8303231-65-Lab-Chem

8303231-65-Lab-Chem - Chem Factsheet...

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Calorimetry Experiments Number 65 1 C hem F actsheet www.curriculumpress.co.uk To succeed in this topic you need to:- Have a good understanding of AS-level Energetics covered so far in Factsheet 08 - Energetics I - Hess's Law; be familiar with basic apparatus used in Chemistry. After working through this Factsheet you will:- be familiar with the main types of experiment used in calorimetry at AS-level; be able to calculate values of enthalpy changes from experimental data. Experiment 1: Enthalpy changes on displacement and the reactivity series Introduction The relative positions of metals in the reactivity series can be investigated by adding metals to solutions of salts of other metals, for example Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) This confirms that zinc is ‘above’ copper in the reactivity series. The enthalpy change for this and similar reactions can lead to an illustration of Hess’s Law. Method Place 100 cm 3 of copper(II) sulphate solution (0.2 mol dm –3 ) in a polystyrene cup (or vacuum flask if available). Record the temperature (to the nearest 0.1 o C) of the solution every 30 seconds for two minutes. At the two minute mark add 2 g (an excess) of zinc powder and stir the mixture. Continue recording the temperature for a further five minutes. Plot a graph of temperature against time and use the plot to determine the maximum temperature change ( T). Calculation The heat energy exchanged in the experiment is 100 × T × 4.18 joules. In the reaction 0.02 (= 1 / 50 ) mole of copper ions reacted so the heat exchanged per mole would be 50 × (100 × T × 4.18) joules or 5 × T × 4.18 kJ. As the reaction is exothermic the value for H (in kJ mol –1 ) will need a negative sign. Alternative Method Carry out the procedure above to determine the change in temperature. Then place a further 100 cm 3 of copper(II) sulphate solution in the reaction vessel and insert a low voltage heater connected through a joulemeter. Switch on the current and allow the temperature to rise by the same number of degrees. Record the reading on the joulemeter to determine the heat exchanged in the experiment and thus calculate a value for H. Extension If similar experiments are carried out to determine the enthalpy changes for (say) Cu(s) + 2Ag + (aq) Cu 2+( aq) + 2Ag(s) and Zn(s) + 2Ag + (aq) Zn 2+( aq) + 2Ag(s) then Hess’s Law can be illustrated. For the zinc/copper reaction H is –216 kJ mol –1 ; for the copper/silver reaction H is –147 kJ mol –1 ; and for the zinc/silver reaction H is –363 kJ mol –1 . Experiment 2: Enthalpy change of neutralisation Introduction Acidic and alkaline solutions mix exothermically. The ionic equation for neutralisation is H + (aq) + OH (aq) H 2 O(l) Method Put 50 cm 3 of hydrochloric acid (1.0 mol dm –3 ) and 50 cm 3 of sodium hydroxide solution (1.0 mol dm –3 ) into separate measuring cylinders Wait for the temperatures of the two solutions to equalise.
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8303231-65-Lab-Chem - Chem Factsheet...

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