Topic 4 Flashcards

Metal Oxides & Oxidation/Reduction

Metal Oxides & Oxidation/Reduction: read all 6 questions and answers

1. What happens when metals react with oxygen?
   They form metal oxides. The vigour of the reaction depends on the metal's position in the reactivity series. E.g. 2Mg + O₂ → 2MgO.
2. What is oxidation (basic definition)?
   The gain of oxygen. For example, when magnesium burns in air, it gains oxygen to form MgO - magnesium is oxidised.
3. What is reduction (basic definition)?
   The loss of oxygen. For example, when copper oxide reacts with carbon: 2CuO + C → 2Cu + CO₂. Copper oxide is reduced (loses oxygen).
4. What is a reducing agent?
   A substance that reduces another substance by removing its oxygen. The reducing agent itself is oxidised. For example, carbon is the reducing agent in 2CuO + C → 2Cu + CO₂.
5. What type of oxide are metal oxides?
   Metal oxides are basic oxides. They react with acids in neutralisation reactions to produce a salt and water.
6. A piece of iron is left exposed to air and moisture. What chemical process occurs, and what is the common name for the product formed?
   Oxidation occurs, specifically the reaction of iron with oxygen and water. The common name for the product is rust (hydrated iron(III) oxide).

Reactivity Series

Reactivity Series: read all 7 questions and answers

1. What is the reactivity series?
   A list of metals ranked in order of how vigorously they react. More reactive metals lose their outer shell electrons more readily to form positive ions.
2. State the reactivity series (most to least reactive).
   Potassium > Sodium > Lithium > Calcium > Magnesium > Aluminium > (Carbon) > Zinc > Iron > (Hydrogen) > Copper > Silver > Gold.
3. Why are carbon and hydrogen included in the reactivity series?
   Carbon is the benchmark for extraction methods (metals below carbon can be extracted by reduction with carbon). Hydrogen is included because only metals ABOVE hydrogen react with dilute acids.
4. Describe how potassium reacts with water.
   Violently - it ignites with a lilac flame, moves rapidly across the surface, and produces potassium hydroxide and hydrogen gas. Too dangerous for acid reactions.
5. Why does magnesium react with dilute acid but copper does not?
   Magnesium is above hydrogen in the reactivity series, so it can displace hydrogen from the acid. Copper is below hydrogen, so it cannot react with dilute acids.
6. How do you test for hydrogen gas?
   Hold a lighted splint near the mouth of the test tube. Hydrogen burns with a squeaky pop.
7. Aluminium is a reactive metal, yet it is widely used for drinks cans and window frames without corroding easily. Explain this apparent contradiction.
   Aluminium forms a thin, tough, unreactive layer of aluminium oxide on its surface when exposed to air. This oxide layer protects the underlying metal from further corrosion.

Displacement Reactions

Displacement Reactions: read all 5 questions and answers

1. What is a displacement reaction?
   A more reactive metal displaces (pushes out) a less reactive metal from a compound in solution. The more reactive metal has a greater tendency to form positive ions.
2. What happens when iron is placed in copper sulfate solution?
   Iron displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. The blue solution fades to pale green, and a brown coating of copper appears on the iron nail.
3. Will zinc react with magnesium chloride solution?
   No. Magnesium is MORE reactive than zinc, so zinc cannot displace it. No reaction occurs.
4. Will magnesium react with zinc sulfate solution?
   Yes. Magnesium is MORE reactive than zinc, so it displaces zinc: Mg + ZnSO₄ → MgSO₄ + Zn. Grey zinc metal appears.
5. How can displacement reactions be used to determine the reactivity order of metals?
   Test pairs of metals with each other's salt solutions. If a reaction occurs, the added metal is more reactive. By comparing all pairs, you can rank them.

Extraction of Metals

Extraction of Metals: read all 6 questions and answers

1. What is an ore?
   A rock containing enough metal to make extraction economically worthwhile.
2. How are metals below carbon in the reactivity series extracted?
   By reduction with carbon (e.g. In a blast furnace). Carbon removes the oxygen from the metal oxide. E.g. 2Fe₂O₃ + 3C → 4Fe + 3CO₂.
3. How are metals above carbon in the reactivity series extracted?
   By electrolysis. They are too reactive for carbon to remove the oxygen. Examples: aluminium, sodium, potassium.
4. What is phytomining?
   Using plants to absorb metal compounds from the soil. The plants are harvested and burned, and the metal is extracted from the ash. Useful for low-grade ores.
5. What is bioleaching?
   Using bacteria to produce a leachate solution containing dissolved metal compounds. The metal ions can be recovered by displacement or electrolysis.
6. Why are phytomining and bioleaching important?
   They are more environmentally friendly than traditional mining and can extract metals from low-grade ores, reducing the need for quarrying.

Redox & Electron Transfer (Higher Tier)

Redox & Electron Transfer: read all 6 questions and answers

1. Define oxidation in terms of electrons (HT).
   Oxidation Is Loss of electrons (OIL). A substance that loses electrons is oxidised.
2. Define reduction in terms of electrons (HT).
   Reduction Is Gain of electrons (RIG). A substance that gains electrons is reduced.
3. What is a redox reaction?
   A reaction where both oxidation and reduction occur simultaneously. One substance loses electrons (oxidised) while another gains them (reduced).
4. Write the half-equations for Mg displacing Cu²⁺ (HT).
   Oxidation: Mg → Mg²⁺ + 2e⁻ (Mg loses electrons). Reduction: Cu²⁺ + 2e⁻ → Cu (Cu²⁺ gains electrons). Electrons lost = electrons gained.
5. What is an oxidising agent?
   A substance that causes another substance to be oxidised. The oxidising agent is itself reduced (gains electrons). E.g. Cu²⁺ is the oxidising agent in the Mg/Cu reaction.
6. What is the general term for the process where a metal gains electrons during a chemical reaction, and at which electrode does this occur during electrolysis?
   The process is called reduction. During electrolysis, reduction occurs at the cathode (negative electrode).

Reactions of Acids

Reactions of Acids: read all 6 questions and answers

1. What are the four types of acid reaction?
   1. Acid + metal → salt + hydrogen. 2. Acid + metal oxide → salt + water. 3. Acid + metal hydroxide → salt + water. 4. Acid + metal carbonate → salt + water + CO₂.
2. Which acid produces chloride salts?
   Hydrochloric acid (HCl). E.g. Mg + 2HCl → MgCl₂ + H₂.
3. Which acid produces sulfate salts?
   Sulfuric acid (H₂SO₄). E.g. Zn + H₂SO₄ → ZnSO₄ + H₂.
4. Which acid produces nitrate salts?
   Nitric acid (HNO₃). E.g. NaOH + HNO₃ → NaNO₃ + H₂O.
5. How do you test for carbon dioxide?
   Bubble the gas through limewater (calcium hydroxide solution). If CO₂ is present, the limewater turns milky (cloudy white).
6. Why do only metals above hydrogen react with dilute acids?
   Only metals more reactive than hydrogen can displace hydrogen from the acid. Metals below hydrogen (e.g. Copper, silver, gold) cannot react with dilute acids.

Neutralisation & Making Salts

Neutralisation & Making Salts: read all 6 questions and answers

1. What is neutralisation?
   The reaction between an acid and a base to produce a salt and water. Acid + base → salt + water.
2. What is the ionic equation for neutralisation?
   H⁺(aq) + OH⁻(aq) → H₂O(l). Hydrogen ions from the acid combine with hydroxide ions from the base to form water.
3. What is the difference between a base and an alkali?
   A base is any substance that neutralises an acid. An alkali is a SOLUBLE base that produces OH⁻ ions in solution. All alkalis are bases, but not all bases are alkalis.
4. Describe how to make a pure dry sample of a soluble salt (Required Practical 1).
   1. Warm dilute acid. 2. Add excess insoluble base, stir. 3. Filter to remove excess solid. 4. Heat filtrate to evaporate half the water. 5. Cool and crystallise. 6. Pat dry with filter paper.
5. Why do you add excess base when making a soluble salt?
   To ensure all the acid has reacted, so the salt solution is not contaminated with unreacted acid. The excess base is then removed by filtration.
6. What salt is produced from CuO + H₂SO₄?
   Copper sulfate (CuSO₄). The copper comes from CuO, sulfate comes from sulfuric acid. CuO + H₂SO₄ → CuSO₄ + H₂O.

pH, Strong & Weak Acids

pH, Strong & Weak Acids: read all 6 questions and answers

1. What does the pH scale measure?
   How acidic or alkaline a solution is. PH 0-6 = acidic, pH 7 = neutral, pH 8-14 = alkaline.
2. What is a strong acid?
   An acid that fully ionises (dissociates) in water. Every molecule splits to release H⁺ ions. This is irreversible. Examples: HCl, H₂SO₄, HNO₃.
3. What is a weak acid?
   An acid that only partially ionises in water. A reversible equilibrium exists, with most molecules remaining undissociated. Examples: ethanoic acid, citric acid.
4. What is the difference between strong/weak and concentrated/dilute?
   Strong/weak = degree of ionisation (how much the acid splits into ions). Concentrated/dilute = amount of acid dissolved per unit volume. A dilute strong acid and a concentrated weak acid are both valid.
5. How does the pH scale relate to H⁺ concentration? (HT)
   The pH scale is logarithmic. Each decrease of 1 pH unit means the H⁺ concentration increases by a factor of 10. A decrease of 2 pH units = 100× more H⁺ ions.
6. At the same concentration, which has a lower pH: a strong or weak acid?
   A strong acid has a lower pH because it fully ionises, producing more H⁺ ions in solution. A weak acid only partially ionises, so fewer H⁺ ions are produced.

Titrations (Higher Tier / RP)

Titrations: read all 5 questions and answers

1. What is a titration?
   A technique to find the precise volume of one solution needed to react completely with a known volume of another. Used to determine unknown concentrations.
2. Describe the titration method (Required Practical 2).
   1. Pipette a fixed volume of alkali into a conical flask. 2. Add indicator. 3. Fill burette with acid. 4. Add acid dropwise, swirling. 5. Stop at the end point (permanent colour change). 6. Record titre. 7. Repeat for concordant results.
3. What are concordant results?
   Titre values that are within 0.10 cm³ of each other. You need at least two concordant results to calculate a reliable mean.
4. Why use a white tile during a titration?
   To see the colour change of the indicator more clearly against a white background, making it easier to identify the exact end point.
5. How do you read a burette correctly?
   Read at the bottom of the meniscus, at eye level. The titre = final reading − initial reading.

Electrolysis

Electrolysis: read all 9 questions and answers

1. What is electrolysis?
   Using electricity to decompose an ionic compound. The compound must be molten or dissolved in water so the ions are free to move.
2. What happens at the cathode and anode?
   Cathode (−): attracts cations (positive ions). Reduction occurs (cations gain electrons). Anode (+): attracts anions (negative ions). Oxidation occurs (anions lose electrons). Remember: AN OX, RED CAT.
3. What are the products of electrolysing molten lead(II) bromide?
   Cathode: lead metal (Pb²⁺ + 2e⁻ → Pb). Anode: bromine gas (2Br⁻ → Br₂ + 2e⁻).
4. Why is cryolite used in the extraction of aluminium?
   Pure Al₂O₃ has a very high melting point (>2000°C). Dissolving it in molten cryolite lowers the operating temperature to ~950°C, significantly reducing energy costs.
5. Why must the carbon anodes be replaced during aluminium extraction?
   The oxygen produced at the anode reacts with the hot carbon electrodes to form CO₂, gradually wearing them away.
6. In aqueous electrolysis, what determines the product at the cathode?
   If the metal is MORE reactive than hydrogen → hydrogen gas is produced. If the metal is LESS reactive than hydrogen → the metal is deposited. E.g. CuSO₄ gives copper; NaCl gives hydrogen.
7. When aqueous copper(II) sulphate is electrolysed using inert electrodes, what product is observed at the anode?
   Oxygen gas is observed at the anode. Hydroxide ions (from water) are preferentially discharged over sulphate ions, losing electrons (oxidation) to form oxygen and water.
8. A student wants to electroplate a steel spoon with silver. Which electrode should the spoon be connected to, and what should the electrolyte contain?
   The steel spoon should be connected to the negative electrode (cathode). The electrolyte should contain silver ions, typically from a solution of silver nitrate.
9. Consider the electrolysis of concentrated aqueous sodium chloride solution. What are the products at the anode and cathode, respectively?
   At the anode (positive electrode), chlorine gas is produced (preferential discharge of chloride ions over hydroxide ions due to high concentration). At the cathode (negative electrode), hydrogen gas is produced (preferential discharge of hydrogen ions from water over sodium ions).