Acids and Bases

 

This section is a very cool and complex part of Chemistry. It is essential that we understand acids and bases as it is an essential part of many chemical and biological processes.

Bases are water-soluble alkalis. The terms alkali and base are often used inter-changeably, but this is a key point to remember. All bases are alkalis, but not all alkalis are bases.

Acids and alkalis can be dangerous when pH is too low or too high, so let’s get into it.

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The pH scale

The pH scale is a quantitative measure of acidity and alkalinity based off of the concentration of H+ and OH- ions. It ranges from 0 to 14. Compounds with a higher concentration of H+ have a lower pH and are acids. Compounds with a higher concentration of OH- have a higher pH and are alkalis.

The concentration of H+ ions and OH- ions in water is equal.

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Common Acids and Alkalis

You need to be aware of a few common acids and alkalis, knowing these makes big scary acids and bases questions less intimidating in exams.

Common acids:

  • Hydrochloric acid (HCl)

  • Nitric acid (HNO3)

  • Sulphuric acid (H2SO4)

Common alkalis

  • Calcium hydroxide (NaOH)

  • Sodium hydroxide (Ca(OH)2)

  • Lithium hydroxide (LiOH)

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Diluting and Creating Acids and Alkalis

When an acid is diluted, the pH will rise towards 7 as the concentration of H+ ions will decrease.

When an alkali is diluted, the pH will fall towards 7 as the concentration of OH- ions will decrease.

When dissolved in water, soluble non-metal oxides produce acidic solutions.

When dissolved in water, soluble metal oxides produce alkaline solutions.

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Neutralisation and Reactions with Acids

A neutralisation reaction is a reaction between an acid and a base in an aqueous solution which produces a salt and water.

A reaction between a metal oxide and an acid produces a salt and water.

A reaction between a metal hydroxide and an acid produces a salt and water.

A reaction between a metal carbonate and an acid produces a salt, water and carbon dioxide.

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Salts

You need to know different salts produced form different acids:

  • Chloride salts are produced from hydrochloric acid.

  • Sulphate salts are produced from sulphuric acid.

  • Nitrate salts are produced from nitric acid.

Spectator Ions

Spectator ions are ions that do not take part in a chemical reaction but are present in the solution both before and after the reaction.

In your exams, you may be asked to identify the spectator ions in reactions and eliminate them from the equation. The net ionic equation is created by eliminating spectator ions.

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Titrations

Titrations are used to accurately measure the volume of a solution required to reach the end point of a chemical reaction.

Before discussing the method of titrations, there a couple of things you need to be aware of:

  • A standard solution is a solution of accurately known concentration.

  • An indicator is used to show when the end point of the reaction has been reached.

  • The titre volumes are deemed concordant when the measurements are between 2cm^3.

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Titration Method

Method for determining the concentration of an acid.

  1. Clean and set up equipment.

  2. Using a pipette, measure a known volume of acid into a conical flask.

  3. Add an indicator to an alkali standard solution.

  4. Wash the burette with some of the standard solution alkali.

  5. Pour the standard solution of alkali into the burette.

  6. Record the volume of alkali on the burette from the bottom of the meniscus.

  7. Open the tap, little by little, while swirling the conical flask to mix the solutions.

  8. When the acid changes colour, measure the volume on the burette from the bottom of the meniscus.

  9. Repeat this process 3 times until results are concordant and take an average result.

  10. Use this information to calculate the concentration of the acid.

Key Points!

  • The pH scale

    The pH scale is a quantitative measure of acidity and alkalinity based off of the concentration of H+ and OH- ions. It ranges from 0 to 14.

    The concentration of H+ ions and OH- ions in water is equal.

  • Common Acids and Alkalis

    Common acids:

    * Hydrochloric acid (HCl)

    * Nitric acid (HNO3)

    * Sulphuric acid (H2SO4)

    Common alkalis

    * Calcium hydroxide (NaOH)

    * Sodium hydroxide (Ca(OH)2)

    * Lithium hydroxide (LiOH)

  • Diluting and Creating Acids and Alkalis

    When an acid is diluted, the pH will rise towards 7 as the concentration of H+ ions will decrease.

    When an alkali is diluted, the pH will fall towards 7 as the concentration of OH- ions will decrease.

    When dissolved in water, soluble non-metal oxides produce acidic solutions.

    When dissolved in water, soluble metal oxides produce alkaline solutions.

  • Neutralisation and Reactions with Acids

    A neutralisation reaction is a reaction between an acid and a base in an aqueous solution which produces a salt and water.

    A reaction between a metal oxide and an acid produces a salt and water.

    A reaction between a metal hydroxide and an acid produces a salt and water.

    A reaction between a metal carbonate and an acid produces a salt, water and carbon dioxide.

  • Salts

    You need to know different salts produced form different acids:

    Chloride salts are produced from hydrochloric acid.

    Sulphate salts are produced from sulphuric acid.

    Nitrate salts are produced from nitric acid.

  • Spectator Ions

    Spectator ions are ions that do not take part in a chemical reaction but are present in the solution both before and after the reaction.

  • Titrations

    Know the method of titrations.

    A standard solution is a solution of accurately known concentration.

    An indicator is used to show when the end point of the reaction has been reached.

    The titre volumes are deemed concordant when the measurements are between 2cm^3.

Any Questions?