Experiment – Getting Hotter & Getting Colder
Background
Chemicals exist in a particular form because it is energetically right for them. If they have too much
energy it is released into the surrounding environment (in our case – water). However, if they don’t have
enough energy they will absorb some from their surrounding environment. So essentially, if we monitor
the temperature of the surroundings we can deduce what energy changes have occurred during a reaction.
Dissolving
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Aim
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Dissolving
To observe what occurs to the temperature of water as a compound dissolves
Method
1.
Accurately measure 10mL of water into a test tube
2.
Accurately measure and record the temperature of the water.
3.
Record the mass of a small amount of sodium thiosulfate (Na2S2O3)
4.
Add the sodium thiosulfate to the water.
5.
Stir using the thermometer to dissolve the solid.
6.
Measure the maximum temperature change that occurs.
7.
Record your results.
8.
Repeat steps 1 – 6 using sodium hydroxide pellets and ammonium chloride instead of sodium
thiosulfate.
Analysis
1.
Identify any risks involved with the method.
2.
Describe the procedure for minimizing this risk.
3.
Comment on the accuracy of the temperature measurements gained.
4.
Suggest how the accuracy could be improved.
5.
Identify the reactions as either endothermic or exothermic.
6.
Outline the criteria you used to designate a reaction as exothermic.
7.
Outline the criteria you used to designate a reaction as endothermic.
8.
Use suitably labeled diagrams to show what happened in each reaction.
Let’s Think
The reaction for the dissolving of sodium hydroxide in water is shown.
Complete the equation by adding the term endothermic or exothermic.
NaOH (s) + H2O (l)  Na+ (aq) + OH- (aq) …………………………..
1.
Would the evaporation of water from the sodium hydroxide solution to produce sodium
hydroxide solid be exothermic or endothermic? Explain.
2.
Write an appropriate equation to show the formation of the solid sodium hydroxide from the
solution, including an expression to show whether it is endothermic or exothermic.
3.
Write similar equations to show the dissolving of sodium thiosulfate (Na2S2O3), and the
formation of solid sodium thiosulfate from its solution.
4.
For both reactions calculate the amount of energy gained or lost in the reaction.
5.
Assess the validity of the results. Is there a way to improve the method?
6.
Define the term reversible reaction in terms of dissolving and crystallizing.