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 strength using something called 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)` just means it's "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
An acid only shows its acidic properties when it's dissolved in water. Without water, the H⁺ ions are not released. For example, pure, dry hydrogen chloride (HCl) gas is not acidic. But the moment you dissolve it in water, it forms hydrochloric acid and releases H⁺(aq) ions, making the solution acidic.
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 when you add water.
Common Acids to Remember
In the Lab:
Hydrochloric acid (HCl)
Sulphuric acid (H₂SO₄)
Nitric acid (HNO₃) (Good to know, but HCl and H₂SO₄ are the main examples in the syllabus)
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 chemical reactions. These are their "characteristics".
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):
We'll cover this more in the next section, but the key reaction is: Acid + Base → Salt + Water.
What is the "Basicity" of an Acid?
This sounds confusing, but it's simple! Basicity refers to the number of H⁺ ions that one molecule of an acid can release when dissolved in water.
Monobasic acid: Releases ONE H⁺ ion per molecule. (e.g., HCl, HNO₃, CH₃COOH)
Dibasic acid: Releases TWO H⁺ ions per molecule. (e.g., H₂SO₄)
Tribasic acid: Releases THREE H⁺ ions per molecule. (e.g., H₃PO₄)
Key Takeaway for Acids
Acids are all about the H⁺(aq) ion! This single ion is responsible for all their characteristic properties and reactions. Water is essential to release it.
2. Understanding Alkalis (and Bases!)
Base vs. Alkali: What's the Difference?
This is a very common point of confusion, so let's clear it up.
A base is a substance that reacts with an acid to form a salt and water only. Bases can be soluble or insoluble in water.
An alkali is a soluble base. When it dissolves in water, it produces hydroxide ions (OH⁻(aq)).
Memory Aid: All alkalis are bases, but not all bases are alkalis. Think of it like this: All terriers are dogs, but not all dogs are terriers. Alkalis are just a specific group of bases (the ones that dissolve in water).
The Alkali Defining Ion: OH⁻(aq)
Just like H⁺(aq) defines an acid, the hydroxide ion, OH⁻(aq), defines an alkali. An alkali is a substance that produces OH⁻(aq) ions when dissolved in water.
Common Alkalis to Remember
In the Lab:
Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)
Aqueous ammonia (NH₃(aq))
In Everyday Life:
Baking soda solution (sodium hydrogencarbonate)
Soap and detergents
Oven cleaner (often contains NaOH)
Typical Reactions of Alkalis
Alkalis have their own set of characteristic reactions, all thanks to the OH⁻(aq) ion.
1. Alkalis react with acids (Neutralisation):
This is the classic acid-base reaction. The H⁺ from the acid and the OH⁻ from the alkali combine to form neutral water.
Example: Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water
Chemical Equation: $$NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l)$$
Ionic Equation: $$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$
(This ionic equation is the heart of ALL neutralisation reactions between acids and alkalis!)
2. Alkalis react with ammonium salts when heated:
This reaction produces a salt, water, and ammonia gas. Ammonia has a characteristic pungent smell.
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: $$OH^-(aq) + NH_4^+(aq) \xrightarrow{heat} H_2O(l) + NH_3(g)$$
Key Takeaway for Alkalis
Alkalis are soluble bases that produce OH⁻(aq) ions in water. This ion is responsible for their properties, including the ability to neutralise acids.
3. Safety First! Corrosive Nature
Both concentrated acids and concentrated alkalis are highly corrosive. This means they can cause severe damage to skin, eyes, and clothes. Always handle them with care!
ALWAYS wear safety goggles.
When diluting concentrated acids (especially sulphuric acid), slowly add ACID to WATER, not the other way around. This is because a huge amount of heat is released. Adding water to acid can make it boil and splash dangerous acid everywhere!
Memory Aid: "Do as you oughta, add acid to water."
Did you know? Concentrated sulphuric acid is such a powerful dehydrating agent that it can remove hydrogen and oxygen atoms (as water) from sugar, leaving behind a black block of carbon!
4. The pH Scale: A Ruler for Acidity
We need a way to measure how acidic or alkaline a solution is. We use the pH scale for this. It's a numerical scale that usually runs from 0 to 14.
A solution with pH < 7 is acidic. (The lower the pH, the more acidic.)
A solution with pH = 7 is neutral. (e.g., pure water.)
A solution with pH > 7 is alkaline. (The higher the pH, the more alkaline.)
The Maths Behind pH (For Strong Acids)
The pH scale is a logarithmic scale. The formula that connects pH to the concentration of H⁺(aq) ions is:
$$pH = -\log[H^+(aq)]$$
Where [H⁺(aq)] is the concentration of hydrogen ions in mol dm⁻³.
Don't worry if this seems tricky at first! For the strong acids you need to know (like HCl and H₂SO₄), calculating pH is straightforward. "Strong" means the acid fully breaks apart (dissociates) in water.
Step-by-Step pH Calculation
Example 1: Find the pH of 0.1 mol dm⁻³ HCl(aq).
Identify the acid type. HCl is a strong, monobasic acid. This means 1 molecule of HCl gives 1 H⁺ ion.
Find [H⁺(aq)]. Because it's a strong monobasic acid, [H⁺(aq)] is the same as the acid's concentration. So, [H⁺(aq)] = 0.1 mol dm⁻³.
Use the formula. $$pH = -\log(0.1)$$
Calculate. On your calculator, -log(0.1) = 1. The pH is 1.
Example 2: Find the pH of 0.05 mol dm⁻³ H₂SO₄(aq).
Identify the acid type. H₂SO₄ is a strong, dibasic acid. This means 1 molecule of H₂SO₄ gives 2 H⁺ ions.
Find [H⁺(aq)]. Because it's dibasic, we must multiply the acid's concentration by 2. So, [H⁺(aq)] = 2 x 0.05 = 0.1 mol dm⁻³.
Use the formula. $$pH = -\log(0.1)$$
Calculate. The pH is 1.
Common Mistake Alert! Always remember to check if an acid is dibasic (like H₂SO₄) or tribasic (like H₃PO₄) before calculating pH. Forgetting to multiply the concentration by 2 or 3 is a very common error!
5. How to Determine pH in the Lab
There are a few ways to find the pH of a solution, ranging from a rough guess to a precise measurement.
Acid-Base Indicators
These are special dyes that change colour in acidic or alkaline conditions. They give a simple "yes/no" answer but don't tell you the exact pH value.
Litmus: Turns RED in acidic solution and BLUE in alkaline solution.
Methyl Orange: Turns RED in acidic solution and YELLOW in alkaline solution.
Memory Aid: "My Apple is Red, my Banana is Yellow" (MAR for Methyl-Acid-Red, BY for Base-Yellow).Phenolphthalein: Is COLOURLESS in acidic solution and PINK in alkaline solution.
Universal Indicator
This is a mixture of several different indicators. Instead of just one colour change, it produces a whole spectrum of colours, from red for very strong acids (pH 1) to dark purple for very strong alkalis (pH 14). By matching the colour to a chart, you can estimate the pH value.
pH Meter
This is the most accurate method. A pH meter is an electronic instrument with a probe that you place in the solution. It gives a precise digital reading of the pH value, often to two decimal places.
Quick Review: Choosing a Method
To simply check if something is an acid or an alkali? Use Litmus or another simple indicator.
To get a rough idea of the pH value (e.g., pH 4 or pH 10)? Use Universal Indicator.
To get a very accurate and precise pH reading for an experiment? Use a pH Meter.