Topic 5: Energetics & Thermochemistry

• heat energy is released - energy released in bond formation on product side is greater than energy consumed in bond breaking on reactant side

ΔH = - x

• heat energy is absorbed - energy released in bond formation on product side is less than energy consumed in bond breaking on reactant side

ΔH = + x

• a specified part of the universe - under observation/where a chem rxn is taking place

• the remaining portion of the universe - NOT part of the system

heat content of the system symbol: H

• heat absorbed/evolved during a rxn • measured at a constant temp/pressure symbol: ΔH unit: kJ mol^-1

ΔH = Hp - Hr ΔH = enthalpy change Hp = enthalpy of products Hr = enthalpy of reactants

-273 degrees Celsius the temp at which all particle movements cease completely

• form of energy • measure of total energy in a given amount of substance • depends on the amount of substance present

• measures the ‘hotness’ of a substance • avg kinetic energy of the substance • independent of the amount of substance present

heat required to raise the temp of 1g of a substance by 1°C/K unit: J K^-1 g^-1 j = joules k = kelvin g = grams

heat needed to increase the temp of the object by 1°C/K

enthalpy change when 1 mol of the gaseous bond is broken or formed

technique of measuring enthalpy change

• a well-insulated container (e.g. polystyrene cup) • in which temp change of a liquid is measured • before and after the change

• no heat transfer between the soln, the thermometer, the surrounding air, and the calorimeter itself • the solution’s density and specific heat capacity are equivalent to water’s • rxn occured fast enough for max temp to be achieved before cooling begins

• not having the desired rxn occur (e.g. incomplete combustion) • loss of heat to surroundings in exothermic reactions • absorption of heat from surroundings in endothermic reactions • using incorrect specific heat capacity value in enthalpy calculations

• for slow rxns (e.g. metal ion displacement), the results will be less accurate • due to heat loss over time • this can be compensated by plotting a temp-time graph (cooling graph) to extrapolate backwards

• draw a line of extrapolation backwards from cooling curve • draw a vertical line where the reactants were mixed (i.e. when the curve begins to rise) • the y-coordinate of the point of intersection is the temp that would’ve been reached if no heat was lost to surroundings

• the total enthalpy change for a chem rxn doesn’t depend on the pathway it takes • only considers initial and final states

AKA: law of conservation of energy • energy can neither be created nor destroyed • it can only be converted from one form to another

• denoted by ΔHf • enthalpy change when 1 mole of a substance is formed from its elements • all substances being in standard states • enthalpy of formation of every element in its standard state is assumed to be 0!