Thermochemistry

Key terms Thermodynamics – study of energy and its interconversions Energy – capacity to do work or produce energy Law of conservation of energy – energy can be neither created nor destroyed but only changed in form. Potential energy (PE) – energy due to position or composition Kinetic energy (KE) – energy due to motion Heat – transfer of energy between objects of different temperature Work – force acting over a distance Pathway - the way the energy is transferred State function/state property – depends on current (present) state (ex = energy)

Heat transfer Exothermic = energy given off; heat flows out of the system Endothermic = energy absorbed; heat flows into the system Where does the energy come from? – Difference in potential energies between reactants and products – Energy gained by the surroundings must be equal to the energy lost by the system (reaction) – Exothermic reaction - lowers potential energy; reactants higher than products – Endothermic reaction – increase potential energy; reactants lower than products – See energy diagrams

Energy Diagrams Exothermic Reaction Reactants ∆PE ENERGY Products Feels warm Change in energy negative - ∆E < 0 Endothermic Reaction Products ∆PE ENERGY Reactants Feels cool Change in energy positive - ∆E > 0

Internal energy in a system – E = sum of potential and kinetic energies of all the particles in the system ∆E = q + w ∆E = change in systems internal energy q = heat w = work (system) Or if we look at surroundings ∆E = q – w’ w’ = work (surroundings view) Units for E = Joules - ?

Enthalpy Enthalpy = H (a state property) H = E + PV E = internal energy of system P = pressure of system V = volume of system ∆H = q p At constant pressure, the change in enthalpy (heat of reaction) is equal to energy flow as heat. ∆H = H products – H reactants ∆H positive = endothermic ∆H negative = exothermic

Calorimetry Key Terms Calorimetry – science of measuring heat Calorimeter – device used to determine heat of reaction; p.255 for diagram Constant pressure calorimetry – atmospheric pressure constant used for determining change in enthalpy (heats of reaction) ∆H = q p = q Constant volume calorimetry – use a bomb calorimeter ∆E = q v = q ∆E = ∆T * heat capacity of calorimeter

Specific Heat

Hess’s Law Change in enthalpy is same whether occurs in one step or series of steps Get a net reaction if occurs in series of steps Reaction 1H1 Reaction 2H2. Net reaction∆H = H 1 + H 2 If reverse the reaction, sign of ∆H reversed

Standard enthalpy of formation Change in enthalpy that accompanies formation of 1 mole of a compound from its elements in its standard states; ∆H° f – Under standard conditions/standard states Enthalpy of formation of compounds = chart Enthalpy of formation of an element = 0 Apply Hess’s Law to figure out overall ∆H° f ∆H° f = ∑n p ∆H° f (products) - ∑n r ∆H° f (reactants) Multiply the enthalpies by the coefficients in balanced equation

Sources of Energy Fossil fuels – lead to greenhouse effects – Coal – Petroleum – Natural gas New energy sources - costly – Coal conversion – Hyodrogen – Alternate fuels Oil shale Ethanol kerogen