Topic 5: Energetics

• measure of the ability to do work - conserved in chemical reactions

• measure of the amount of energy contained in a substance • stored in chemical bonds and intermolecular forces as potential energy • when substances react, the different in the enthalpy between the reactants and products results in a heat change

a mode of energy transfer which occurs as a result of temperature difference • increases the average kinetic energy of molecules • its the TOTAL kinetic energy of a system

• heat absorbed by system • breaking bonds; separation of particles which are held together by a force of attraction • enthalpy is positive • reactants more stable than products

• heat give out by system • making bonds; bringing together of particles which haven an attrative force between them • enthalpy is negative • products more stable than reactants e. g. combustion and neutralization reactions

• a pressure of 100 kPa • concentrations of 1 mol dm^-3 for all solutions • all substances in their standard states

pure form of the substance under stnadard condtions

• meaure of the average kinetic energy of the particles - Kelvin scale; measures relationship between kinetic energy and temperature (298K usually given as standard)

• mass of the object (shared vaue of kinetic energy among particles) • heat added (amount of kinetic energy) • nature of the substance

• property of a substance which gives the heat needed ot icnrease the tempreature of unit mass by 1 k • depends on number of particules in unit mass sample

q=mc@T q= heat change in joules m= mass in grams c= specific heat capacity @T= change in temperature (in kelvin)

4.18 J g^-1 K^-1

heat change (q)/temperature change (@T)

enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions

• not all heat produced by combustion transferred to water • incomplete combustion due to imited oxygen • experiment not performed under standard conditions

carrying out reaction in insulated system (e.g. polysterene cup) as heat release or absorved by reaction can be measured from the water temperature change

states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and final conditions, and reactants and products are the same

you reverse the sign of the enthalpy value

enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard condtions of • 298 K (25 C) • 1.00 x 10^5 Pa

its zero, as no enthalpy change occurs when an element is formed from itself

AH of reaction= AH of Products- AH of Reactants

energy needed to break one mole of bonds in gaseous molecules under standard conditions

• to take into account bond enthalpies and compare them which exist in different environments • different enthalpies of atoms bonded in different ways

energy needed to break one mole of bonds in gasesous molecules under standard conditions averaged over similar compounds