + 4H 2 O (l) ΔH = – 2220kJ mol –1

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) X Temperature Rise ( °C )

Specific Heat Content For Water this would become 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 Stirrer Electric Heater to ignite sample WaterInsulated Container Sample in Crucible Pressurised Vessel 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 Temperature Change

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