C3.1 - Introducing Chemical reactions (AI generated with instructions by moi)

What is a chemical formula?

A shorthand representation of a substance showing the symbols and number of each element it contains. The number of atoms is shown as a subscript e.g. H₂O

What is the formula of a simple covalent compound?

It shows the actual number of atoms of each element in one molecule e.g. H₂O has 2 hydrogen atoms and 1 oxygen atom bonded together

How do you write the formula of an ionic compound?

Combine positive and negative ions so the overall charge is zero. The charges must balance — use brackets when more than one of a compound ion is needed e.g. Ca(NO₃)₂

What are the common polyatomic ions you must know?

Ammonium NH₄⁺, hydroxide OH⁻, nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻, hydrogen carbonate HCO₃⁻, phosphate PO₄³⁻

What charge do Group 1, 2 and 3 metals form?

Group 1 → 1+, Group 2 → 2+, Group 3 → 3+

What charge do Group 5, 6 and 7 non-metals form?

Group 5 → 3−, Group 6 → 2−, Group 7 → 1−

How do you deduce the formula of an ionic compound from ion charges?

Write the symbols and charges of each ion, then swap the numbers of the charges to find the ratio of ions needed so total charge = zero e.g. Al³⁺ and O²⁻ → Al₂O₃

What is a balanced chemical equation?

A representation of a reaction using chemical symbols where the number of atoms of each element is the same on both sides. Balancing is done by changing coefficients only — never change the formulae

What are state symbols and what do they mean?

Symbols added to equations to show physical state: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water)

How do you determine the state of a substance in an equation?

Generally: ionic compounds will be (aq) if dissolved, though there are exceptions. Ionic compounds formed in solution are often (s) if they precipitate out

What is the Law of Conservation of Mass?

In a closed system, mass is never created or destroyed in a chemical reaction — the total mass of reactants equals the total mass of products

Why might mass appear to change in a reaction?

If a gas is produced and escapes (e.g. CO₂) the mass of the container appears to decrease. In a closed system, mass is always conserved. If a gas is a reactant (e.g. O₂ from air), mass can appear to increase

[HIGHER] What are ionic equations and when are they used?

Ionic equations show only the ions that actually change in a reaction — spectator ions (present but unchanged on both sides) are cancelled out e.g. Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

[HIGHER] What are half equations?

Half equations show either the oxidation or reduction part of a reaction separately, showing electron transfer e.g. Zn → Zn²⁺ + 2e⁻ (oxidation). The number of electrons lost must equal the number gained

[HIGHER] What is the Avogadro Constant?

6.02 × 10²³ — the number of particles (atoms, molecules or ions) in one mole of any substance

[HIGHER] What is a mole?

The unit of amount of substance. One mole of any element has a mass equal to its relative atomic mass in grams (the molar mass). Symbol: mol

[HIGHER] What is the equation linking moles, mass and molar mass?

Moles = mass ÷ molar mass (n = m ÷ M). Mass in grams, molar mass in g/mol

[HIGHER] How do you use moles to balance equations?

The mole ratio in a balanced equation = the ratio of moles of each substance in the reaction. Divide masses by molar mass to find moles, then compare the ratio to the coefficients in the balanced equation

[HIGHER] How do you calculate the mass of a product from a balanced equation?

1. Find moles of the given substance (moles = mass ÷ M). 2. Use the molar ratio from the balanced equation to find moles of the target substance. 3. Multiply moles by its molar mass to get the mass

[HIGHER] What is the limiting reactant?

The reactant that is completely used up first in a reaction — it determines the maximum amount of product that can form. The other reactant is in excess

[HIGHER] How do you identify the limiting reactant?

Calculate the moles of each reactant, then divide each by its coefficient in the balanced equation. The one with the smaller value is the limiting reactant