I am going to do an investigation on how much acid is needed to neutralise a certain amount of alkali. The acid that I will use is hydrochloric acid and the alkali I will use is sodium hydroxide.
Hydrochloric acid (HCl) contains hydrogen and chlorine and is ionically bonded.
Hydrochloric acid looks like this:
Sodium Hydroxide (NaOH) is made up from Sodium, Hydrogen and oxygen. NaOH is ionically bonded and it looks like this:
Acids, Alkalis and Neutralisation
An acid is a proton donor and an alkali is a proton receiver. An Acid will have a pH less than 7 and an alkali will have a pH of more than 7. An acid with a pH of 1 is the strongest as ALL of the Hydrogen ions are ionised. The strongest alkali, with a pH of 14, is when ALL the hydroxide ions are ionised.
Neutralisation is when an acid and an alkali of equal strengths are mixed, the pH will be 7 and salt and water is produced. In more detail it is when ionised hydroxide and hydrogen meet and share electrons.
When a substance is diluted the concentration is weakened. The molarity changes when the water is added.
The indicator will change colour when neutralisation occurs, also the temperature will stop rising.
Choosing an Indicator
I will find the best indicators to use in my experiment, the three indicators that I will compare are:
I will find which indicator is more beneficial for my experiment by filling a test tube with about 1/3 of Sodium Hydroxide and then adding a small amount of an indicator in, the first indicator I used was the Universal Indicator. As I added the hydrochloric acid to the sodium hydroxide it became neutralised and the colours kept changing as the substance changed from acid to alkali, it was very difficult to establish when you have reached neutralization as the colour changes to easily.
When I added the Methyl Orange to another sodium hydroxide, it went red for the acid but wasn’t clear when neutralization had occurred.
When I used Phenol Phtalein the substance turned purple when an alkali and went clear when neutralization took place. It was very obvious when the substance was neutralised to the single drop. Therefore the indicator that I will use to judge when the substance is neutralised is the Phenol Phtalein
Measurement of Liquids
My most accurate method to measure the acid is by using a syringe pipette to create negative pressure and absorbing the water and releasing it into the measuring cylinder.
Neutralization of an Alkali with an Acid
HCl + NaOH =; NaCl + H2O
From this equation we know that we need the same molarity and the same volume of sodium hydroxide and hydrochloric acid, and this should reach neutralization. I will do a small experiment to find how much hydrochloric acid will neutralise 20ml of sodium hydroxide. I will do three attempts.
My first attempt showed that 20.50ml of acid neutralised 20.00ml of alkali. My second attempt showed that 20.40ml of acid neutralises 20.00ml of alkali.
My third attempt showed that 20.30ml of acid neutralises 20.00ml of alkali.
With a mean of 20.40ml of acid neutralising the alkali this is quite accurate as there is likely to have been human error and too much alkali put in the conical flask.
I predict that if equal volumes of acid and alkali are mixed and both are of the same molarity; neutralization will occur and heat will stop being given off. Neutralization is the point where no bonds are being made or broken, therefore no heat is given out. This in my opinion will make the temperature drop to room temperature.
These are the apparatus I will use in my experiment:
2 beakers (one large, one small)
I used a clamp stand to prevent the burette from falling and smashing. The burette was used to add small amounts of fluids so I could be more precise. I used an airsyringe to absorb the alkali safely. A conical flask was used because it is less likely to tip over and spill the substances. I used lots of pipettes because they are easy and simple to use, I could not use one as contamination would occur. I used a measuring cylinder so that I could measure the amount of liquid in it accurately.
My next diagram will show how I read the liquids in measuring cylinders and burettes:
Health and Safety
* The sodium hydroxide is very soapy so do not get it on the outside of beakers and other equipment as it is easy to drop.
* Place apparatus at a safe height so that people know its there and don’t accidentally knock it over.
* Use a conical flask when measuring the temperature with a thermometer as beakers will tip over.
* Wear a white coat to protect yourself from spillage of acids or alkalis.
* Put a piece of white paper underneath the beakers so that you know where you have spilt something and what you have spilt.
* Make sure the clamp stand is tight enough to support the burette but not smash it.
Exothermic and Endothermic
An exothermic reaction is when heat is given out during the reaction and heats up itself and the surroundings. Also in an Exothermic Reaction the products have less energy than the reactants.
This graph shows an exothermic reaction, the neutralization reaction gave out heat as we added the acid to the alkali. We did a second set of experiments to measure the heat produced from the reaction and to see if heat was no longer produced after neutralization.
In an Endothermic Reaction everything is made colder because heat is absorbed. Heat is absorbed because the products have more energy than the reactants.
When the acid and the alkali dissociate less energy is used to produce the salt and water, therefore heat energy is given off.
We did some preliminary experiments on the burette’s tap to increase accuracy by practicing at what speed we turn the tap as one drop can effect the pH by quite a lot.
The burette was filled with hydrochloric acid to the 0 mark and I gradually added it to the 20.0ml Sodium Hydroxide (with 5 drops of Phenyl Pthalein in it) in the conical flask. The Phenyl Phtalein started off pink but as almost an equal amount (20.0ml) of acid was added it turned colourless. I shaked the conical flask continuously so that I could see when it went colourless. This is how the experiment looked:
Results: Temperature, Attempt 1
Acid added to 20ml of Alkali (ml)
Acid added to 20ml of Alkali
Acid added to 20ml of Alkali
Acid added to 20ml of Alkali (ml)
Hydrochloric Acid (ml)
Sodium Hydroxide (ml)
My conclusion fully supports my prediction: an equal ratio of Alkali and Acid is needed for titration. This is shown in the titration experiment as all of the bars are near 20ml. The average acid needed to neutralise 20ml of alkali was 20.40, the extra 0.40ml was caused by human error. Also I found out that neutralization is exothermic; it gives off heat. I found this out because the line on the graph for the average temperature change during titration showed a steady increase in temperature until neutralization. I found on my graph that when neutralization is complete less heat is given off because the line goes down after 20ml; This happens because the reaction has finished and no more products are being made, the collisions stop, or are less frequent, and are not charged with energy.
The end point was at 19oC and 20ml of acid, when more acid was added after neutralization took place the temperature did not increase instead dropped towards room temperature. This is because the reaction has finished, no more bonds are being broken and no more heat energy is being given off. The collision theory explains this:
Particles are faster at high temperatures so there are more collisions and these collisions are more energetic because they are faster. The entire reaction is therefore faster.
In the higher temperature you can see that collisions are more vigorous and more likely to cause a reaction. Because temperature affects reaction rate, the reaction must affect the temperature, so when all of the molecules have formed another compound (NaCl + H2O in my investigation) the temperature must decrease because there are no more molecules able to break bonds.
If there was no human error I believe that the titration experiment would show that 20.00ml of acid is needed to neutralise 20.00ml of alkali.
I could have done better in my experiment by spilling less, I should have began using a conical flask rather then a beaker and this would have stopped me from spilling the alkali. The spillages took time to clean up making me have to rush my experiment and therefore made me more likely to make mistakes. However I do believe that my results were overall consistent and accurate, the human error was minimal. For example my tables showed that after 20ml of acid was added; the temperature always decreased. There were not any anomalies in the results.
Using an ammeter and a simple circuit I could investigate when neutralisation occurs by passing a current through it. This is a diagram of the experiment I could undertake:
I will be able to tell exactly when neutralisation occurs because the Ammeter will show a sudden drop in current. This is because the H+ and OH- molecules ionically bond to form water which will weaken the electrolyte therefore weakening the current passing through it.