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Topic 5 Energetics/thermochemistrylevelHigherlevelIB CHEMISTRY
5.1 Measuring energy changes• Heat is a form of energy.• Temperature is a measure of the average kinetic energy of the particles.• Total energy is conserved in chemical reactions.• Chemical reactions that involve transfer of heat between the system andthe surroundings are described as endothermic or exothermic.• The enthalpy change (∆H) for chemical reactions is indicated in kJ mol-1.• ∆H values are usually expressed under standard conditions, given by ∆H°,including standard states.• Calculation of the heat change when the temperature of a pure substanceis changed using q=mc∆T.• A calorimetry experiment for an enthalpy of reaction should be coveredand the results evaluated.OBJECTIVES
Temperature and HeatTemperatureis the average kinetic energyof molecules(K).Heatis the amount of energyexchanged due to atemperature difference between two substances (J).
Enthalpy (H)Enthalpy (H)– the amount of energyor heatcontent of a substance. The energy is stored inthe chemical bonds. It includes kinetic andpotential energy and is not measured directlybut changes are measured (J).Standard enthalpy change of reaction (ΔH⁰)-the difference between the enthalpy of theproducts and the enthalpy of the reactants at298K and 1.00 x 105Pa.
EnthalpyChange in enthalpy (ΔH)– enthalpy of products lessreactants. It is the heat energy change per mole.ΔH = HP– HRForexothermic reactionsthe temperature of thesurroundings increases andΔH is negative, energy isgiven outForendothermic reactionsthe temperature of thesurroundings decreases andΔH is positive, energy isabsorbedThe higher the enthalpy the less stable the substance
Enthalpy level diagrams
The energy is measured by temperature. We cancalculate the energychangeonly. Not the energystored in the resultant products or reactants.Enthalpy level diagrams
Exothermic reactionsExamples include:1.Combustion reactions2.Neutralization reactionsIn an exothermic reaction products:•.have less stored energy•.are more stable•.have stronger bonds (harder to break)
Heat energy change (Q)Q = mcΔTm = mass in gc = specific heat capacity of the substance JK-1g-1ΔT = change in temperature in KQ = Heat energy changeΔH = Heat energy change per moleSo,ΔH= Q /n
CalorimetryA technique used to measure enthalpychange.Heat absorbed, Q = (m x c x ΔT)liquidcwater= 4.18 JK-1g-1mwater= volume in cm31 cm3= 1g
Styrofoam Calorimeter
BombCalorimeter
Assumptions and sources of errorThat no heat is lost to or gained from the surroundings.Significant errors are associated with reactions:involving the evolution of gasinvolving combustion where hot gas is used to heat liquid in acalorimeter.

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Term
Spring
Professor
carlson

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