Higher Tier: hydrogen ions are reduced in acid–metal reactions
| English | 中文 | Pinyin |
|---|---|---|
| hydrogen ion | 氢离子 | qīng lí zi |
| net ionic reaction | 净离子反应 | jìng lí zi fǎn yìng |
What would explain this observation?
- Higher Tier: The hydrogen in bubbles from magnesium and acid has come from aqueous hydrogen ions 氢离子. Those ions gain electrons supplied by the magnesium.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- For Mg + 2H⁺ → Mg²⁺ + H₂, magnesium loses two electrons and is oxidised. The hydrogen ions gain electrons and are reduced. The half equations are Mg → Mg²⁺ + 2e⁻ and 2H⁺ + 2e⁻ → H₂. Combining cancels the transferred electrons. Chloride or sulfate ions remain in solution as appropriate spectators.
- hydrogen ion: The H⁺ ion present in an aqueous acid in this GCSE model; net ionic reaction 净离子反应: A reaction representation showing participating ions while omitting unchanged spectators.
Which species is reduced when Mg reacts with dilute hydrochloric acid?
Charge is +2 on both sides of the net equation. Two H⁺ ions are needed to form one H₂ molecule; gaining one electron each makes their total gain two electrons. The metal product is an ion, while the hydrogen product is a neutral diatomic molecule. Electron gain must be assigned to hydrogen ions, not to chloride ions just because hydrochloric acid was used.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Charge is +2 on both sides of the net equation. Two H⁺ ions are needed to form one H₂ molecule; gaining one electron each makes their total gain two electrons. The metal product is an ion, while the hydrogen product is a neutral diatomic molecule. Electron gain must be assigned to hydrogen ions, not to chloride ions just because hydrochloric acid was used.
- Separate the reactant species before identifying redox. In a supplied equation locate any metal atom becoming a positive ion and hydrogen ions becoming gas. Balance each half equation by atoms and charge. Do not generalise the simple net equation to every acid or every metal; the source specifies Mg, Zn and Fe with dilute hydrochloric/sulfuric acids.
Which two habits make the investigation or model in this case more defensible?
Separate the reactant species before identifying redox. In a supplied equation locate any metal atom becoming a positive ion and hydrogen ions becoming gas. Balance each half equation by atoms and charge. Do not generalise the simple net equation to every acid or every metal; the source specifies Mg, Zn and Fe with dilute hydrochloric/sulfuric acids.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: Fe → Fe²⁺ + 2e⁻ combines with 2H⁺ + 2e⁻ → H₂. Five represented Fe atoms release ten electrons, which reduce ten H⁺ ions to five H₂ molecules. The electron count explains the two-to-one H⁺:H₂ ratio without changing the number of iron nuclei.
How many electrons reduce twelve H⁺ ions to hydrogen molecules? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
How many electrons reduce twelve H⁺ ions to hydrogen molecules?
The result is 12 electrons. Known: Fe → Fe²⁺ + 2e⁻ combines with 2H⁺ + 2e⁻ → H₂. Five represented Fe atoms release ten electrons, which reduce ten H⁺ ions to five H₂ molecules. The electron count explains the two-to-one H⁺:H₂ ratio without changing the number of iron nuclei.
Check the conclusion and its limits
- Hydrogen gas formation is reduction in this account, even though no oxygen has been removed. Oxidation does not mean gaining positive charge by adding protons. This embedded HT explanation does not make common-tier salt names and product equations HT-only.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Hydrogen ions lose electrons when they become hydrogen gas. This claim is false: Hydrogen gas formation is reduction in this account, even though no oxygen has been removed. Oxidation does not mean gaining positive charge by adding protons. This embedded HT explanation does not make common-tier salt names and product equations HT-only.
Higher Tier: hydrogen ions are reduced in acid–metal reactions: Charge is +2 on both sides of the net equation. Two H⁺ ions are needed to form one H₂ molecule; gaining one electron each makes their total gain two electrons. The metal product is an ion, while the hydrogen product is a neutral diatomic molecule. Electron gain must be assigned to hydrogen ions, not to chloride ions just because hydrochloric acid was used.
Hydrogen ions lose electrons when they become hydrogen gas.
Hydrogen gas formation is reduction in this account, even though no oxygen has been removed. Oxidation does not mean gaining positive charge by adding protons. This embedded HT explanation does not make common-tier salt names and product equations HT-only.
The H⁺ ion present in an aqueous acid in this GCSE model: write the technical term.
hydrogen ion means The H⁺ ion present in an aqueous acid in this GCSE model.