Acids And Alkalis

Question 1

pH and indicators

Answer 1

The pH scale ranges from about 0 to about 14 and it tells you how acidic or how alkaline a solution is.

0-3 is known as strongly acidic. Examples include hydrochloric acid

4-6 is known as weakly acidic. Examples include ethanoic acid (vinegar).

7 is known as neutral. Example is sodium chloride

8-10 is known as weakly alkaline. Example is ammonia

11-14 is known as strongly alkaline. Example is sodium hydroxide.

Question 2

Measuring pH

Answer 2

Universal inddiactor is made from a mixture of indicators, which change colour in a gradual way over a range of pH values. It can be used as a solution or as paper. The colour of the paper or solution is checked against a chart to find the pH.

Question 3

Acid and alkali indicators

Litmus
Methyl Orange
Phenolphthalein
Universal indicator

^ for Acid and Alkalis

Answer 3

Different indicators show different colours. These type of examples are shown below.

Litmus:
Acid - Red
Alkali - blue

Methyl Orange:
Acid - red
Alkali - yellow

Phenolphthalein:
Acid - colourless
Alkali - pink

Universal indicator:
Acid - red
Alkali - blue

Question 4

Acids

Answer 4

All acids contain hydrogen
All acids contain H^+ ions

When a acid reacts with a base or alkali, it is called a neutralisation reaction and a salt is formed.

Salts are when the H^+ ions in an acid is replaced by a metal. They are neutral.

For example hydrochloric acid. The type of salt formed is chloride.
Or sulfuric acid. The type of salt formed is sulphate.
Or nitric acid. The type of salt formed is nitrate.

We can define acids as substances that act as a source of hydrogen ions in solution.

Question 5

General reactions of acids

Acid + Metal hydroxide —>
Acid + metal —>
Acid + metal oxide —>
Acid + metal carbonate —>

Answer 5

Acid + Metal hydroxide —> metal salt + water
Acid + metal —> metal salt + hydrogen
Acid + metal oxide —> metal salt + water
Acid + metal carbonate —> metal salt + water + carbon dioxide

Question 6

Bases

Answer 6

Bases are substances that neutralises acids by combing with the hydrogen ions in them. When referring to a base, we mean a metal oxide, a metal hydroxide or ammonia.

Some bases dissolve in water to form solutions containing hydroxide ions. These are alkalis. Alkalis are a source of hydroxide ions in solution.

Examples of alkalis are sodium hydroxide and potassium hydroxide. When sodium hydroxide is in water, it breaks apart to form sodium hydroxide ions.

The other alkali is a solution of ammonia. The ammonia reacts with the water to form ammonium ions and hydroxide ions.
These alkalis all have a pH greater than 7.

Question 7

Neutralisation - reacting acids with bases

Answer 7

Acids react with bases or alkalis in a neutralisation reaction. Metal oxides such as copper oxides and magnesium oxides are bases.

Copper oxide reacts with dilute sulfuric acid in a neutralisation reaction to produce a solution of copper sulfate and water.

Question 8

Reacting acids with alkalis

Answer 8

Sodium hydroxide solution reacts with hydrochloric acid to for, sodium chloride and water.

When an acid is added to an alkali, the pH decreases.
When an alkali is added to an acid, the pH increases.
When an neutral solution is formed, the pH will be 7.

Question 9

Titration

Answer 9

We can use titration to follow the course of a neutralisation reaction between an acid and alkali. Titration can be used to find out how much of the acid/alkali reacts with a certain volume of the alkali/acid.

Method: Using phenolphthalein as an indicator

1. Measure out 25cm^3 of HCL using a pipette and pipette fliter.
2. Transfer to a conical flask
3. Fill burette with sodium hydroxide solution. Take initial reading on burette.
4. Add a few drops of phenolphthalein indicator
5. Add sodium hydroxide until indicator changes colour
6. Take final reading on burette

The alkali is added to the acid from the burette until the indicator changes colour. You usually do a rough titration first in order to find out approximately how much alkali is required to neutralise the acid. On the second turn, we would add the alkali slowly once knowing how alkali was required from the rough turn.

The volume of alkali added it obtained by subtracting the initial reading from the final reading.

Various other indicators can also be used, such as methyl Orange, but not universal indicator because it has a range of colours and changes gradually between them, this means we would not be able to see a clear endpoint for the titration.