HKDSE Chemistry Study Notes: Acids and Alkalis
Hello! Welcome to your study notes for "Acids and Alkalis". Don't worry, this topic is all around you, from the lemon in your drink to the soap you use to wash your hands. Understanding acids and alkalis is a fundamental part of chemistry. In these notes, we'll break everything down into simple, easy-to-understand parts. We'll learn what makes an acid an acid, what alkalis are, and how we can measure their acidity using the pH scale. Let's get started!
1. Getting to know Acids
What's the Big Idea? The H⁺(aq) Concept
The single most important thing to know about acids is this: An acid is a substance that, when dissolved in water, produces hydrogen ions as the only positive ions. We write the hydrogen ion as H⁺(aq). The `(aq)` means it is "aqueous", or dissolved in water.
Think of it like this: An acid molecule is like a package holding onto one or more H⁺ ions. When you put it in water, the water "unwraps" the package and sets the H⁺ ions free!
The Crucial Role of Water and Electrical Conductivity
An acid only shows its acidic properties when it is dissolved in water. Without water, the H⁺ ions are not released. For example, pure, dry hydrogen chloride (HCl) gas exists as covalent molecules and is not acidic. But the moment you dissolve it in water, it ionises to release H⁺(aq) and Cl⁻(aq) ions, making the solution acidic.
Because aqueous acids contain free-moving (mobile) ions, they are conductors of electricity.
Analogy: A teabag has the potential to make tea, but you only get the tea (the flavour, the colour) when you add hot water. Similarly, an acid only shows its properties and mobile ions when you add water.
Common Acids to Remember
In the Lab:
Hydrochloric acid (HCl)
Sulphuric acid (H₂SO₄)
Nitric acid (HNO₃)
In Everyday Life:
Ethanoic acid (in vinegar)
Citric acid (in lemons and oranges)
Carbonic acid (in fizzy drinks)
Typical Reactions of Acids
Because all acids produce H⁺(aq) in water, they have similar characteristic chemical reactions.
1. Acids react with reactive metals:
This reaction produces a salt and hydrogen gas.
Example: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
Chemical Equation: \(Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)\)
Ionic Equation: \(Mg(s) + 2H^+(aq) \rightarrow Mg^{2+}(aq) + H_2(g)\)
(Notice how the essential reaction is between the metal and the H⁺ ions!)
2. Acids react with carbonates and hydrogencarbonates:
This reaction produces a salt, water, and carbon dioxide gas.
Example: Calcium carbonate (limestone) + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide
Chemical Equation: \(CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)\)
Ionic Equation: \(CaCO_3(s) + 2H^+(aq) \rightarrow Ca^{2+}(aq) + H_2O(l) + CO_2(g)\)
3. Acids react with bases/alkalis (Neutralisation):
Acid + Base → Salt + Water.
What is the "Basicity" of an Acid?
Basicity refers to the maximum number of H⁺ ions that one molecule of an acid can produce when completely ionised in water.
Monobasic acid: Can release ONE H⁺ ion per molecule. (e.g., HCl, HNO₃, CH₃COOH)
Dibasic acid: Can release TWO H⁺ ions per molecule. (e.g., H₂SO₄)
Tribasic acid: Can release THREE H⁺ ions per molecule. (e.g., H₃PO₄)
Strength vs. Concentration: Don't Mix Them Up!
Strength: Refers to the degree of ionisation. A strong acid (like HCl or HNO₃) ionises completely in water. A weak acid (like CH₃COOH) ionises only slightly in water.
Concentration: Refers to the amount of solute dissolved per unit volume of solution (mol dm⁻³). You can have a dilute solution of a strong acid (e.g. 0.001 mol dm⁻³ HCl) or a concentrated solution of a weak acid (e.g. 6 mol dm⁻³ CH₃COOH).
2. Understanding Alkalis (and Bases!)
Base vs. Alkali: What's the Difference?
A base is a substance (metal oxide or metal hydroxide) that reacts with an acid to form a salt and water only.
An alkali is a soluble base. When dissolved in water, it produces hydroxide ions (OH⁻(aq)) as the only negative ions. Alkaline solutions also conduct electricity due to their mobile ions.
Memory Aid: All alkalis are bases, but not all bases are alkalis. Alkalis are simply the subgroup of bases that dissolve in water.
Common Alkalis to Remember
In the Lab:
Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)
Aqueous ammonia (NH₃(aq))
Calcium hydroxide solution / Limewater (Ca(OH)₂)
In Everyday Life:
Soap and detergents
Oven and drain cleaners (contain concentrated NaOH)
Typical Reactions of Alkalis
1. Alkalis react with acids (Neutralisation):
Example: Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water
Chemical Equation: \(NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)\)
Ionic Equation: \(H^+(aq) + OH^-(aq) \rightarrow H_2O(l)\)
2. Alkalis react with ammonium salts when heated:
This reaction liberates ammonia gas, which turns moist red litmus paper blue.
Example: Sodium hydroxide + Ammonium chloride → Sodium chloride + Water + Ammonia
Chemical Equation: \(NaOH(aq) + NH_4Cl(aq) \xrightarrow{heat} NaCl(aq) + H_2O(l) + NH_3(g)\)
Ionic Equation: \(OH^-(aq) + NH_4^+(aq) \xrightarrow{heat} H_2O(l) + NH_3(g)\)
3. Alkalis react with solutions of metal ions to form insoluble metal hydroxides (Precipitation):
Dilute alkalis precipitate distinctively coloured insoluble metal hydroxides:
\(Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)\) (Pale blue precipitate)
\(Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)\) (Dirty green precipitate)
\(Fe^{3+}(aq) + 3OH^-(aq) \rightarrow Fe(OH)_3(s)\) (Reddish-brown precipitate)
\(Mg^{2+}(aq) + 2OH^-(aq) \rightarrow Mg(OH)_2(s)\) (White precipitate)
3. Safety First! Corrosive Nature
Both concentrated acids and concentrated alkalis are highly corrosive, causing severe burns to skin and eyes. Always wear safety goggles and gloves.
When diluting concentrated acids (especially sulphuric acid), always slowly add ACID to WATER with constant stirring, never water to acid. Dilution is strongly exothermic; adding water to concentrated acid can cause local overheating, violent boiling, and dangerous acid splatters!
Memory Aid: "Do as you oughta, add acid to water."
4. The pH Scale: A Measure of Acidity
We use the pH scale to express the acidity or alkalinity of an aqueous solution, typically running from 0 to 14 at 25 °C:
pH < 7: Acidic solution (lower pH = higher [H⁺(aq)])
pH = 7: Neutral solution (pure water at 25 °C)
pH > 7: Alkaline solution (higher pH = higher [OH⁻(aq)])
The Maths Behind pH (For Strong Monobasic Acids)
The mathematical definition of pH is:
\(pH = -\log[H^+(aq)]\)
Where [H⁺(aq)] is the molar concentration of hydrogen ions in \(mol\ dm^{-3}\).
Step-by-Step pH Calculations
Example 1: Find the pH of 0.1 mol dm⁻³ HCl(aq).
Identify the acid: HCl is a strong monobasic acid that completely ionises: \(HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)\).
Determine [H⁺(aq)]: \([H^+(aq)] = 0.1\ mol\ dm^{-3}\).
Calculate pH: \(pH = -\log(0.1) = 1\).
Example 2: Find the pH of 0.01 mol dm⁻³ HNO₃(aq).
Identify the acid: HNO₃ is a strong monobasic acid that completely ionises: \(HNO_3(aq) \rightarrow H^+(aq) + NO_3^-(aq)\).
Determine [H⁺(aq)]: \([H^+(aq)] = 0.01\ mol\ dm^{-3} = 1.0 \times 10^{-2}\ mol\ dm^{-3}\).
Calculate pH: \(pH = -\log(0.01) = 2\).
5. How to Determine pH in the Lab
There are several methods to test acidity or determine pH in school laboratories:
Acid-Base Indicators
Acid-base indicators are weak organic acids or bases that display different colours in acidic and alkaline solutions:
Litmus: Red in acid, Blue in alkali.
Methyl Orange: Red in acidic solution (pH < 3.1), Yellow in alkaline solution (pH > 4.4).
Phenolphthalein: Colourless in acidic/neutral solution, Pink in alkaline solution (pH > 8.3).
Universal Indicator and pH Paper
A mixture of indicators showing continuous colour changes across the whole pH scale (pH 1 to 14). By matching the resulting solution colour to a standard pH colour chart, we can estimate the approximate pH value.
pH Meter or Data-Logger with pH Sensor
The most precise method. A calibrated pH meter measures the electrode potential to give an accurate, quantitative digital reading (e.g. pH 3.45).
Quick Summary: Choosing a Method
Qualitative check (acid vs alkali): Simple indicators like Litmus.
Approximate pH estimate: Universal Indicator.
Accurate, continuous quantitative pH measurement: pH Meter.