Exothermic Reaction
Exothermic reactions are those that give off heat energy. Some energy is needed to start the reaction, to break apart the bonds of the reacting compounds (the reactants), but then as the chemicals recombine to form the products of the reaction so even more energy (usually as heat) is given off; there is a net amount of energy given out to the surroundings. The enthalpy of the products is lower than that of the reactants, as indicated in this energy profile. The equation for the enthalpy (H) or, rather, the enthalpy change (ΔH) of an exothermic reaction is also included; ΔH is taken to be negative for an exothermic reaction. Exothermic reactions are more common than endothermic reactions precisely because they only require activation energy to start, then they will give off energy rather than require it.
Prediction:
I predict that the lower the concentration of the hydrochloric acid the less heat will be given off. This is because there is a smaller probability that hydrogen ions will collide with hydroxide ions with enough energy to form bonds forming water. There is less probability as there are less partials per litre.
Preliminary work:
Hydrochloric acid and sodium hydroxide with the hydrochloric acids concentration changing.
This shows me that there is still enough of a change when the concentration of the hydrochloric acid is 0.125M to use this concentration. It also shows me that with a concentration of hydrochloric acid at 2M the temperature does not go of the scale.
Method:
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Measure out 50cm3 of 2M hydrochloric acid
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Measure out 50cm3 of 1M sodium hydroxide
- Take the temperature of both hydrochloric acid and sodium hydroxide then average the temperatures
- Add the hydrochloric acid and sodium hydroxide to a polystyrene cup
- Take the peak temperature
- Find the difference between the average start temperature and the peak temperature
- Repeat steps 1-6 changing the concentration of hydrochloric acid to 1M, 0.5M, 0.25M and 0.125M
- Repeat steps 1-7 twice.
Fair test:
The variable I am using for the experiment is concentration. I will not alter any other variables such as pH (acid/alkali used) and volume as they will affect the results I get.
Safety:
To keep safe I will wear goggles all the time and I will read all risk assessments relevant to the chemicals I am using.
Equipment list:
- Measuring cylinders x2
- Thermometer
- Polystyrene cup
- 1M sodium hydroxide (NaOH)
- 2M, 1M, 0.5M, 0.25M, 0.125M hydrochloric acid (HCl)
Results
Graph
Analysis
From my results I can see that I was right in my prediction when I said that the lower the concentration of the hydrochloric acid the less heat will be given off. This is because there is a smaller probability that hydrogen ions will collide with hydroxide ions with enough energy to form bonds forming water. There is less probability as there are less partials per litre. I also used the equation E=MC0 to find out the energy produced. I can also see that there were some anomalous results but the general trend fits my prediction.
