Half term

GCSE Chemistry Study Notes

Chemical Formulae, Equations, and Calculations

Empirical and Molecular Formulae

  • Empirical formula: The simplest whole-number ratio of atoms in a compound.Example: The empirical formula for C₂H₇ is CH₃.

  • Molecular formula: The actual number of atoms in a molecule of a compound.Example: The molecular formula of glucose is C₆H₁₂O₆.

Calculating Empirical Formula

  1. Find the mass or percentage composition of each element.

  2. Divide by the relative atomic mass (Ar) of each element.

  3. Divide each by the smallest value.

  4. Round to the nearest whole number.

Practical: Determining the Formula of a Metal Oxide by Combustion

  • Heat a known mass of magnesium in a crucible until it reacts completely with oxygen.

  • Measure the mass of the product (magnesium oxide).

  • Use mass differences to determine the empirical formula.

Relative Formula Mass (Mr)

  • Relative formula mass (Mr): Sum of the Ar values of all the atoms in a compound.Example: Mr of H₂O = (2 × 1) + 16 = 18.

Balancing Chemical Equations

  • Write the unbalanced equation.

  • Balance each element by adjusting coefficients.

  • Ensure that the total number of atoms of each element is equal on both sides.

Calculations Involving Moles

  • Formula:

  • Concentration formula:

Group 1 (Alkali Metals)

General Properties

  • Soft metals, low density.

  • Highly reactive, stored under oil to prevent reaction with air.

  • Reactivity increases down the group.

Reactions with Water

  • Form a metal hydroxide and hydrogen gas:

  • Observations: fizzing, floating, and sometimes flames.

Reactions with Air

  • Form metal oxides:

Trend in Reactivity

  • Reactivity increases down the group because:

    • Atomic radius increases.

    • Outer electron is further from the nucleus, making it easier to lose.

    • Shielding by inner electron shells increases.

Group 7 (Halogens)

General Properties

  • Diatomic non-metals (e.g., F₂, Cl₂).

  • Trend in physical states:

    • Fluorine (yellow gas)

    • Chlorine (green gas)

    • Bromine (brown liquid)

    • Iodine (purple solid)

  • Reactivity decreases down the group.

Trends in Group 7 Properties

  • Melting and boiling points increase down the group because of stronger intermolecular forces.

  • Reactivity decreases because:

    • Atomic size increases, making it harder to gain an electron.

    • Shielding increases.

Displacement Reactions

  • A more reactive halogen will displace a less reactive halogen from a compound:

Trend in Reactivity (Electronic Configurations)

  • Elements at the top (like fluorine) have a stronger pull on electrons due to a smaller atomic radius and less shielding.