Types of Chemical Reactions (OCR GCSE Chemistry A (Gateway)): Flashcards

Reduction is the gain of electrons by a species during a chemical reaction.

OIL RIG stands for: Oxidation Is Loss of electrons, Reduction Is Gain of electrons. It is a memory aid for electron transfer in redox reactions.

False.

Magnesium loses two electrons to form Mg²⁺, so it is oxidised, not reduced. The copper ions gain electrons and are reduced.

In a displacement reaction, the metal that displaces another is oxidised because it loses electrons.

The half equation for the reduction of copper ions is: Cu²⁺ + 2e^– → Cu. The copper ions gain two electrons.

If electrons appear on the right-hand side of the half equation, the species has lost electrons and been oxidised. If electrons appear on the left-hand side, the species has gained electrons and been reduced.

True.

Electrons cannot be created or destroyed. Every electron lost by the oxidised species must be gained by the reduced species.

Zinc is oxidised in the reaction Zn + Cu²⁺ → Zn²⁺ + Cu. The half equation for this is: Zn → Zn²⁺ + 2e^–.

An acid is a substance that, when dissolved in water, produces hydrogen ions (H⁺) in solution.

An alkali is a substance that, when dissolved in water, produces hydroxide ions (OH^–) in solution.

True.

A pH of 7 sits at the midpoint of the scale and is neither acidic nor alkaline — it is neutral.

Acids have pH values below 7. The lower the pH, the more acidic the solution and the higher the concentration of H⁺ ions.

The pH scale runs from 1 to 14. Acids have a pH below 7 and alkalis have a pH above 7.

1. Using an indicator, which changes colour depending on the pH and is matched to a chart.\n2. Using a digital pH meter, which gives a precise numerical reading.

False.

Universal indicator is not suitable for titrations because it changes colour gradually over a wide pH range. A sharp colour change is needed, so a synthetic indicator with a narrow pH range should be used instead.

Synthetic indicators are more useful because they have narrow pH ranges and produce a sharp colour change at a specific pH. Natural indicators change colour gradually over a broader range.

The presence of H⁺ ions makes a solution acidic, while the presence of OH^– ions makes a solution alkaline.

A digital pH meter uses an electrode with a thin glass membrane that allows hydrogen ions to pass through. The ions alter the voltage detected by the electrode, which is converted to a pH reading.

A neutralisation reaction occurs when an acid reacts with a base to produce a salt and water.

The overall ionic equation is H⁺ + OH^– ⟶ H₂O. It shows that the H⁺ ions from the acid react with the OH^– ions from the base to produce water.

acid + base ⟶ salt + water

In a neutralisation reaction, the acid and base always produce a salt and water.

False.

Sulfuric acid produces sulfate salts. Chloride salts are produced by hydrochloric acid.

The products are a salt, water and carbon dioxide. The general equation is: acid + metal carbonate ⟶ salt + water + carbon dioxide.

A salt is a compound formed when the hydrogen ions of an acid are replaced by metal ions. Its name has two parts: one from the metal used and one from the acid used.

Only metals above hydrogen in the reactivity series react with dilute acids. The higher a metal is in the reactivity series, the more vigorous the reaction.

When hydrochloric acid reacts with copper carbonate, the products are copper chloride, carbon dioxide and water.

Hydrochloric acid always produces chloride salts.

Effervescence occurs because carbon dioxide gas is produced during the reaction. The bubbles of CO₂ are responsible for the fizzing observed.

False.

Neutralisation requires the production of salt and water. When a metal reacts with an acid, hydrogen gas is produced instead of water, so it does not fit the definition of neutralisation.

A strong acid is an acid that completely dissociates in water, producing a high concentration of H⁺ ions and a very low pH.

A weak acid is an acid that only partially ionises in water, producing a relatively low concentration of H⁺ ions and a pH typically between 4 and 6.

False.

Strength describes how completely an acid dissociates, while concentration describes how many acid particles are present per dm³ of solution. A strong acid can be dilute, and a weak acid can be concentrated.

A weak acid only partially ionises in water, so it produces fewer H⁺ ions than a strong acid of the same concentration. Fewer H⁺ ions means a higher pH.

For a weak acid, the equilibrium lies to the left, meaning there is a high concentration of intact acid molecules in solution.

This results in a low concentration of H⁺ ions and a pH closer to 7 than a strong acid.

The ⇌ symbol indicates that the reaction is reversible. The acid molecules and their ions can interconvert, establishing a dynamic equilibrium.

True.

A strong acid undergoes complete dissociation, producing more H⁺ ions even when dilute. A concentrated weak acid only partially ionises, so it can still have a higher pH than a dilute strong acid.

It means that at equilibrium, most of the acid exists as intact molecules rather than dissociated ions. The concentration of H⁺ ions is therefore low, giving a relatively high pH.

Strong acids such as HCl and H₂SO₄ dissociate completely in water, producing a high concentration of H⁺ ions.

This gives strong acids a very low pH.

Strong acids:\n\n1. Hydrochloric acid (HCl)\n2. Sulfuric acid (H₂SO₄)\n\nWeak acids:\n\n1. Ethanoic acid (CH₃COOH)\n2. Hydrofluoric acid (HF)

pH is a measure of the concentration of H⁺ ions in solution. A lower pH indicates a higher concentration of H⁺ ions.

It means that each change of 1 on the pH scale represents a change in H⁺ ion concentration by a factor of 10. A pH of 3 has ten times the H⁺ concentration of a pH of 4.

An acid with a pH of 2 has 100 times the concentration of H⁺ ions compared to an acid with a pH of 4.

The pH scale is logarithmic, so each unit represents a factor of 10, and two units represents a factor of 10 × 10 = 100.

True.

The pH scale is logarithmic, so each 1-unit decrease in pH represents a 10-fold increase in H⁺ ion concentration.

Adding a base to an acid increases the pH. The base provides OH^– ions that react with and reduce the concentration of H⁺ ions in solution, so the pH rises rapidly.

A strong acid completely dissociates in water, releasing more H⁺ ions per mole than a weak acid, which only partially dissociates. More H⁺ ions in solution means a lower pH.

False.

Concentration and acid strength are independent. A dilute strong acid can have a lower pH than a concentrated weak acid, because the strong acid dissociates completely while the weak acid only partially ionises.

A pH titration curve shows that when a base is added to an acid, the pH increases rapidly due to a decrease in the concentration of hydrogen ions.

This is because the OH^– ions from the base neutralise the H⁺ ions from the acid.

The pH decreases rapidly. Adding acid increases the concentration of H⁺ ions in solution, which lowers the pH, as shown by a pH titration curve.