Exothermic neutralisation reaction.

Exothermic Neutralisation Reaction.

Introduction.

The aim of this investigation is to try to find out how concentration can effect an exothermic neutralisation reaction. I will be reacting sodium hydroxide with hydrochloric acid, which neutralises to form sodium chloride a salt with water. Sodium hydroxide is an alkali and hydrochloric acid in an acid so therefore when reacted together neutralisation occurs. In this case when both substances are reacted, thermal energy is produced hence an exothermic reaction as thermal energy is radiated out.

The reaction will look like this:

NaOH + HCl H2O + NaCl

The factors that might affect the results of this experiment are:

Volume
Concentration
Type of substance used.
Catalyst
Pressure

Volume can affect the experiment because if it is not kept constant them results will be varied and there would be no proof that concentration can effect this exothermic neutralisation reaction, as the test would not be fair. Also if substances used were different this could affect the end result produced in this case a salt and water, as more reactive acids and alkalis can perhaps can produce a more vigorous reaction and therefore a higher temperature rise as more thermal energy is produced.

The input variable that I will change will be concentration. Concentration increases the number of collisions because there are more particles in the solution to collide with each other. So if a solution is more concentrated more particles are bumping into each other. Which makes more collisions so there are more reactions between particles, therefore more reactants that produce more water. When water is produced there bonds release energy therefore the more water produced by the reacting OH- and H+ ions the bigger the temperature rise. This is called collision theory examples of this are shown by the diagrams below.

I will record the temperature change of each experiment to find out how concentration can affect the output variable. I will keep the concentration of the alkali constant and change the concentration of the acid. I will be able to do this by recording the temperature before the start of the experiment and the temperature at the end of the experiment and then subtracting the two results away from each other. So as there will be different concentrations in different reactions this will mean different temperature changes. Therefore a different number of atoms in substances when volume is constant so different number of atoms react with each other. This means thermal energy produced by ions in different reactions will produce different temperature changes caused by formation of water.

Prediction

I predict that the experiment with the highest concentration of acid used will produce the highest temperature rise once reacted with alkali. This is because there are more ions in the higher concentration acid solution than the lower concentration solution, in the same volume. The diagrams below can show this:

2M alkali 2M acid

Complete + Complete

neutralisation

has occurred

2M alkali 1M acid

Complete

neutralisation

+ has not occurred,

spare OH- ion.

For example if I reacted 2 mol/l of NaOH with 2 mol/l Hydrochloric acid there will be an equal amount of ions that will react to form an exothermic neutralisation reaction likewise with 1 mol/l of each. But the 2 mol/l solution will produce the biggest temperature rise, as there will be double thermal energy produced in the volume of liquid, as the solution is more concentrated and there are more H+ and OH- ions to produce ...

This is a preview of the whole essay

2M alkali 2M acid

Complete + Complete

neutralisation

has occurred

2M alkali 1M acid

Complete

neutralisation

+ has not occurred,

spare OH- ion.

But if the concentrations reacted were different then this too would affect the exothermic neutralisation reaction. For example if I reacted 2 mol solution of NaOH with 1 mol of HCl like diagram two, only half of the OH- ions would be able to react to form water as the acid is at half the concentration to the alkali, therefore producing less of a temperature rise. As shown by the second diagram as water is produced but there is an OH- ion left in the solution so complete neutralisation has not occurred, which is the case in diagram one.

Moreover I predict that the most energy released will be when complete neutralisation occurs meaning that most water is produced will be 2 mol of acid and alkali. This leads me to think that if 2M of alkali were reacted with 1M of acid only half the ions are reacted to form the neutralisation reaction so less energy is produced. So 2mol/l of both substances will produce double the temperature rise as double the amount of ions are reacting in the same volume of solution. I know this because according to Avagrado’s law one mol of any element is 6.02x1023. So if I react equal volumes of the same molar concentrations of NaOH and HCl I am reacting the same number of atoms together therefore equal concentrations produce the highest temperature rise.

Method.

To test the temperature rise of each exothermic neutralisation reaction I will need:

Safety goggles
Paper towels
Beakers
Measuring cylinders
Sodium hydroxide
Hydrochloric acid
Thermometer
Deionised water

When starting this experiment firstly I need to gather all the equipment in the list above. In this experiment especially it is very important to wear safety goggles, as the acid and alkali are corrosive and irritant. It is important to have hair tied back and a clear work surface when conducting this experiment. Following this I shall place paper towels on the table so if there are any spillages they will be easy to clean up. I will measure the acid and alkali is separate measuring cylinders to prevent contamination and then record the starting temperature of the alkali before adding the acid. Once the reaction has completed I shall record the finishing temperature. Each experiment will be repeated twice in order to get a broader range of results so an average can be determined, making the results more reliable.

Diagram

Preliminary experiment One.

Using 2mol/l of HCl and NaOH I will conduct this experiment in order to determine what volume shall be used in the final experiment. I shall test 5ml, 10ml, 15ml and 20ml volume to see if there is any difference. The results I got were:

From these results I can see that volume did not affect the temperature rise for 10, 15 and 20ml of acid and alkali as it was 9oC but it did for 5ml, which was 5oC. This could be due to a number of factors that could be that a smaller amount of energy was created and a bigger surface area to volume ratio meant that more heat is lost. Also the 5ml volume does not cover the bottom of the thermometer so thermal energy is lost this way. I have decided to use 20ml of acid and alkali for the final experiment, as it will be easier to calculate 0.2 mol/l of acid etc.

Preliminary experiment two.

Using 20ml volume I will test 1mol/l of NaOH with HCl and vice versa. The aim of this experiment is to find out if it will make a difference which way the neutralisation reactions occurs. When using 1mol/l I will use 10ml of acid/alkali with 10ml of deionised water in order to create a 1mol/l solution with 20ml volume.

As you can see it makes no difference if the acid is added to the alkali or the alkali to acid. I have decided to add acid to alkali as instead of using a measuring cylinder, which was stated in the method I would measure the acid using a burette. But this is not possible to do with the alkali as it blocks the burette and sediments are left behind as the solution separates.

Preliminary experiment three.

The third preliminary experiment will help determine whether I will be able to test very small concentrations of the acid with the 2mol/l concentration of alkali, this will also help me find a range to work in from my results.

As there was a temperature rise at 0.2mol/l I will measure my results from 2mol/l to 0.2mol/l. There would be no point in going to 0.1mol/l as there degree of temperature would not be readable on the thermometer that I will be using. This will provide me with ten results, which I will have two sets of so that I can compare temperature rise and use the average to plot a graph and see if there are any patterns occurring in the results.

Results.

Analysis/Conclusion.

From my results I can see that when the concentration increases so does the temperature rise. When the concentration of the acid was 2mol/l the temperature rise was 11oC. I would expect that at 1mol/l acid concentration the temperature rise would half as only half the numbers of ions are able to react to form the exothermic neutralisation reaction. So using the 2mol/l result I would say that the temperature rise for 1mol/l would be 5.5oC, but in this case it was 6oC. This shows a degree of inaccuracy in the results, but as it was only half a degree out I would say that the results are fairly accurate. Overall there seems to be a direct correlation between concentration and temperature rise. The results form a very thin ‘s’ shape line that increase as the concentration increases. Showing that concentration does affect the exothermic neutralisation reaction.

I predicted that the experiment with the highest concentration of acid used would produce the highest temperature rise once reacted with alkali. This is because there are more ions in the higher concentration acid solution than the lower concentration solution, in the same volume. This was shown in my results proving the prediction correct as there was on overall increase in temperature rise as the concentration was increased, with 2mol/l with an 11oC temperature compared to the next result of 1.8mol/l acid of 10.5oC. However there were some dissimilarities between my results and my prediction. I also said that the most energy released would be when complete neutralisation occurs meaning that most water is produced will be 2 mol of acid and alkali. This leads me to think that if 2mol/l of alkali were reacted with 1mol/l of acid only half the ions are reacted to form the neutralisation reaction so less energy is produced. My results were half a degree out between 2mol/l concentration and 1mol/l concentration. This could be due to inaccuracy of reading of the thermometer as it is very hard to read the small scale that determines each degree, so it would be very easy to make the mistake to half a degree.

There were not any results that I did not expect to get as I predicted scientifically that there would be an increase in temperature as the concentration increases because complete neutralisation could occur producing the most water and releasing the most thermal energy from these bonds.

So the higher the concentration of acid the more neutralisation occurs, therefore more water if formed. This means more energy is released that results in a bigger temperature rise. Therefore a 2mol/l concentration of acid and alkali will produce twice as much temperature rise than the 1mol/l of acid and 2mol/l of alkali, as it has double the amount of ions reacting in the same volume that neutralise to form water.

Evaluation.

In order to keep the experiment a fair test I only change done variable that was the acid and kept everything else the same. In order to make readings more accurate a burette was used to measure the volume of acid. However this was not possible to use with the alkali so a measuring cylinder was used. I made sure that the different liquids we kept separate to prevent confusion and also to prevent contamination because a reaction would already take place if it did happen which would cause the results to change. Also after each individual test the measuring cylinder, beaker and thermometer were washed.

Overall I am happy with the results that I obtained as they fir the pattern that I suggested would occur in my prediction. However there is a degree of anomalous results as I thought the results would go up steadily as continuous data from 1oC to 10oC for example. Instead the average results went from 1oC, 3oC, 4oC, 5.5oC, 6oC, 7oC, 8oC, 9.5oC, 10.5oC, and 11oC. So there were no regular intervals between each 0.2mol/l difference in concentration, which is what I would have expected. This could have been due to inaccurate readings when measuring out the levels of concentrations of the different liquids. Or inaccurate readings from the thermometer as the degrees are hard to read on a small scale. An improvement to this to get an accurate reading could be to use a digital thermometer, as it is accurate to 0.1 of a degree rather then reading the thermometer itself. This could make the experiment more accurate; with results that show the pattern of double the temperature rise at double the concentration. Also I have mentioned how a burette was used to measure the acid only as NaOH will block the burettes with sediments, so there is some inaccuracy with measuring, that may be why the results were slightly out. Another possibility could be to filter the NaOH and the use a burette to measure it.

If I were to do the experiment again I would test concentrations above 2mol/l to prove my prediction correct beyond doubt. I would predict that after 2mol/l of acid instead of there being OH- ions left in the solution there will be H+ ions left from the excess HCl. This will change the shape of the graph as it will no longer increase but decrease as no more complete neutralisation will occur. This means the graph will level off and produce the same temperature rise as 2mol/l concentration of each. I could further the testing by after reacting each different molar concentration of HCl with NaOH I would test it with litmus paper. This will enable me to show whether complete neutralisation has occurred. If it has occurred then the litmus paper would turn green, if not then an acidic solution would turn red and alkali purple.

To further the idea of my prediction I could use a different acid for example sulphuric acid (H2SO4). Sulphuric acid has two hydrogen bonds compared to HCl. Therefore instead of getting a bigger temperature rise at 2mol/l of each you would get it at 1mol/l of acid and 2mol/l of alkali. This is because of the two hydrogen bonds, so at 1mol/l there is the same amount of hydrogen bonds as the alkali.

Overall I am pleased with the results that I obtained as they agreed with my prediction that 2 molar concentrations of acid and alkali would produce complete neutralisation and therefore the biggest temperature rise. This is due to the most formation of water, when it occurs the bonds release thermal energy producing the exothermic reaction causing the temperature rise.

By Amareen Bhambra 11B.