1.7 Energy changes in chemical reactions  (Page 2/5)

Method:

1. Cut a piece of plastic from the ice-cream container lid that will be big enough to cover the top of the beaker. Cut a small hole in the centre of this piece of plastic and place the thermometer through it.
2. Pour some citric acid (H ${}_3$ C ${}_6$ H ${}_5$ O ${}_7$ ) into the glass beaker, cover the beaker with its 'lid' and record the temperature of the solution.
3. Stir in the sodium bicarbonate (NaHCO ${}_3$ ), then cover the beaker again.
4. Immediately record the temperature, and then take a temperature reading every two minutes after that. Record your results in a table like the one below.

The equation for the reaction that takes place is:

$H_3{C}_6{H}_5{O}_7\left(aq\right)+3NaHC{O}_3\left(s\right)\to 3C{O}_2\left(g\right)+3H_{2}O\left(l\right)+N{a}_3{C}_6{H}_5{O}_7\left(aq\right)$

Results:

• Plot your temperature results on a graph of temperature against time. What happens to the temperature during this reaction?
• Is this an exothermic or an endothermic reaction?
• Why was it important to keep the beaker covered with a lid?
• Do you think a glass beaker is the best thing to use for this experiment? Explain your answer.
• Suggest another container that could have been used and give reasons for your choice. It might help you to look back to Grade 10 for some ideas!

Demonstration : endothermic and exothermic reactions 2

Apparatus and materials:

Vinegar, steel wool, thermometer, glass beaker and plastic lid (from previous demonstration).

Method:

1. Put the thermometer through the plastic lid, cover the beaker and record the temperature in the empty beaker. You will need to leave the thermometer in the beaker for about 5 minutes in order to get an accurate reading.
2. Take the thermometer out of the jar.
3. Soak a piece of steel wool in vinegar for about a minute. The vinegar removes the protective coating from the steel wool so that the metal is exposed to oxygen.
4. After the steel wool has been in the vinegar, remove it and squeeze out any vinegar that is still on the wool. Wrap the steel wool around the thermometer and place it (still wrapped round the thermometer) back into the jar. The jar is automatically sealed when you do this because the thermometer is through the top of the lid.
5. Leave the steel wool in the beaker for about 5 minutes and then record the temperature. Record your observations.

Results:

You should notice that the temperature increases when the steel wool is wrapped around the thermometer.

Conclusion:

The reaction between oxygen and the exposed metal in the steel wool, is exothermic , which means that energy is released and the temperature increases.

The heat of reaction

The heat of the reaction is represented by the symbol $\Delta H$ , where:

• In an exothermic reaction, $\Delta H$ is less than zero because the energy of the reactants is greater than the energy of the product. For example, $H_2+C{l}_2\to 2HCl$ $\Delta$ H = -183 kJ
• In an endothermic reaction, $\Delta H$ is greater than zero because the energy of the reactants is less than the energy of the product. For example, $C+H_{2}O\to CO+H_2$ $\Delta$ H = +131 kJ

Some of the information relating to exothermic and endothermic reactions is summarised in [link] .

Enthalpy

Enthalpy is the heat content of a chemical system for a given pressure, and is given the symbol 'H'.