Topic 5: Energetics & Thermochemistry

• a form of energy • transferred from a warmer body to cooler body • due to temp equilibrium

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

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

• 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 - denoted by H

• heat absorbed/evolved during a rxn • at constant temp/pressure • is denoted by ΔH

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

ΔH = negative

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

ΔH = positive

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

heat required to raise the temp of 1g of a substance by 1 degree Celsius/Kelvin unit: J/kg j = joules k = kelvin g = grams

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

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

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

• there is no heat transfer between the soln, the thermometer, the surrounding air, and the calorimeter itself • the solution is dilute enough so that its density and specific heat capacity are equal to water’s • the rxn is assumed to have occurred sufficiently rapidly for max temp to be achieved before rxn mixture begins cooling to room temp

• 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 an incorrect specific heat capacity in enthalpy calculations

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

• 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!

signs are reversed e.g. negative value turns positive and vice versa

• Strong covalent double bonds in normal O2 is broken by high-energy UV radiation with λ O•(g) + O•(g) • The oxygen atoms have unpaired electrons, so they are reactive, and will react to form ozone. O•(g) + O2 (g) -> O3 (g) The rxn is exothermic and the energy given raises the temp of the stratosphere • Bonds in ozone are weaker than the double bond in oxygen, so lower energy UV rays can break them (λ O•(g) + O2(g) • The free atomic oxygen reacts with another ozone molecule. O3 (g) + O•(g) -> 2O2 (g) This is another exothermic reaction, which produces heat that maintains the relatively high temp of the stratosphere.