Heat of neutralization

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Heat of neutralization

  1. 1. Chapter 4 Thermochemistry 4.4 Heat Of Neutralization ITeach – Chemistry Form 5
  2. 2. Heat of neutralization Is 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. Chapter 4 Thermochemistry Heat Of Neutralization ITeach – Chemistry Form 5 Energy change = mc In which, m = the mass of the aqueous reaction mixture c = the specific heat capacity of the aqueous reaction mixture = the change in temperature
  3. 3. <ul><li>50 cm 3 of 2.0 mol dm -3 sodium hydroxide is measured using a 100 m l measuring cylinder and poured into a plastic cup. </li></ul>2. The initial temperature of the sodium hydroxide is determined using a thermometer. <ul><li>The measuring cylinder is washed with water and then rinsed with the hydrochloric acid solution. </li></ul><ul><li>50 cm 3 of 2.0 mol dm -3 hydrochloric acid is measured using a 100 m l measuring cylinder. Its initial temperature is recorded. </li></ul><ul><li>The HCl solution is then poured into the NaOH solution. </li></ul><ul><li>The mixture is stirred and the maximum temperature attained is recorded. </li></ul>Determining the heat of neutralisation between a strong acid and a strong alkali Chapter 4 Thermochemistry Heat Of Neutralization ITeach – Chemistry Form 5
  4. 4. Calculation Energy Level Diagram Determining the heat of neutralisation between a strong acid and a strong alkali Initial temperature of NaOH solution = 28.0 o C Initial temperature of HCl solution = 28.0 o C Maximum temperature of mixture = 41.4 o C <ul><li>Average initial temperature of reactants is = 28 o C </li></ul><ul><li>Rise in temperature during reaction is (41.4 - 28) = 13.4 o C. </li></ul>2. Heat given out during reaction is mc θ = (50 + 50)(4.2)(13.4) Joule = 5628 J = 5.628 kJ 5. Ionic equation for reaction : H + (aq) + OH - (aq)  H 2 O( I ) Chapter 4 Thermochemistry Heat Of Neutralization Past Year Question ITeach – Chemistry Form 5 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
  5. 5. The heat of neutralisation involving a weak acid and a weak alkali <ul><li>A strong acid and a strong alkali ionizes completely in water. </li></ul><ul><li>HCl(aq)  H + (aq) + Cl - (aq) </li></ul><ul><li>KOH(aq)  K + (aq) + OH - (aq) </li></ul><ul><li>When these solutions are added together the only change that occurs is the formation of water molecules. </li></ul><ul><li>K + OH - (aq) + H + Cl - (aq)  K + Cl - (aq) + H 2 O( l ) </li></ul><ul><li>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 . </li></ul><ul><li>H + (aq) + OH - (aq)  H 2 O( l )  H= -57.4 kJ mol -1 </li></ul><ul><li>Hence the heat of neutralization of any strong acid and strong alkali is the same. For example </li></ul><ul><li>Na + OH - (aq) + H + NO 3 - (aq)  Na + NO 3 - (aq) + H 2 O( l )  H= -57.4 kJ mol -1 </li></ul>Chapter 4 Thermochemistry Heat Of Neutralization Past Year Question ITeach – Chemistry Form 5
  6. 6. 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 alkali However 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 . Chapter 4 Thermochemistry Heat Of Neutralization Past Year Question ITeach – Chemistry Form 5
  7. 7. The End i - Teach

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