OCR GCSE Chemistry - Paper 3

C1 - What is the definition of isotopes?

Atoms of the same element with the same number of protons but different numbers of neutrons (or different mass numbers)

C1 - How do you define atomic number?

The number of protons in the nucleus of an atom

C1 - What are the 3 key limitations of the simple particle model (inelastic spheres)?

It does not show the forces between particles, the actual particle sizes, or the space between them

C2 - What is the formula to calculate the Rf value in chromatography?

Distance moved by Substance / Distance moved by Solvent

C2 - Why is the start line drawn in pencil during paper/thin-layer chromatography?

Pencil lead (graphite) is insoluble and will not dissolve or run in the solvent

C2 - Describe metallic bonding in terms of structure and particles

A giant lattice of positive metal ions surrounded by a sea of delocalised electrons

C2 - Why do metals conduct electricity?

They contain delocalised electrons that are free to move and carry charge through the structure

C2 - Why are metals malleable and ductile?

Layers of positive metal ions can slide over each other

C2 - What holds giant ionic structures together?

Strong electrostatic forces of attraction between oppositely charged ions

C2 - Why do ionic compounds have high melting points?

Large amounts of energy are required to break the strong electrostatic forces of attraction between oppositely charged ions

C2 - Why do ionic compounds conduct electricity when molten or aqueous, but not when solid?

Ions are free to move and carry charge when molten/aqueous, but are fixed in a lattice when solid

C2 - Why does diamond have a very high melting point?

It contains many strong covalent bonds that require a large amount of energy to break

C2 - Why does graphite conduct electricity?

It has delocalised electrons between layers that can move and carry charge

C2 - Why is graphite soft and slippery?

It has weak intermolecular forces between layers, allowing the layers to slide over each other

C2 - Why do simple molecular substances have low boiling points?

They have weak intermolecular forces that require little energy to overcome (covalent bonds within molecules are not broken)

C2 - What size range defines nanoparticles?

Particles with a diameter between 1 nm and 100 nm (typically 10⁻⁹ m to 10⁻⁷ m)

C2 - Why do nanoparticles often react much faster than bulk materials?

They have a very high surface area to volume ratio

C3 - What is an endothermic reaction in terms of temperature change?

A reaction that takes in energy from the surroundings, causing the temperature of the surroundings to decrease

C3 - What is an exothermic reaction in terms of temperature change?

A reaction that gives out energy to the surroundings, causing the temperature of the surroundings to increase

C3 - What is activation energy?

The minimum amount of energy required for a chemical reaction to start (or for a successful collision to occur)

C3 - Why is bond breaking endothermic and bond making exothermic?

Energy is absorbed/taken in to break bonds, and energy is released/given out when new bonds form

C3 - How do you explain an exothermic reaction in terms of bond energies?

More energy is released during bond making than is taken in during bond breaking

C3 - What defines a strong acid?

An acid that completely ionises (or dissociates) in aqueous solution

C3 - What defines a weak acid?

An acid that only partially ionises (or dissociates) in aqueous solution

C3 - What is the difference between acid strength and acid concentration?

Strength is the degree of ionisation; concentration is the amount of acid solute dissolved per unit volume of solution

C3 - As the pH of a solution decreases by 1 unit, what happens to the hydrogen ion concentration?

The concentration of hydrogen ions (H⁺) increases by a factor of 10

C3 - What ionic equation represents neutralisation between an acid and an alkali?

H⁺ (aq) + OH⁻ (aq) →H₂O (l)

C3 - What is formed at the cathode during the electrolysis of a molten binary ionic compound?

The metal (or positive cation is reduced to form metal atoms)

C3 - In aqueous electrolysis, when is hydrogen gas produced at the cathode instead of a metal?

When the metal in solution is more reactive than hydrogen

C3 - In aqueous electrolysis using inert electrodes, when is oxygen gas produced at the anode?

When no halide ions (chloride, bromide, iodide) are present in the solution

C3 - What is oxidation in terms of electron transfer?

Loss of Electrons

C3 - What is reduction in terms of electron transfer?

Gain of Electrons

C3 - CRAO (Electrolysis)

Cathode = Reduction / Anode = Oxidation

C3 - Cations are…

Paws-ative (Positive)

C7 - What equipment is used to accurately measure a fixed volume of alkali and add variable acid during a titration?

A pipette for the alkali and a burette for the acid

C7 - How can you obtain pure, dry crystals of a soluble salt from a salt solution?

Gently heat/evaporate the solution to reach saturation, leave it to crystallise, filter the crystals, and dry them on filter paper or in a warm oven