Working out stabdard enthalpy change Flashcards
equipment
hydrated copper(II) sulfate crystals anhydrous copper(II) sulfate powder polystyrene cups with lids 250 cm3 or 4003 cm beaker (as appropriate for holding the cup) stand and clamp 0 °C to 50 °C glass or digital thermometer two 25 cm3 measuring cylinders two weighing bottles stopwatch graph paper stirrer deionised or distilled water access to a digital mass balance (measuring to 2 decimal places).
method
a) Weigh out between 3.90 g and 4.10 g of anhydrous copper(II) sulfate in a dry stoppered weighing bottle, keeping the stock of solid in a closed container during weighing. Take care to avoid skin contact. The precise mass should be recorded. b) Construct a suitable table of results to allow you to record temperatures at minute intervals up to 15 minutes. c) Using a measuring cylinder, place 25 cm3 of deionised water into a polystyrene cup and record its temperature at the beginning (t=0), start the timer and then record the temperature again every minute, stirring the liquid continuously.
d) At the fourth minute, add the powdered anhydrous copper(II) sulfate rapidly to the water in the polystyrene cup and continue to stir, but do not record the temperature. At the fifth minute and for every minute up to 15 minutes, stir and record the temperature of the solution in the polystyrene cup. e) Plot a graph of temperature (on the y-axis) against time. Draw two separate best fit lines; one, which joins the points before the addition, and one, which joins the points after the addition, extrapolating both lines to the fourth minute. f) Use your graph to determine the temperature change at the fourth minute, which theoretically should have occurred immediately on addition of the solid.
* If the stock of anyhydrous copper(II) sulfate powder is not white, place in an evaporation dish in an oven at a low temperature or heat gently over a Bunsen burner before storing in a desiccator.
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alternative method
a) Weigh out between 6.20 g and 6.30 g of hydrated copper(II) sulfate in a dry stoppered weighing bottle. The precise mass should be recorded. b) Construct a suitable table of results to allow you to record temperatures at minute intervals up to 15 minutes as you did for Experiment 1. c) Using a measuring cylinder, place 24 cm3 of deionised water into a polystyrene cup. Since the hydrated crystals contain water, the total amount of water will be approximately the same as in Experiment 1. d) Repeat the procedure adopted in Experiment 1. Add the copper(II) sulfate crystals to the water in the polystyrene cup and obtain temperature data for 15 minutes. e) Plot a graph similar to that in Experiment 1 and determine the temperature change in this experiment
why results aren’t the same as the textbook values
Results obtained could vary considerably depending on the type of cup used, whether or not a lid was used of if there is a high temperature in the laboratory on the day. Lids on the polystyrene cups help to minimise heat loss and maximise change in temperature.
