Synthesising the concepts of strong acids, weak acids, strong alkalis and weak alkalis
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Malaysia SPM Form 4 Chemistry, Chapter 7: Acid and Base.
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[edit] Strong and Weak Acids: Fully or Partially Ionised
[edit] Strong acids
- Strong acids are fully ionised in solution; for example, for sulphuric acid:
[edit] Weak acids
- Weak acids are only partially ionised in solution. Some of the ions recombine, and remain as molecules. This is shown by the reversible reaction sign in the equation. An example is carbonic acid.
- It follows that strong acids, because they are fully ionised, are strong electrolytes, and weak acids are weak electrolytes (see Unit 9.3) as they have few ions to migrate towards the anode and cathode. Table 4 is a list of some common strong and weak acids.
[edit] Example Of Strong Acid And Weak Acid
| Strong acid | Weak acid |
|---|---|
| Sulphuric acid H2SO4 | Carbonic acid H2CO3 |
| Nitric acid HNO3 | Ethanoic acid CH3COOH |
| Hydrochloric acid HC1 | Sulphurous H2SO3 |
| Citric acid C6H8O7 | |
| Phosphoric acid H3PO4 |
- Organic acids like ethanoic acid are weak acids. This is because in a 1 mol dm-3 solution only 4 out of every 1 000 molecules ionise to produce hydrogen ions.
e.g. CH3COOH(aq) − − > CH3COO − (aq) + H + (aq)
- The other 996 remain unchanged in solution as molecules
[edit] pH Scale: The Strength of an Acid or Alkali
- The pH scale is a measure of the relative acidity or alkalinity of a solution.
pH = -log[H+]
[H+] = concentration of hydrogen ion
- pH is an abbreviation for power of the Hydogen and has values from 0 to 14.
- Any pH value below seven is acidic; the lower it is, the stronger the acid, and the higher the hydrogen ion concentration.
- Any pH value above seven is alkaline; the higher the value, the stronger the alkali, and the lower the hydrogen ion concentration.
- The value of seven is the neutral value, which means that the solution is neither acidic nor alkaline.
- Theoretically there is no limit to the pH scale, but most solutions are between pH 1 and pH 14.
- For example, looking at the 'extremes', 1M hydrochloric acid (HCl) has a pH of 0 and 10M HCl has a pH of -1, and 1M sodium hydroxide (NaOH) has a pH of 14, but 10M potassium hydroxide (KOH) has a pH of 15!
- However the solubility limits of substances in water ensures that its almost impossible to get below -1 or above 15!
[edit] Universal Indicator
- To find the pH of a solution an indicator is used like Universal Indicator.
- An indicator is a substance or mixture of substances that when added to the solution gives a different colours depending on the pH of the solution.
- Universal indicator is a very handy indicator for showing whether the solution is acid, neutral or alkaline and gives the pH to the nearest pH unit.
- The best indicator of all is the universal indicator.
- This is really a mixture of many indicators, and can be used to show not only whether a substance is acidic or alkaline, but also whether it is a strong acid or a weak acid.
- It does this by having colours in between the extremes of red and violet (e.g. orange, yellow, green, blue and indigo).
- These are in the same order as the spectral colours and each colour is associated and a particular pH value.
