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September 27, 2020 12:15 pmpts 30 pts
AH=4 T.2. Measurement of AH Taking the example of heat of neutra- lisation of an acid with a base or vice versa, it can be apolythene bottle fitted with a cork having two holes, FIGURE 6.12 THERMOMETER one for the thermometer and the other for the stirrer, foamed polystyrene cup with a cover is also sometimes POLYTHENE THERMOMETER determined using a simple calorimeter consisting of BOTTLE STIRRER FOAMED POLYSTYRENE CUP WITH shown in Fig. 6.12a. Instead of polythene bottle,a COVER Hsed. It is called coffee-cup calorimeter (Fig. 6.12b). Taking the example of neutralisation of HC1 with NaOH, the method consists of the following steps (i) A known volume of HCI of known concen- tration (say 100 cm of 0-5 N) is taken in one beaker and an equal volume of NaOH of the same concentration (i.e., 100 cm of 0.5 N) is taken in another beaker. REACTION MIXTURE (a) Polythene bottle as calorimeter (b) Foamed polystyrene cup with cover (coffee-cup calorimeter) (it) Both the beakers are kept in water bath till the solutions attain the same temperature. (iil) HCl solution kept in the first beaker is transferred into the polythene bottle. Immediately NaOH solution kept in the second beaker is added into the polythene bottle. Stirring is done to mix HCl and NaOH. The highest temperature attained is noted. Calculations. Assuming that the density of the solution is same as that of water (i.e., 1 g/cm°) and the specific heat of the solution is nearly equal to that of water (i.e., 4.184 J K g 4-2 J K g) and neglecting the heat capacity of the polythene bottle, the result is calculated as follows Suppose the initial temperature of the acid and the base = C Final tempearature of the solution after mixing = I2C . Rise in temperature = (12-1C Total mass of the solution = 100 +100 200 g Heat producedMass)x Specific heat x Rise in temp. = 200x 4.184 x (t - t1) J =xJ (say). Ma This is the heat produced by neutralisation of 100 cc of 0-5 N HC1. Heat produced by neutralisation of 1000 cc of 1 N HCl (containing one gram equivalent of HCl) 1 x1000xjoules 0-5 100 This gives the heat of neutralization per equivalent. As will be discussed later, heat of neutralization a strong acid by a strong base is always equal to 57.1 kJ equil or 57-1 kJ mol of Ht ions or OH ions. Measurement of AH or q, for any reaction. For any reaction occurring in solution, the heat change (generally under atmospheric pressure), i.e., the enthalpy i to take
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