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THERMOCHEMISTRYEnergy Changes in Chemical ReactionsEnergy changes occur in almost all chemical reactions.There are two kinds of chemical reactions that undergo changes in heat. They are:Exothermic reactionsEndothermic reactionsExothermic ReactionChemical reaction that release heat into its surroundings.The total energy contained in the product is less than that of the reactants.Heat energy is released from the reactant into the surroundings to increase.The reaction mixture becomes hot, so does the container that holds it.The rise in temperature can be felt by hand. A thermometer can be used to measure the rise in temperatureEndothermic ReactionAn endothermic reaction is a chemical reaction that absorbs heat from the surroundings.The total energy contained in the products is more than that of the reactants.Heat energy is absorbed by the reactants form the surroundings, causing the surroundings temperature to drop.The reaction mixture becomes cold, so does the container that holds it.The reduction in temperature can be detected by hand or by using a thermometerExothermic reaction H negative

Endothermic reaction

Relationship between energy change with the formation and breaking of chemical bondsWhen chemical reactions occur, heat energy might be absorbed or released.Heat energy is absorbed when chemical bonds in a compound are broken.On the other hand, heat energy is released when chemical bonds in a compound are formed.In an exothermic reaction, the energy that is released from the formation of bonds exceeds the energy required to break the bonds.

Application of exothermic and endothermic reactions.

Exothermic and endothermic reactions are applied in the making of hot and cold packs.Figure below briefly shows the principle behind the making of hot and cold packs.

Making of hot packs and cold packsThe hot or cold packs contain two plastic bags.The outer plastic bag is made of a strong thick plastic and is filled with chemical powder.The inner plastic bag is made of weaker thin plastic that contains water.When the beg is punched, the inner plastic bag will break open. The water inside will then mix with and dissolve the chemical powder, causing the solution to become either very hot or cold.Hot packsCold packsThe chemicals that are suitable for use to produce an exothermic reaction when mixed with water are:Dry copper(II) sulphateDry potassium chloridemagnesium chloridecalcium chloridemagnesium sulphateThe chemicals that are suitable for use to provide an endothermic reaction when mixed with water are:ammonium nitratepotassium nitratesodium thiosulphateHot packs are widely used by mountain climber or explorer to areas with a cold climate to warm their hands and feet.Cold packs are widely used by athletes to help them reduce pain in swellings that results from sporting injuries.

Heat of neutralizationIs the heat energy released when one mole of water is formed from the neutralization between one mole of hydrogen ions, H+ from an acid and one mole of hydroxide ions, OH- from an alkali.Energy change = mc

In which, m = the mass of the aqueous reaction mixturec = the specific heat capacity of the aqueous reaction mixture = the change in temperature

Heat Of Neutralization2250 cm3 of 2.0 mol dm-3 sodium hydroxide is measured using a 100 ml measuring cylinder and poured into a plastic cup.2. The initial temperature of the sodium hydroxide is determined using a thermometer.The measuring cylinder is washed with water and then rinsed with the hydrochloric acid solution.50 cm3 of 2.0 mol dm-3 hydrochloric acid is measured using a 100 ml measuring cylinder. Its initial temperature is recorded.The HCl solution is then poured into the NaOH solution.The mixture is stirred and the maximum temperature attained is recorded.Determining the heat of neutralisation between a strong acid and a strong alkaliHeat Of Neutralization

23CalculationEnergy Level DiagramDetermining the heat of neutralisation between a strong acid and a strong alkali

Initial temperature of NaOH solution = 28.0 oCInitial temperature of HCl solution = 28.0 oCMaximum temperature of mixture = 41.4 oCAverage initial temperature of reactants is = 28 oCRise in temperature during reaction is (41.4 - 28) = 13.4 oC.2.Heat given out during reaction is mc = (50 + 50)(4.2)(13.4) Joule = 5628 J = 5.628 kJ5.Ionic equation for reaction : H+(aq) + OH-(aq) H2O(I) 3.Number of moles of NaOH used = = 0.1 mole

4.0.1 mole of NaOH reacts with 0.1 mole of HCl releases 5.628 kJ of heat. Therefore if 1 mole of reactants react, the heat energy released is x 5.628 kJ = 56.28 kJ. Heat of neutralisation, H = -56.28 kJ mol-1

Heat Of Neutralization24The heat of neutralisation involving a weak acid and a weak alkali A strong acid and a strong alkali ionizes completely in water.HCl(aq) H+(aq) + Cl-(aq)KOH(aq) K+(aq) + OH-(aq) When these solutions are added together the only change that occurs is the formation of water molecules.K+OH-(aq) + H+Cl-(aq) K+Cl-(aq) + H2O(l) Hence the heat of neutralization is the heat released from the formation of 1 mole of water and its value is approximately 57.4 kJ mol-1.H+(aq) + OH-(aq) H2O(l) H= -57.4 kJ mol-1 Hence the heat of neutralization of any strong acid and strong alkali is the same. For exampleNa+OH-(aq) + H+NO3-(aq) Na+NO3-(aq) + H2O(l) H= -57.4 kJ mol-1Heat Of Neutralization25

Heat energy is required to break this O-H bond before the H+ can react with the OH- to form water. Thus the net heat change is less than 57 kJ mol .

The heat of neutralisation involving a weak acid and a weak alkaliHowever when weak acids or alkalis are involved, the heat of neutralization is lower than 57.4 kJ mol-1.These weak acids or alkalis are partially dissociated in water. Some energy is needed to break the -O-H bonds to release the hydrogen ions or the hydroxide ions before they can react. Thus the net heat change will be less than 57.4 kJ/mol. For example the heat of neutralization of ethanoic acid and sodium hydroxide is only 54 kJ mol-1.Heat Of Neutralization26