## Wednesday, May 18, 2011

### Form 5 Chapter 4 Thermochemistry

Describe an experiment to determine the heat of precipitation of silver chloride

Diagram:

Apparatus:
Measuring cylinders, thermometer and plastic cup

Materials:
0.5 mol dm-3 silver nitrate solution, 0.5 mol dm-3 sodium chloride solution

 Procedure: 1.      25cm3 of 0.5 mol dm-3 sodium chloride solution is measured and poured into a clean and dry plastic cup using a measuring cylinder.2.      The initial temperature of sodium chloride solution is measured and then recorded.3.      Using another measuring cylinder, 25 cm3 of 0.5 mol dm-3 silver nitrate solution is measured. The initial temperature of the silver nitrate solution is measured after 5 minutes and recorded.4.      The silver nitrate solution is poured quickly and carefully into the sodium chloride solution.5.      The mixture is stirred with a thermometer throughout the experiment and the highest temperature obtained is recorded. Result: Mass, m c q = (25 + 25) g = 50 g 4.2 Initial temperature of sodium chloride solution     = T1oCInitial temperature of the silver nitrate            = T2 oCHighest temperature reached                                            = T4 oC=> Average initial temperature = (T1+T2)/2                       = T3 oCè Increase in temperature,  q = (T4 - T3) oC Calculation: No of moles of sodium chloride used No of moles of silver nitrate used = MV/1000 = 0.0125 mole = MV/1000= 0.0125 mole Chemical equation: AgNO3 (aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)Ionic equation:              Ag + (aq)                              +              Cl- (aq) à           AgCl (s)                                                             1 mole                                  1 mole                  1 mole Heat released =              mcq                                             =              (25 + 25 ) x 4.2 x (T4 - T3)                                             =              x J0.0125 mole of AgCl precipitated releases x J of heat energy. Therefore, 1 mole of AgCl precipitated will released Heat of precipitation, DH = x/0.0125 J = y J Thermochemical equation:AgNO3 (aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)                             DH = - y/1000 kJmol-1