Welcome to the World of Acid Reactions!

In this chapter, we are exploring one of the most exciting parts of Chemical changes: how acids behave. Whether it's the acid in your stomach helping you digest lunch or the reactions used to make fertilisers for our food, understanding acids is like learning the "instruction manual" for the world around us.

Don't worry if some of the formulas look a bit intimidating at first – we'll break them down step-by-step. By the end of these notes, you’ll be a pro at predicting what happens when chemicals mix!


1. What Makes an Acid an Acid?

In Chemistry, the "secret ingredient" in every acid is the hydrogen ion. When we dissolve an acid in water, it releases these ions.

  • Acids produce hydrogen ions \( (H^+) \) in aqueous solutions.
  • Alkalis (which are soluble bases) produce hydroxide ions \( (OH^-) \) in aqueous solutions.

The pH Scale

The pH scale (from 0 to 14) is our measuring stick for how acidic or alkaline a solution is. You can measure this using a Universal Indicator (which changes colour) or a pH probe (which gives a precise digital number).

  • pH 0–6: Acidic (Red/Orange/Yellow)
  • pH 7: Neutral (Green) - Example: Pure water
  • pH 8–14: Alkaline (Blue/Purple)

The Neutralisation Equation

When an acid meets an alkali, they cancel each other out to move toward pH 7. This is called neutralisation. The hydrogen ions from the acid and the hydroxide ions from the alkali react to form water.

The Equation: \( H^+(aq) + OH^-(aq) \rightarrow H_2O(l) \)

Quick Review: Acids = \( H^+ \), Alkalis = \( OH^- \). When they mix, they make water (\( H_2O \))!

2. Making Salts: The "Partner Swap"

When an acid reacts, it usually produces a salt. Making a salt is a bit like a dance where the chemicals swap partners. The name of the salt depends on which acid you use.

The Naming Rule (Memorise this!)

  • Hydrochloric Acid makes Chlorides
  • Sulfuric Acid makes Sulfates
  • Nitric Acid makes Nitrates

The Four Key Reactions

You need to know what happens when acids react with different substances. Here are the simple "recipes":

1. Acid + Metal \( \rightarrow \) Salt + Hydrogen

Mnemonic: Remember MASH (Metal + Acid \( \rightarrow \) Salt + Hydrogen).
Example: Magnesium + Hydrochloric Acid \( \rightarrow \) Magnesium Chloride + Hydrogen

2. Acid + Alkali \( \rightarrow \) Salt + Water

Example: Sodium Hydroxide + Nitric Acid \( \rightarrow \) Sodium Nitrate + Water

3. Acid + Base \( \rightarrow \) Salt + Water

A base is just a chemical that neutralises an acid (like a metal oxide). If it dissolves in water, we call it an alkali.
Example: Copper Oxide + Sulfuric Acid \( \rightarrow \) Copper Sulfate + Water

4. Acid + Metal Carbonate \( \rightarrow \) Salt + Water + Carbon Dioxide

Tip: If you see "Carbonate," you will always get bubbles because Carbon Dioxide gas is made!
Example: Calcium Carbonate + Hydrochloric Acid \( \rightarrow \) Calcium Chloride + Water + Carbon Dioxide

Key Takeaway: The acid always provides the second part of the salt's name (e.g., Sulfate), and the metal provides the first part (e.g., Copper).

3. Required Practical: Making a Pure Salt

In your exam, you might be asked how to make a pure, dry sample of a soluble salt (like Copper Sulfate crystals) from an insoluble oxide or carbonate.

Step-by-Step Guide:

  1. Add: Add the solid metal oxide (or carbonate) to the acid. Keep adding it until no more reacts (this is called adding it in excess).
  2. Filter: Use a funnel and filter paper to remove the leftover solid that didn't react. Now you have a clear solution of the salt.
  3. Evaporate: Gently heat the solution in an evaporating basin over a water bath. Stop heating when crystals start to form.
  4. Crystallise: Leave it to cool down so the rest of the salt can form large, pretty crystals.
  5. Dry: Pat the crystals dry with filter paper.
Common Mistake: Don't evaporate all the water until it's dry with the Bunsen burner! This can cause the salt to decompose. Always let it finish crystallising slowly.

4. Strong and Weak Acids (Higher Tier Only)

Many students think "strong" just means "concentrated," but in Chemistry, they are very different things!

Strong vs. Weak

  • Strong Acids: These completely ionise in water. This means every single molecule splits up to release \( H^+ \) ions.
    Examples: Hydrochloric, Sulfuric, and Nitric acids.
  • Weak Acids: These only partially ionise. Only a small fraction of the molecules split up.
    Examples: Ethanoic (vinegar), Citric, and Carbonic acids.

Concentration vs. pH

Concentration tells you how much acid is dissolved in 1 \( dm^3 \) of water. pH tells you the concentration of \( H^+ \) ions.

The Rule: As the pH decreases by 1 unit (e.g., from pH 3 to pH 2), the concentration of hydrogen ions increases by a factor of 10.


5. Titrations (Chemistry Only)

A titration is an experiment used to find out exactly how much acid is needed to neutralise an alkali. It is used to find unknown concentrations.

Key Equipment:

  • Pipette: Used to measure a very accurate, fixed volume of the alkali into a flask.
  • Burette: A long tube used to drop the acid into the flask slowly, one drop at a time.
  • Indicator: We use Phenolphthalein (turns pink in alkali, colourless in acid) or Methyl Orange (yellow in alkali, red in acid). We do NOT use Universal Indicator because its colour change is too blurry!

How to do it:

  1. Measure the alkali into a flask using a pipette.
  2. Add a few drops of indicator.
  3. Slowly add acid from the burette until the indicator just changes colour (the end-point).
  4. Record the volume of acid used and repeat until you get concordant results (results within 0.1 \( cm^3 \) of each other).

6. Oxidation and Reduction (Higher Tier Only)

When an acid reacts with a metal, it's actually a Redox reaction. This means electrons are being moved around.

  • Oxidation: Loss of electrons.
  • Reduction: Gain of electrons.

Memory Aid: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

In the reaction \( Mg + 2HCl \rightarrow MgCl_2 + H_2 \):
1. The Magnesium atoms lose electrons (Oxidation): \( Mg \rightarrow Mg^{2+} + 2e^- \)
2. The Hydrogen ions gain electrons (Reduction): \( 2H^+ + 2e^- \rightarrow H_2 \)


Chapter Summary - Key Takeaways

  • Acids contain \( H^+ \); Alkalis contain \( OH^- \).
  • Neutralisation: \( H^+ + OH^- \rightarrow H_2O \).
  • Salts: Named after the metal and the acid (e.g., Sodium Chloride).
  • Reactions: Know your MASH (Metal + Acid) and Carbonate (makes bubbles) reactions.
  • Practical: Excess base \( \rightarrow \) Filter \( \rightarrow \) Evaporate \( \rightarrow \) Crystallise.
  • HT Only: Strong acids split up completely; Weak acids only split up a little bit.