The Lewis Theory:
The American chemist Gilbert Lewis has put another theory that provides a very broad definition of acids and bases forth. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF3, can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.
Neutralization, chemical reaction, according to the Arrhenius theory of , in which a water solution of acid is mixed with a water solution of base to form a and water; this reaction is complete only if the resulting solution has neither acidic nor basic properties. Such a solution is called a neutral solution. Complete neutralization can take place when a strong acid, such as hydrochloric acid, HCl, is mixed with a strong base, such as sodium hydroxide, NaOH. Strong acids and strong bases completely break up, or dissociate, into their constituent ions when they dissolve in water. In the case of hydrochloric acid, hydrogen ions, H+, and chloride ions, Cl-, are formed. In the case of sodium hydroxide, sodium ions, Na+, and hydroxide ions, OH-, are formed. The hydrogen and hydroxide ions readily unite to form water. If the number of hydrogen ions in the hydrochloric acid solution is equal to the number of hydroxide ions in the sodium hydroxide solution, complete neutralization occurs when the two solutions are mixed. The resulting solution contains sodium ions and chloride ions that unite when the water evaporates to form sodium chloride, common table salt. In a neutralization reaction in which either a weak acid or a weak base is used, only partial neutralization occurs. In a neutralization reaction in which both a weak acid and a weak base are used, complete neutralization can occur if the acid and the base are equally weak. The heat produced in the reaction between an acid and a base is called the heat of neutralization. When any strong acid is mixed with any strong base, the heat of neutralization is always about 13,700 calories for each of acid and base neutralized.
Acids and bases can be classified as organic or inorganic. Some of the more common organic acids are: , , , salicylic acid, , and . Some examples of organic bases are: and ethylamine. Some of the common inorganic acids are: hydrogen sulfide, , , and sulphuric acid. Some common inorganic bases are: , sodium carbonate, sodium bicarbonate, , and .
Acids, such as hydrochloric acid, and bases, such as potassium hydroxide, that have a great tendency to dissociate in water are completely ionized in solution; they are called strong acids or strong bases. Acids, such as acetic acid, and bases, such as ammonia, that are reluctant to dissociate in water are only partially ionized in solution; they are called weak acids or weak bases. Strong acids in solution produce a high concentration of hydrogen ions, and strong bases in solution produce a high concentration of hydroxide ions and a correspondingly low concentration of hydrogen ions. Strong acids and strong bases make very good electrolytes i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes.
Chemists use methyl orange as an indicator in the titration of weak bases with strong acids. It changes from red (at pH 3.1) to orange-yellow (at pH 4.4). pH-related color changes result from changes in the way electrons are confined in a molecule when hydrogen ions are attached or detached.
A titration is a method of analysis that will allow you to determine the precise endpoint of a reaction and therefore the precise quantity of reactant in the titration flask. A burette is used to deliver the second reactant to the flask and an indicator or pH Meter is used to detect the endpoint of the reaction.
Apparatus:
-Volumetric flask (250cm³)
-Conical flask (250cm³)
-Burette
-Bulb pipette (25cm³)
-Pipette
-Glass rod
-Funnel
-Beaker (250cm³)
-Top pan balance (accurate to 2 decimal places)
-Methyl orange indicator
-White tile
-Retort stand
-Boss + clamp
-Distilled water
-Anhydrous sodium carbonate
Diagram:
Method:
Put a beaker on the weighing scales and accurately add 2.65g of anhydrous sodium carbonate, weighing by difference reduces the percentage error. Then dissolve the powdered sodium carbonate in a beaker by distilled water, stirring with a glass rod. Then make the solution up to 250cm³ in a volumetric flask washing all the equipment used with distilled water. To get the volume up to the marker you must use a pipette for the last bit. The volumetric flask must be inverted to make the concentration equal. Set up a burette and first of all rinse through with acid so that the acid is not diluted or contaminated. Then fill the burette with the unknown acid solution. Transfer the sodium carbonate to a beaker. Use a bulb pipette to transfer 25cm³ of this solution to a conical flask you must touch the end of the bulb pipette on the side of the conical flask to get the last drop. Add 3 drops of methyl orange solution to the conical flask as to many drops will make the colour change harder to see. Add the acid solution from the burette while swirling the flask as you add the acid until the indicator changes colour, this allows you to find an approximate value. A white tile was put under the conical flask so it was easier to see the colour change. Repeat the titration until you get 3 values with in o.1 of each other. To get an accurate reading you must run the acid in until about 3cm³ before your test titre value swirling as you do. Then add the acid drop by drop and when you get a colour change that last a while before changing back to orange then you need to add half a drop of acid and rinse the sides of the conical flask with distilled water. To add half a drop you must get the acid to start forming an acid drop on the end of the burette, and then bring the conical flask so the side touches the burette then rinse with distilled water.
Risk assessment:
I shall be using glassware that has the potential to be a hazard I will be wearing safety specs throughout, to stop chemicals or foreign objects from making contact with my eye. If there is an accident with glass then it will be swept up and put in a special broken glass bin. As we are only using a very week acid no special precautions will be needed but safety specs will be worn and if it comes in contact with the skin then it will be washed off immediately.
Results:
Evaluation:
In the investigation that was carried out there were no anomalous results. There was a result that was a bit more than the other titres; this was because the tap was left open after the colour change had happened so there was an excess of acid used. I did not use this result in my average. There is a percentage error involved when weighing items. To reduce the percentage error when I weighed the sodium carbonate, I weighed the sodium in a glass beaker so as to increase the weight decreasing the error involved. There is also a percentage error in the measuring equipment, but it is very small as I used precision-made glass wear. The biggest error is the human error when recording measurements, or noticing the colour change of the indicator. To reduce this error I used a spotting tile and when it got close to the colour change I only added half a drop of the acid of unknown concentration. To reduce this error more effectively you could use a colorimeter to detect the change in colour. I carried out accurate measurements using the pipette by touching it on the surface of the liquid to get the last drop. I worked out the percentage error for each piece of equipment that I used; I came to the conclusion that as they were so small they would not have had a great enough effect to make my readings not accurate.
Bibliography:
http://www.factmonster.com/ce6/sci/A0835329.html
http://www.dartmouth.edu/~chemlab/techniques/titration.html
http://antoine.frostburg.edu/chem/senese/101/acidbase/faq/methyl-orange.shtml
Chemical ideas, Salter’s advanced chemistry, second edition, Heinemann
Chemical storylines, Salter’s advanced chemistry, second edition, Heinemann
