Exothermic and Endothermic Reactions » Required Practical: Temperature Changes

The aim of the practical

This is required practical 4. You investigate the variables that affect temperature changes in reacting solutions. The spec gives these reaction types as examples: acid plus metals, acid plus carbonates, neutralisations and displacement of metals.

In each one you mix two things, stir, and watch the thermometer. A rise means the reaction is exothermic. A fall means it is endothermic. The practical is about measuring that change well, and finding out what makes it bigger or smaller.

Key terms:

• Temperature change: the difference between the starting temperature and the highest (exothermic) or lowest (endothermic) temperature reached. A fall is a decrease, for example a fall of 8 °C.
• Insulation: a material that slows down the loss of heat to the surroundings.

Apparatus and method

Here is a standard method for a neutralisation, using dilute hydrochloric acid and sodium hydroxide solution.

1. Put a polystyrene cup inside a glass beaker. The beaker stops the cup tipping over.
2. Use a measuring cylinder to measure 25 cm³ of dilute hydrochloric acid. Pour it into the cup.
3. Put a thermometer in the acid. Wait, then record the starting temperature.
4. Measure 5 cm³ of sodium hydroxide solution and add it to the acid.
5. Stir gently with the thermometer or a stirrer. Record the highest temperature reached.
6. Keep adding the sodium hydroxide, 5 cm³ at a time, recording the highest temperature each time.
7. Repeat the whole experiment at least twice more.

For acid plus a metal or carbonate, you add a measured mass of the solid (for example 1.0 g of magnesium ribbon) instead of the alkali. For displacement, you add a metal powder such as zinc to copper sulfate solution. In every version the steps are the same: measure the start temperature, mix, stir, record the highest temperature.

This links to the spec skills: measuring mass, temperature and volume accurately, and careful mixing of reagents under controlled conditions.

Why insulation matters

Even with a cup, some heat is still lost. So the temperature rise you measure is usually a little smaller than it should be. Remember this when you evaluate your results.

Variables

You can only change one thing at a time, or the test is not fair. You choose which variable to investigate. Here are some options.

• Changing the volume or concentration of the acid and keeping the alkali the same.
• Changing the mass of the metal added to a fixed volume of acid.
• Changing the type of metal, such as magnesium, zinc and iron, with the same acid.

Start with a prediction, such as: the more zinc you add, the bigger the temperature rise, because more particles react and release more energy.

The dependent variable is always the temperature change. Everything else is a control variable and must stay the same. These are the starting temperature, the volumes, the concentrations, the mass of solid, the type of cup and how you stir.

For a metal, use the same surface area each time. Powder reacts faster than a lump, so mixing powder with ribbon would not be fair.

Recording and processing results

Draw a results table before you start. Include a column for the starting temperature, the highest temperature, the temperature change and each repeat.

Work out the temperature change for each repeat first, then find the mean. Give your answer to a sensible number of significant figures, matching your thermometer readings. If one repeat is very different from the others, it is an anomaly. Check it and repeat it.

To show a pattern, plot a graph. Put the thing you changed (the independent variable) on the x-axis and the temperature change on the y-axis. If you added more and more zinc to the same copper sulfate solution, the temperature change rises and then levels off once all the copper sulfate has reacted. Draw a line of best fit. If the temperature change rises then levels off, draw two straight lines that meet where the temperature stops rising.

Safety

• Wear eye protection. Acids and alkalis can damage eyes.
• Dilute acids and alkalis can irritate skin. Wash splashes off with plenty of water.
• Acids with metals and carbonates give off gas. Add the solid slowly, because carbonates can froth up and spill.
• Mop up spills straight away and keep glass thermometers away from the table edge.

Evaluating the method

The spec asks you to evaluate methods and suggest improvements. Common ideas are below.

• Use a lid and a polystyrene cup to cut heat loss.
• Use a digital thermometer or a temperature probe for more precise readings.
• Stir the same way each time so the reaction mixes fully.
• Repeat the experiment and use the mean to reduce the effect of random errors.

Exam-style question

A student adds 2.0 g of magnesium ribbon to 30 cm³ of dilute hydrochloric acid in a glass beaker. The temperature rises from 20.0 °C to 31.0 °C. The student repeats the experiment with 2.0 g of zinc powder and gets a rise of only 6.0 °C.

(a) Calculate the temperature change for magnesium. (b) Suggest two ways the student could improve the method. (c) Give two variables the student must control to compare the metals fairly. (d) Is the student's comparison fair? Explain your answer.