To find out how dilute hydrochloric acid is needed to neutralise 25cm³ of an alkali called sodium hydroxide.
Neutralisation Investigation
Aim:
To fin out how dilute hydrochloric acid is needed to neutralise 25cm³ of an alkali called sodium hydroxide.
Background information
Substances that neutralise acids are called bases. Bases are alkali - they have pH's above 7; acids are acidic, and have pH's below 7. Bases that can dissolve in water are also known as alkalis.
Introduction:
Acid
Acids are compounds of non-metals with simple molecular structures. They all contain hydrogen (H) covalently bonded to other elements, like HCl, or H2SO4 (hydrochloric and sulphuric acid respectively). When an acid is dissolved in water, its molecules ionise. The hydrogen present in every acid has the potential to ionise; if it does, it becomes an H+ ion. It is the H+ ions that gives acids their acidic properties. The amount of an acid's molecules that ionise in water determines the strength of its acidity. Vinegar, for instance, is a weak acid, an only 1 in about 100,000 of its molecules ionise; on the other hand, almost all molecules of HCl ionise. This means that in an acid-base reaction involving vinegar, there are less H+ ions available to react, and so less bonds are made compared to an acid-base reaction involving HCl. Bond making is exothermic, and so less energy is given out. Stronger acids (those with a higher number of H+ particles that can dissolve in water) have a lower pH.
Bases
Just as all acids contain H+ ions, all alkalis contain OH- ions, and it is this OH- (hydroxide) ion that gives alkalis their alkali properties.
Bases are usually oxides, hydroxides, or carbonates of metal. Ammonia is unusual in this respect, as it contains no metal element. Alkali substances are also very corrosive, and can do even more damage to living cells than acids.
The neutralisation reaction
NaOH(aq) + HCl (aq) NaCl(aq) + H2O(l)
The above is an example of a neutralisation reaction, involving an alkali and an acid. The result is a salt and water. In every neutralisation reaction, the metal in the base (Na+ here) takes the place of the Hydrogen in the acid, forming a metal compound called a salt. The term salt is used to describe any compound formed by the reaction between a base and an acid.
From the above equation, we can break up these molecules into this:
Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) Na+(aq) + Cl-(aq) + HHO(l)
The Na+ and Cl- on the left side of the equation are present at the end of the equation (on the right side), and are known as spectator ions, as they have not chemically reacted. Therefore, the equation can be rewritten, with just the reactants, as:
OH-(aq) + H+(aq) H2O(l)
This equation occurs in all acid/alkali neutralisation reactions where the salt is soluble - hydrogen ions from the acid react with hydroxide ions from the alkali to form water. The neutralisation reaction is exothermic because of the bonds being made (OH- + H+ bonding together to make H2O). This reaction can be generalised to apply to any reaction between a base and an acid:
acid + base salt + water
If the base happens to be a carbonate, then carbon dioxide is formed as well (e.g. calcium carbonate, CaCO3):
CaCO3 + 2HCl CaCl2 + CO2 + H2O
Calcium + Hydrochloric Calcium + carbon + water
carbonate acid chloride dioxide.
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 ...
This is a preview of the whole essay
acid + base salt + water
If the base happens to be a carbonate, then carbon dioxide is formed as well (e.g. calcium carbonate, CaCO3):
CaCO3 + 2HCl CaCl2 + CO2 + H2O
Calcium + Hydrochloric Calcium + carbon + water
carbonate acid chloride dioxide.
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 (25cm3) and a fixed quantity of acid and water (25cm3). 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.
Prediction:
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.
Safety:
To keep this test safe for my self and others around me I must ensure that all precautions are taken to avoid accidents.
Firstly during the practical I will be using Hydrochloric acid, which could be potentially harmful. Hydrochloric acid, if it comes into contact with the eye, cause blindness. It can also irritate the skin.
Finally after the experiment has been carried out I must ensure that hands are washed thoroughly so I do not eat etc: with chemicals in my hands.
To make sure I will be reasonable safe...
> Hair should be tide back
> Bags etc should be under the table
Apparatus list:
I have found I need the following apparatus
Equipments:
Beakers
Burettes/stand
Conical flasks
Safety goggles
Measuring cylinder
pH charts
Polystyrene cup small funnel
Universal indicator etc
Chemicals:
Hydrochloric acid (HCl);
Sodium hydroxide (NaOH);
Fair Testing:
To make sure that the test is fair I will measure the amounts of each liquid very carefully. Each time I start a new test, I will have washed and dried the beaker out as not to leave any of the liquids from the last experiment in. Instead of measuring the amount from a certain temperature, I will take the start temperature and then one at the end. The difference will then be taken. I will use a different measuring cylinder for each liquid, and the water will be taken from the tap. The Hydrochloric acid and the Sodium Hydroxide will be taken from a bottle. I will time the experiments for approximately 3 minutes each. This should give time for the different liquids to mix together and hopefully neutralise. There are many things in the experiment that could be considered not fair but that is why we are doing the experiment, to see how the different amounts of each liquid react. In each of the experiments the total amount of liquid in the mixtures will be 50ml. One of the experiments does not contain water, but otherwise, the lowest amount in any one is 20.7ml and the highest 20.7ml. Having 50ml of liquid in each mixture will keep the experiment fair.
Method:
. I will add 25cm of sodium hydroxide (NaOH) to the conical flask and then add few drops of hydrochloric acid (HCl) the colour will be change.
2. I will add hydrochloric acid to the burette and note down the volume at a certain point.
3. I will be according the temperature, pH, colour and volume of acid
4. I will start adding the acid slowly when I am near the end point
5. I will know when the alkali neutralised, when the colour turns green.
6. I will make several repeats to make my results accurate.
7. I will use a polystyrene cup because I will use lose less heat
8. I can make the experiment accurate by cleaning all the equipments
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.
Table: 1
Volume of acid
Added in cm³
Colour of
Universal indicator
pH
Temperature
°C
O
Blue
11
21.5
20.1
Blue
11
27.7
20.5
Blue
11
28.1
21
Blue
11
27.9
21.3
Blue
11
28.1
21.8
Blue
11
28.1
22.2
Blue
11
28.1
22.4
Blue
11
28.1
22.8
Blue
11
28.1
23
Blue
11
27.9
23.1
Blue
11
27.9
23.2
Green
6
27.6
Table: 2
Volume of acid
Added in cm³
Colour of
Universal indicator
pH
Temperature
°C
0
Purple
12
20.7
15
Blue
11
26
17
Blue
11
26.4
19
Blue
11
26.7
20
Blue
11
26.8
20.3
Blue
11
26.7
20.7
Blue
11
26.7
21
Blue
11
26.7
21.4
Blue
11
26.7
21.6
Blue
11
26.7
21.9
Blue
11
26.7
2 2.1
Blue
11
26.7
22.4
Blue
11
26.7
22.6
Blue
11
26.7
23
Blue
11
26.7
23.3
Blue
11
26.6
23.6
Blue
11
26.6
24
Blue
11
26.6
24.2
Blue
11
26.6
24.5
Blue
11
26.6
24.9
Blue
11
26.5
25.1
Dark green
8
26.4
24.5
Green
6
26.2
Analysing evidence:
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 as I 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. 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. 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 2 concentration experiments to do I had to fill up 2 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.
I think that I did take enough results. I would repeat the results so that I would have a total of 2 results. 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. I don't really know what had caused it- maybe I had not cleaned the polystyrene cup out before I added the new solutions. I think that my results are consistent with each other. There weren't really any that were so different that I should have ignored.
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. 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.
Izza Chaudhry
(11y1)
