ned}\na + b &= 2c \\\\\n\\frac{d}{2} + \\frac{2d}{3} &= 2c \\\\\n\\frac{3d + 4d}{6} &= 2c \\\\\n\\frac{7d}{6} &= 2c \\\\\nc &= \\frac{7d}{12}\n\\end{aligned}$$"},{"type":"text","order":5,"title":"Find integer coefficients","content":"To obtain the smallest positive integer coefficients, we look at the denominators of our expressions: $2$, $3$, and $12$. The least common multiple is $12$. \n\nLet's choose $d = 12$:\n- $a = \\frac{12}{2} = 6$\n- $b = \\frac{2(12)}{3} = 8$\n- $c = \\frac{7(12)}{12} = 7$\n- $d = 12$\n\nSubstituting these back gives our balanced equation:\n\n$$6\\text{NO}_2 + 8\\text{NH}_3 \\rightarrow 7\\text{N}_2 + 12\\text{H}_2\\text{O}$$"},{"type":"text","order":6,"title":"Final check and markscheme breakdown","content":"Let's double-check the final answers against the requirements:\n- **Oxidation states of N:** $+4$ in $\\text{NO}_2$, $-3$ in $\\text{NH}_3$, and $0$ in $\\text{N}_2$. B1\n- **Balanced chemical equation:** $6\\text{NO}_2 + 8\\text{NH}_3 \\rightarrow 7\\text{N}_2 + 12\\text{H}_2\\text{O}$. B1"}]}],"generatedAt":"2026-06-11T09:31:29.260Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]},{"id":"95302d4d-a114-4fce-b408-a9a2baf5f9da","label":"4","content":"Platinum in catalytic scrubbers acts as a heterogeneous catalyst to convert environmental pollutants. State the meaning of the term *heterogeneous catalyst*.","markscheme":"- A catalyst that is in a different physical state or phase from the reactants B1\n- Speeds up the reaction by providing an alternative reaction pathway with lower activation energy, without being consumed/used up B1","marks":2,"order":3,"markingRubric":null,"explanation":{"methods":[{"id":"direct-definition","label":"Direct Definition","steps":[{"type":"text","order":0,"title":"Define the physical state","content":"The prefix **hetero-** means different. In chemistry, a **heterogeneous catalyst** is a catalyst that is in a **different physical state (or phase)** from the reactants. For example, in a catalytic converter, the platinum catalyst is a solid, while the polluting reactant gases are in the gas phase. This difference in phase is what makes it 'heterogeneous'.","highlights":[{"text":"heterogeneous catalyst","location":"specification"}]},{"type":"text","order":1,"title":"Explain how it functions","content":"Next, we explain how it acts as a catalyst. A catalyst increases the rate of a chemical reaction. It does this by providing an **alternative reaction pathway** that has a **lower activation energy** ($E_a$). Crucially, the catalyst is **not consumed or used up** during the chemical reaction."},{"type":"text","order":2,"title":"State the final definition","content":"To secure both marks on your exam, write a complete definition combining both parts: 'A heterogeneous catalyst is a catalyst in a different physical state or phase from the reactants, which speeds up the reaction by providing an alternative reaction pathway with lower activation energy without being used up.' This addresses both marking points clearly."}]}],"generatedAt":"2026-06-11T09:31:48.027Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]},{"id":"20e33443-114b-434f-8f99-b483f47b3580","label":"5","content":"Soot (particulate carbon, \$\\text{C}\$) is another pollutant formed in jet engines during fuel combustion. Explain why carbon particulates are formed instead of carbon dioxide.","markscheme":"- Caused by incomplete combustion of the hydrocarbon fuel due to a limited/insufficient supply of oxygen B1","marks":1,"order":4,"markingRubric":null,"explanation":{"methods":[{"id":"incomplete_combustion","label":"Incomplete Combustion Explanation","steps":[{"type":"text","order":0,"title":"Analyze the question","content":"We need to explain why carbon particulates (soot, $\\text{C}$) are produced during the combustion of fuel in jet engines instead of carbon dioxide ($\\text{CO}_2$).","highlights":[{"text":"Explain why carbon particulates are formed instead of carbon dioxide."

Transition metals (A-level only)

Question 37

Transition metals (A-level only) Questions

Transition metals (A-level only)

Question 37

Transition metals (A-level only) Questions