Chemistry - Chemical Reactions

In a chemical reaction, bonds are broken (requiring energy) and new bonds are formed (releasing energy). Explain why some reactions need an initial input of energy to start, even though the overall reaction releases energy.

Model response: All chemical reactions require an initial input of energy to break the bonds in the reactant molecules — this is the activation energy. Even in highly exothermic reactions (like combustion), the reactant bonds must be broken before new, stronger bonds can form in the products. A match must be struck to start a fire because the initial heat provides the activation energy to break bonds in the fuel and oxygen molecules. Once these initial bonds are broken and new bonds form, the energy released by forming product bonds exceeds the energy needed to continue breaking reactant bonds, so the reaction sustains itself. Catalysts work by providing an alternative reaction pathway with lower activation energy — the same overall energy change occurs, but less initial energy is needed. This is fundamentally different from nuclear reactions, where the nuclei of atoms themselves are changed (fission or fusion), releasing millions of times more energy. In chemical reactions, nuclei are unchanged — only the electrons in bonds are rearranged. This distinction between chemical reactions (electron rearrangement) and nuclear reactions (nuclear change) is one of the most important boundaries in science.

The equation 2Mg + O₂ → 2MgO tells us that 2 atoms of magnesium react with 1 molecule of oxygen. Using relative atomic masses (Mg = 24, O = 16), calculate the mass of magnesium oxide produced when 4.8 g of magnesium burns completely.

Model response: From the equation: 2Mg + O₂ → 2MgO. Relative masses: 2 × 24 = 48 g of Mg reacts with 32 g of O₂ to produce 2 × (24+16) = 80 g of MgO. So 48 g Mg → 80 g MgO. We have 4.8 g Mg (which is 48/10 = one-tenth of the equation amount). Therefore MgO produced = 80/10 = 8.0 g. Check with conservation of mass: 4.8 g Mg + 3.2 g O₂ = 8.0 g MgO (4.8 + 3.2 = 8.0). The balanced equation is essentially a recipe: it tells us the exact proportions in which substances react and are produced. In industry, this is critical for calculating how much raw material is needed and how much product to expect. The gap between calculated and actual yield (percentage yield) indicates losses during the process — an important economic consideration.

When ammonium dichromate is heated, it decomposes vigorously, producing nitrogen gas, water vapour, and chromium(III) oxide — it resembles a miniature volcano. Classify this reaction and explain why it is so vigorous.

Model response: This is a thermal decomposition reaction — a single compound breaks down when heated. The equation is: (NH₄)₂Cr₂O₇ → N₂ + 4H₂O + Cr₂O₃. The vigour is explained by two factors: (1) the reaction is exothermic — once initiated by heat, the energy released sustains and accelerates the decomposition (it is self-propagating), and (2) a large volume of gas is produced (nitrogen and steam), which causes the dramatic expansion that resembles a volcanic eruption. This reaction also involves internal redox: the nitrogen in the ammonium ion is oxidised (from -3 to 0 oxidation state) while the chromium is reduced (from +6 to +3). The different reaction types are not always distinct — this single reaction is simultaneously a decomposition and a redox reaction. Understanding reaction types is industrially vital: thermal decomposition of limestone (CaCO₃ → CaO + CO₂) is one of the oldest and most important industrial processes, producing quicklime for cement, steel-making, and agriculture. Displacement reactions are used in metal extraction. Combustion reactions power vehicles and generate electricity. Each type has characteristic energy profiles and practical applications.

Hydrochloric acid (HCl) and ethanoic acid (CH₃COOH) can both be at the same concentration, but HCl is a strong acid and ethanoic acid is a weak acid. Explain the difference and how it affects pH.

Model response: The distinction between strong and weak is about dissociation, not concentration. A strong acid (like HCl) fully dissociates in water — every HCl molecule splits into H⁺ and Cl⁻ ions. A weak acid (like ethanoic acid) only partially dissociates — most molecules remain intact as CH₃COOH, with only a small fraction splitting into H⁺ and CH₃COO⁻ ions. At the same concentration (e.g., 1 mol/dm³), HCl has a much higher H⁺ ion concentration than ethanoic acid, so HCl has a lower pH. This is separate from concentration, which describes how much acid is dissolved per unit volume. You can have concentrated weak acid (lots of ethanoic acid, still only partially dissociated) or dilute strong acid (a little HCl, but fully dissociated). A concentrated weak acid might have a similar pH to a dilute strong acid, but they would behave differently in reactions — the concentrated weak acid has a larger reserve of undissociated molecules that can dissociate as H⁺ ions are used up, giving it a buffering capacity. This distinction is crucial in biology (blood is buffered by weak acid/conjugate base systems) and in industry (choosing the right acid for the right application).

Acid rain has a pH of approximately 4. Normal rain has a pH of approximately 5.6. How many times more acidic is acid rain than normal rain, and why is normal rain not pH 7?

Model response: Since the pH scale is logarithmic, a difference of 1.6 pH units means acid rain has approximately 10^1.6 ≈ 40 times more H⁺ ions than normal rain. Normal rain is not pH 7 because atmospheric CO₂ dissolves in rainwater to form carbonic acid (H₂CO₃): CO₂ + H₂O → H₂CO₃. This weak acid gives normal, unpolluted rain a natural pH of about 5.6. Acid rain (pH < 5.0) is caused by additional pollutants: sulfur dioxide (from burning fossil fuels) dissolves to form sulfuric acid (SO₂ + H₂O + ½O₂ → H₂SO₄), and nitrogen oxides form nitric acid (2NO₂ + H₂O → HNO₃ + HNO₂). The environmental damage from acid rain is significant: it acidifies lakes and rivers (killing fish and aquatic organisms), damages tree foliage (leaching essential minerals from leaves and soil), corrodes limestone buildings and monuments, and depletes soil nutrients. The 40-fold increase in acidity may sound modest, but because the pH scale is logarithmic, this represents a substantial chemical change. International efforts to reduce SO₂ and NOₓ emissions (like the Clean Air Act and catalytic converters) have significantly reduced acid rain in many regions, demonstrating that environmental damage is reversible when emissions are controlled.

Magnesium reacts much more vigorously with acid than iron does. Explain why, and describe how you would investigate the effect of acid concentration on reaction rate.

Model response: Magnesium is higher in the reactivity series than iron — its atoms lose electrons more easily and more rapidly. This means the rate of electron transfer to H⁺ ions is faster, producing hydrogen gas more quickly (more vigorous fizzing). To investigate the effect of acid concentration: react identical pieces of magnesium ribbon with hydrochloric acid at different concentrations (0.5M, 1.0M, 1.5M, 2.0M). Control variables: same length/mass of magnesium, same volume of acid, same temperature, same surface area of magnesium. Measure either the volume of hydrogen gas produced over time (using a gas syringe) or the time taken for the magnesium to dissolve completely. I predict that higher acid concentration increases the reaction rate because there are more H⁺ ions per unit volume, so collisions between H⁺ ions and the metal surface are more frequent. Plotting volume of gas against time gives a curve — the initial gradient represents the initial rate. This collision theory principle is applied industrially in metal processing: pickling (cleaning metal surfaces with acid) uses optimised acid concentrations and temperatures to maximise cleaning rate while minimising metal loss.

In a titration, 20.0 cm³ of 0.10 mol/dm³ NaOH exactly neutralises 25.0 cm³ of HCl. Calculate the concentration of the HCl. Why would phenolphthalein be a better indicator choice than universal indicator for this titration?

Model response: Moles of NaOH = concentration × volume = 0.10 × (20.0/1000) = 0.002 mol. From the equation NaOH + HCl → NaCl + H₂O, the mole ratio is 1:1, so moles of HCl = 0.002 mol. Concentration of HCl = moles/volume = 0.002/(25.0/1000) = 0.080 mol/dm³. Phenolphthalein is better than universal indicator for this titration because it has a sharp, clear colour change at a specific pH (colourless below pH 8.3, pink above) — you can identify the endpoint precisely to one drop. Universal indicator changes colour gradually across a range of pH values, making it impossible to identify the exact endpoint. Universal indicator is useful for estimating pH but unsuitable for accurate titrations. For a strong acid-strong base titration (like HCl + NaOH), any indicator that changes colour near pH 7 would work because the pH change at the endpoint is very steep (jumping from about pH 3 to pH 11 with a single drop). For weak acid-strong base titrations, phenolphthalein (endpoint around pH 8-10) is preferred because the equivalence point is above pH 7.

The Haber process uses an iron catalyst to manufacture ammonia. Without the catalyst, the reaction would need impractically high temperatures. Explain the economic and environmental significance of catalysts in industrial chemistry.

Model response: The Haber process (N₂ + 3H₂ ⇌ 2NH₃) operates at 450°C with an iron catalyst. Without the catalyst, temperatures above 1000°C would be needed to achieve a useful rate — this would be prohibitively expensive in energy costs, require specialised (expensive) equipment, and actually shift the equilibrium away from ammonia (since the forward reaction is exothermic). The catalyst allows the reaction to proceed at a reasonable rate at a lower temperature, saving enormous amounts of energy and making ammonia production (essential for fertilisers feeding billions) economically viable. Catalysts are also environmentally critical: catalytic converters in car exhausts use platinum, palladium, and rhodium to convert CO, NOₓ, and unburnt hydrocarbons into CO₂, N₂, and H₂O — without these catalysts, air pollution in cities would be far worse. Importantly, catalysts do not change the position of equilibrium or the overall yield — they only speed up the rate at which equilibrium is reached. They are not consumed, so they can be used indefinitely in principle, but catalyst poisoning (impurities blocking active sites — e.g., lead poisoning platinum catalysts, which is why leaded petrol was phased out) can deactivate them. Research into new, more efficient, and less expensive catalysts (including nanocatalysts and biocatalysts) is one of the most active areas of chemistry, with implications for green energy, pharmaceutical manufacturing, and pollution control.

The latent heat of vaporisation of water (2,260 kJ/kg) is approximately 6.7 times greater than its latent heat of fusion (334 kJ/kg). Explain why, using the particle model.

Model response: During melting (fusion), particles gain enough energy to overcome some intermolecular forces and move from a fixed, regular arrangement to a close but disordered arrangement. The particles remain close together — they have overcome the forces that hold them in fixed positions but are still attracted to neighbouring particles. During boiling (vaporisation), particles must gain enough energy to completely overcome all remaining intermolecular forces and separate from each other entirely, moving far apart as a gas. This requires much more energy because all intermolecular attractions must be broken, not just the forces maintaining the fixed structure. This has practical significance: steam burns are much more severe than boiling water burns because steam releases its latent heat of vaporisation (2,260 kJ/kg) when it condenses on skin, in addition to the heat from cooling. This large latent heat also explains why steam is used in power stations and heating systems — it carries enormous amounts of energy in the gas phase that is released on condensation. Conversely, sweating cools you efficiently because evaporating water absorbs a large amount of heat energy from your skin (using the latent heat of vaporisation to cool you).

Using bond energies: C-H = 413 kJ/mol, O=O = 498 kJ/mol, C=O = 805 kJ/mol, O-H = 464 kJ/mol, calculate the overall energy change for the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O.

Model response: Bonds broken (energy absorbed): 4 × C-H = 4 × 413 = 1652 kJ; 2 × O=O = 2 × 498 = 996 kJ. Total energy in = 2648 kJ. Bonds formed (energy released): 2 × C=O = 2 × 805 = 1610 kJ; 4 × O-H = 4 × 464 = 1856 kJ. Total energy out = 3466 kJ. Overall energy change = energy in - energy out = 2648 - 3466 = -818 kJ/mol. The negative sign indicates the reaction is exothermic — more energy is released forming new bonds than is required to break the old ones. This 818 kJ per mole of methane is why natural gas is used as a fuel. In reality, the value is approximately -890 kJ/mol — bond energy calculations give approximate values because bond energies are averages across different molecular environments. This calculation method is powerful because it allows prediction of energy changes for reactions that have not been performed, which is valuable in designing fuels, explosives, and industrial processes. The balance between exothermic and endothermic processes also underlies climate science — the greenhouse effect involves the balance between exothermic infrared emission from Earth and endothermic absorption by greenhouse gases.