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CSEC Chemistry: Acids

Updated: Apr 30, 2020

You may think of an acid as a fulminant, corrosive liquid that can melt your clothes, you, and everything you hold dear.

However, not all acids can do those things (that still means you should handle them carefully though).

Acids are the opposite of bases. Acidity and alkalinity is measured on the pH scale which runs from 1-14. Lower pH (1-6) is acidic and higher pH (8-14) is basic/alkaline. A pH of 7 is neutral.

When understanding acids and bases, it all comes down to hydrogen ions, also called protons (abbreviated with the chemical symbol H⁺).

An acid is a substance that donates hydrogen ions, meaning it is a hydrogen donor. When dissolved in water, acids liberate hydrogen ions. These free hydrogen ions (protons) are the cause of the behaviours of acids. These protons do not remain separate when in solution, though. They become attached to the oxygen atoms in the polar water molecule forming hydronium ions:

H₂O + H⁺ → H₃O

This hydronium ion is usually written as H⁺(aq).

Acids only exhibit their acidic properties/behaviours when dissolved in water. When they are anhydrous, i.e. not dissolved in water, they can be in any of the three states of matter:

  • solid (eg citric acid and ascorbic acid)

  • liquid (eg nitric acid and phosphoric acid)

  • gas (eg HCl, SO₃ and NO₂)

Preparing Acids

Acids are generally produced by dissolving oxides of non-metals in water. These acidic oxides are often referred to as acid anhydrides.

An acid anhydride is a non-metallic oxide that dissolves in water to form an acid.

The following are examples of acid anhydrides reacting with water to form an acid:

SO₂(g) + H₂O(l) → H₂SO₃(aq) sulphurous acid

SO₃(g) + H₂O(l) → H₂SO₄(aq) sulphuric acid

CO₂(g) + H₂O(l) → H₂CO₃(aq) carbonic acid

N₂O₅(g) + H₂O(l) → HNO₂(aq) + HNO₃(aq) nitrous acid + nitric acid

Classification of Acids

Acids can be classified in different ways:

  • mineral or organic

  • strong or weak

  • monobasic, dibasic or tribasic

Mineral acids

These are acids originally obtained from minerals, eg hydrochloric acid (HCl) and nitric acid (HNO₃)

Organic Acids

These were originally procured from plant and animal materials (they all contain carbon, hydrogen and oxygen), for example, ethanoic acid or vinegar (CH₃COOH) and tartaric acid in grapes (CHO)

Strong Acids

These are acids that are completely ionized in aqueous solutions. This means that all of the hydrogen ions in the acid are released in the solution. The conduct electricity well when in solution.

For example: hydrochloric acid, sulphuric acid

HCl(aq)  → H⁺(aq) + Cl⁻(aq)

As you can see, all of the hydrogen is released, and hence, the acid is completely ionized.

Weak Acids

These acids are only partially ionized in aqueous solutions, meaning that some hydrogen remains unreleased. They conduct electricity to an extent when in solution.

For example: ethanoic acid (CH₃COOH) and tartaric acid

CH3COOH (aq)

H⁺(aq) CH3COO⁻(aq)

As you can see, all of the hydrogen in the compound is not ionized to free hydrogen ions.

Monobasic Acids

These acids give one free hydrogen ion for each molecule in aqueous solution.

For example: hydrochloric acid, nitric acid, ethanoic acid, lactic acid

Dibasic Acids

These acids give two free hydrogen ion for each molecule in aqueous solution.

For example: sulphuric acid, tartaric acid

Tribasic Acids

These acids give two free hydrogen ion for each molecule in aqueous solution.

For example: phosphoric acid, citric acid

Please note that although some acids contain several hydrogen atoms in their formulae, they don't all necessarily form hydrogen ions. Ethanoic acid has 4 hydrogen atoms in its formula (CH3COOH) but only produces one hydrogen ion in solution, so it is monobasic.

Reactions of Acids

1. Acids react with reactive metals (such as zinc and magnesium) to give hydrogen gas and a salt.

metal + acid → salt + hydrogen

Zn(s) + H2SO4(aq)→ ZnSO4(aq) + H2(g)

2. Acids react with metal oxides and metal hydroxides (metallic bases) to form a salt and water only. (This is a neutralization reaction)

metal oxide + acid → water + salt

CuO(s) + H2SO4(aq) → H2O(l) + CuSO4(aq)

3. Acids react with carbonates and hydrogencarbonates to give a salt, water and carbon dioxide.

hydrogencarbonate + acid → salt + water + carbon dioxide

NaHCO3(aq) + HNO3(aq) → NaNO3(aq) + H2O(l) + CO2(g)]

There are many acids and reactions containing acids that are present in our everyday lives, for example:

1) Antacids comprise ions that react with stomach acid, reducing damage and relieving heartburn and similar pain. For example, Tums is a very commonly used antacid that can be bought over the counter. Other antacids, such as Alka Seltzer, work in similar ways. Below you can see the mechanism and products for reaction in which calcium carbonate (the active ingredient in Tums) neutralizes HCl in the stomach:

CaCO3 ​​​​​​(s) + HCl (aq) → H2CO3 (aq) + CaCl2 (aq)

Following this acid-base reaction, carbonic acid will degrade quickly into CO2 and H2O:

H2CO3 (aq) → CO2(g) + H2O(l)

2) Vinegar is dilute acetic/ethanoic acid, which is what gives salad dressings and pickled vegetables their tart taste.

3) Citrus fruits, i.e. oranges, lemons, and limes contain citric acid, which gives them their sour taste

4) Aspirin contains acetylsalicylic acid, which helps to reduce pain, fever, or inflammation.

5) Ascorbic acid (C6H8O6) is vitamin C. It is found in citrus fruit and some other fruits and juices.

6) You may be familiar with salicylic acid, which is used to treat the outer layer of the skin, and is found in many acne creams and face washes.

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