Harvesting Chemical Energy

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Presentation transcript:

Harvesting Chemical Energy

Combustion Reactions Combustion is a chemical reaction that occurs in the presence of OXYGEN and results in the release of CARBON DIOXIDE AND WATER

Balancing Combustion Rxns Balance the carbons Balance the hydrogens Balance the oxygens (use a fraction if needed) Get rid of the fraction by multiplying by the bottom number (2)

Example The combustion of octane

Example 2 The combustion of ethanol

Combustion Release energy How does this feel??? Draw an energy diagram for this type of reaction

Calorimetry A quantitative lab technique that measures the change in energy of a chemical reaction

Experimental Data The change in temperature of the water in a calorimeter depends on the mass and heat capacity of the water

Q = mcΔt Q = heat (energy) in J m = mass in g c = specific heat capacity (usually of water) = 4.19J/g°C Δt = change in temp (tf – ti) in °C This formula can be found on the bottom of pg 5 in your data booklet!

Enthalpy of a Reaction The THEORETICAL energy released in a chemical reaction can be calculated using the standard heats of formation (found on page 5)

ΔHrxn = Σ(nΔHf)p - Σ(nΔHf)r

Example 1 Calculate the enthalpy of the combustion of octane

Example 2 Calculate the enthalpy of the combustion of ethanol

Example 3 Calculate the enthalpy of combustion of acetic acid

Efficiency Theoretically, all energy from a combustion reaction should go into use… this is not the case!

Example During a trip, a car uses 2.35x107kJ of chemical potential energy supplied by combustion of gasoline. The car’s engine is able to transform 4.73x106kJ of that chemical energy into useful work. Calculate the efficiency of the car:

Use the first law of thermodynamics to determiner the percentage of the car’s input energy that is transformed into non-useful forms of output energy

List some of the non-useful forms of energy produced by the car’s engine

Explain why the non-useful energy can never be completely eliminated?