Neutralisation Investigation

Moles

A single mole of an element contains 6 × 1023 atoms of that element.  It is a convenient unit of measurement as this amount of atoms has a mass equal to that of the relative atomic mass of the element in grams.  This number is called the Avogadro Constant.  Therefore one mole of Hydrogen (6 × 1023 atoms of hydrogen) weighs 1g, and one mole of Magnesium (6 × 1023 atoms of Magnesium) weighs 24g.  When we refer to solutions as having 1M concentration, we mean that it has 1M of hydrogen in every litre of the solution.

Planning

I have been asked to find out how different factors affect the energy released in a reaction between an acid and an alkali.

There are 4 main factors that could be investigated: the temperature of the acid or alkali, the quantities of the acid or alkali, the dilution/concentration of the acid or alkali, and the pH of the acid or alkali (strong/ weak).

The factor I have chosen to investigate is the concentration of the acid. This is the input/ independent variable.

I will have to keep the controlled variables the same in order to ensure that any change in readings obtained are due to the change of my input variable only.  To make it a fair test, I will try to keep the temperature the same throughout the investigation. I will keep it away in the same place for every experiment ensuring that the room temperature is the same. I will use a fixed quantity of alkali (20cm3) and a fixed quantity of acid and water (20cm3). To do this accurately I will use a burette to measure out the quantities. I will be measuring the temperature change as the outcome/ dependent variable.

The problem is to find out how much energy is released as the concentration varies.

I predict that the higher the concentration of the acid solution, the greater the temperature change will be, and the greater the energy released.

I have arranged to get sufficiently accurate and reliable results by using a burette to measure the quantities of solution, I will use polystyrene cups instead of glass cups and wrap cotton wool around it for the reactions to reduce temperature loss, and lastly I will repeat the experiment twice to get more reliable results. When using the burette, I will check the markings before and after at eye level to get the measurements as accurate as possible. The polystyrene cup will also have a lid to reduce heat loss.

Linking prediction with theory: The higher the number of H+ and OH- particles there are, the higher the temperature change. The higher the concentration of acid, the more the acid (H+) ions the solution contains.  By changing the proportion of water to acid, we can change the total number of H+ ions, and therefore the density of H+, in any given area of the solution, in each test. This means that a strong acid will have more H+ ions in any given area than one with a weaker acid. So the more the H+ ions there are, the more chance these ions will come into contact with the OH- ions to react. When they react together, they make new bonds. Creating new bonds create energy, as this is an exothermic reaction. This energy will be shown to us as a temperature rise (change). During the reaction, there will be a sudden increase in temperature- this is the reaction taking place. There will eventually be a time when the temperature change decreases in its acceleration. This slowness in temperature change is when there are no more H+ ions to react with the OH- ions.

There are risks in this investigation as we are using harmful acids and alkaline so we must minimise them. Acids are particularly harmful to the skin and eyes therefore I will be wearing safety goggles, a lab coat, and protective gloves. I will also have to wipe up any spillages.

Method

Equipment list:

Burette

Thermometer

Polystyrene cups

Beakers

Cotton wool

Spillage mats

Glass rod

Diagram:

Procedure:

1. Measuring the required amount of acid (hydrochloric) from a burette into a measuring cylinder
2. Measuring the required amount (20cm3) of alkali (sodium hydroxide) from a burette into a measuring cylinder
3. Measuring the required amount of water (distilled) from a burette into a measuring cylinder
4. Pour the acid and water into a beaker (if necessary) and stir gently with a glass rod
5. Get the polystyrene cup ready with cotton wool taped around the sides
6. Pour the alkali into the cup and record the temperature with the thermometer.
7. Add the solution of acid and water and quickly, put the lid on top of the cup
8. Stir gently to speed up reaction.
9. When the temperature of the mixture has stopped increasing (i.e. the reactants have reacted fully), record the final temperature.
10. Repeat the experiment again for more reliable results
11. Repeat the experiment for other concentrations and record all results

Solution Ratios:

I have selected these values because these are a good range of concentrations and they will hopefully give me good results that I can analyse easily. I will repeat these measurements twice to get more reliable results.

Preliminary Results

I think that these results are very good and they are what I had expected. These results show that the higher the concentration of acid, the higher the energy rise. This means that the more the H+ ions there are, the more the energy there is. I will not change my method because my current one gives me accurate and reliable results.

Obtaining Evidence

My preliminary experiment showed that the different concentrations and quantities I used worked well to give reliable results. The table above shows the results for both the 1st results, and the 2nd results- the repeated ones. I have then taken an average for each of the different molarities.

To make my experiment more accurate, I did several things. In the actual experiment I used a more accurate mercury thermometer to measure the temperature, a thermometer that measured to the nearest tenth of a degree. When measuring the solutions from the burettes, I made sure that I was at eye level to the solution. I.e. my eye was horizontal to the mark where I wanted. I also did this with the mercury thermometer. When I was recording the final temperatures of the reacted/ reacting solution I waited until the temperature was at its very highest so that I could get maximum results for the temperatures.

Analysing Evidence

To calculate the energy released we have to use the formula:

Energy change = Temperature change × total volume × specific heat capacity

The specific heat capacity of the acid and alkali is 4.2J /g /°C. The total volume will always be 40cm3 as we used a total of 40cm3 in each different molarity. Below is a table with the results converted from the temperature change to the energy released.

We can also calculate the number of moles of acid being neutralised using this formula:

                    V        ×        C

1000

C stands for concentration (M), and V stands for volume (cm3).

We then use this formula:

Energy Change

Mole

The table above shows the number of moles that each concentration of acid has. The average of the moles is –58.765 KJ/Mol. The official number for energy given off by moles for an acid and alkali reaction is –57.0 KJ/Mol. My number and the official number is almost the same and so conclude that my results were very accurate altogether.

The results on my graph tell me that as the concentration of acid increases, the energy released increases. The two axes are directly proportional to each other- as x increases y increases. My results agree with my initial prediction that as the concentration of acid increases, the energy released increases. This is because as the acid concentration is increased, there are more H+ ions available.  With more H+ ions available, more bonds can be made; the process of making bonds is exothermic, and so energy is produced, which we measured as heat energy. Looking at the graph you can see that it is straight line through the origin. This tells me that when there is no acid, there is no energy released. This supports my theory that when there are no H+ ions, there is nothing to react with the OH- ions therefore there is no energy given out. From this, we can say that there must be H+ ions present for there to be any energy change – the temperature increase we are investigating is dependent upon the bonds formed between OH- and H+; no bonds can be made without either reactant.

        If I were to extend the graph so the x-axis extended past a molarity of 2.0, we would see a change in the pattern. At the ratio of 20cm3 acid: 20cm3 alkali we get a large output of energy from the reactants. If there were more acid than alkali, we would expect to get the same output of energy from the reactants as a ratio of 20cm3 acid: 20cm3 alkali. The line on the graph would start to flatten to a horizontal line (a gradient of 0), as shown below.

        This is because the H+ ions have fully reacted with the OH- ions; increasing the number of H+ ions (increasing the concentration) will have no effect on the energy released, as there will be no more OH- ions to react with. All the H+ ions will have reacted with the OH- ions therefore there will be no OH- ions for the H+ ions to react with.

        My results are what I not really what I had expected as I thought that the graph would be a curve because the H+ ions would run out of OH- ions to react with. This was the case except it would start to happen after a molarity of 2.0. Maybe this was the case and my graph did not show it. If I took more results and repeated them more I could have come to a more definite conclusion. Having said this, my current graph would be a curve if I had extended it like shown in the above diagram. It is sort of like a curve.

Evaluation

I found my experiment fairly easy to carry out. I didn’t have to change my plan at all having done a preliminary. It all went quite smoothly considering I was pushed for time. This was the only real problem for me. Because I had a total of 12 different concentration experiments to do I had to fill up 12 different beakers of solution from the burette. To do this accurately it wasn’t possible for me to rush this process. In the end I got used to doing it and got all the solutions measured carefully and fairly accurately. Some people had the problem that they could not put the lid onto the cup quick enough. This would have, for a short while, allowed heat from the reaction escape through the top of the cup.  I tried to be as quick as possible to put the lid on, to ensure that the effect of this was insignificant. I had no real problems with this.

I think that I did take enough results and the range of them was large enough. I had a good set of different molarities that had good regular intervals between them. If I were to repeat the experiment, I would repeat the results so that I would have a total of 3 results for each molarity to even out any anomalies. Looking at my graph, I think I would like to take some more results near the 2.0 molarity mark to fill in some gaps. I would look at molarities higher and lower the 2.0 molarity mark so that I could find out whether the line would curve off and flatten for definite.

I think that my results were fairly accurate, as most of them lied close to the line of best fit. If I were to do this experiment again, I would not change the way I measured things. I measured them accurately. There was one anomalous result. This is shown in the table in bold on page 4 and again it is ringed on the graph. I don’t really know what had caused it- maybe I had not cleaned the polystyrene cup out before I added the new solutions. Or maybe I was too slow in putting the thermometer into the solution. I think that my results are consistent with each other. There weren’t really any that were so different that I should have ignored. The anomalous result gave me a temperature change of 7 whereas when I had repeated I got a temperature increase of 12. This is a considerably large difference but I decided that I would not ignore it. I took the average like the other readings and plotted, circling it as an anomalous result. However I did ignore it when plotting the line of best fit.

To extend my inquiry I can look at many things. Having looked at strong acids and alkaline (hydrochloric acid and sodium hydroxide solution) I can also look at weak acids and alkaline. However, the results should be the same except I would expect to get less energy from the weak acids/ alkaline. I could also look at mixing the tow: a weak acid with a strong alkali or vice versa. This could give some interesting results although they might not be fair. Looking at different acids and alkaline could also be done to extend my inquiry. This also links in with strong and weak acids. We could look at sulphuric acid or calcium hydroxide solution. This also links in with pH. We could use acids and alkaline of different pH’s to see whether they follow the same pattern as the one we investigated. Another possibility would be looking at acids reacting with metals.