Overview
This lecture covers Topic 4: Chemical Changes in GCSE Chemistry, including acids, bases, the pH scale, neutralization, the reactivity series, redox reactions, and electrolysis.
The pH Scale
- pH measures how acidic or alkaline a solution is, ranging from 0 (acidic) to 14 (alkaline), with 7 as neutral.
- Universal indicator or pH meters are used to measure pH.
- Acids (pH < 7) include battery acid, lemon juice, and acid rain.
- Alkalis (pH > 7) include bleach, soap powder, and washing-up liquid.
- Pure water is neutral (pH 7).
Acids, Bases, and Alkalis
- Acids produce H⁺ ions in water and have pH below 7.
- Bases react with acids to form salts; not all bases are alkalis.
- Alkalis are bases that dissolve in water and form OH⁻ ions.
- Strong acids or alkalis ionize completely in water; weak ones only partly ionize.
Important Chemical Reactions
- Metal + Water → Metal Hydroxide + Hydrogen.
- Acid + Metal → Salt + Hydrogen.
- Neutralization reactions: Acid + Base/Oxide/Hydroxide → Salt + Water; Acid + Carbonate → Salt + Water + CO₂.
- Sulfuric acid makes sulfates; nitric acid makes nitrates; hydrochloric acid makes chlorides.
Reactivity Series
- Lists metals by how easily they lose electrons (reactivity).
- Group 1 metals (e.g., potassium, sodium) are highly reactive; transition metals (e.g., copper, gold) are less reactive.
- Carbon and hydrogen are included to compare extraction methods.
Redox Reactions
- Involve the transfer of electrons between substances.
- Oxidation is loss of electrons; reduction is gain (remember: OIL RIG).
- Displacement reactions occur when a more reactive metal replaces a less reactive one in a compound.
Electrolysis
- Uses electricity to split ionic compounds (electrolytes) into elements.
- Positive metal ions move to the cathode (negative electrode) and gain electrons.
- Negative non-metal ions move to the anode (positive electrode) and lose electrons.
- Metals collect at the bottom; non-metals may be released as gases.
Key Terms & Definitions
- Acid — substance that forms H⁺ ions in water.
- Alkali — soluble base that forms OH⁻ ions in water.
- Base — substance that reacts with acids to form salts.
- pH Scale — measures acidity/alkalinity from 0 to 14.
- Neutralization — reaction between acid and alkali/base to form salt and water.
- Reactivity Series — ranks metals by how easily they lose electrons.
- Oxidation — loss of electrons.
- Reduction — gain of electrons.
- Redox Reaction — reaction involving electron transfer.
- Electrolysis — splitting compounds using electricity.
Action Items / Next Steps
- Memorize at least three examples each of common acids and alkalis.
- Revise all key reaction types and the salts formed from different acids.
- Review the order of elements in the reactivity series.
- Understand and practice definitions of key terms.