Investigating acid reactions and salt preparation
Chemical changes • Reactions of acids
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Key concepts
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Heat changes: exothermic and endothermic reactions
Exothermic reactions release energy to the surroundings and cause a measurable rise in temperature of the solution. Examples include acid reacting with many metals and neutralisation of acids by bases. Endothermic reactions absorb energy and cause a temperature drop; experimental examples include some dissolution processes and particular displacement reactions under specific conditions. Temperature change is a direct indicator of net energy transfer but does not identify which bonds break or form; energy accounting requires consideration of all steps and the surroundings.
Variables that affect measured temperature changes
Concentration of acid affects the number of reacting particles per unit volume and therefore the total energy released for a fixed volume; higher concentration produces larger temperature changes for the same volume of acid and excess reactant. Mass of reactants and volume of solution change the thermal capacity of the system so larger masses dilute the temperature change for identical energy release. Surface area and particle size of solid reactants alter reaction rate and the rate of heat flow into the solution; increased surface area speeds reaction and concentrates energy release over a shorter time, producing a larger peak temperature change. Initial temperature, type of reactant, and the presence of impurities also influence measured temperature changes.
Experimental method and calorimetry calculations
A simple calorimeter uses an insulated container, a thermometer, and stirring to measure temperature change during a reaction. The heat transferred to the solution is estimated by q = mcΔT, where q is heat energy (J), m is mass of solution (g), c is specific heat capacity (4.18 J g−1 °C−1 for water-based solutions), and ΔT is temperature change (°C). The measured q may be divided by moles of limiting reagent to give energy per mole. Limiting factors include heat loss to the surroundings and calorimeter, incomplete reaction, inaccurate temperature readings, assumptions about specific heat capacity, and neglecting the calorimeter’s heat capacity. Use of a lid, insulation, and short reaction times reduces error.
Reaction types: acid with metals, acid with carbonates, neutralisation and displacement
Acid plus metal reactions produce a salt and hydrogen gas; the reaction is often exothermic, and rate depends on metal reactivity and surface area. Acid plus carbonate reactions produce a salt, carbon dioxide and water; the reaction is often exothermic but gas evolution can cause heat loss and mixing effects that alter measured temperature. Neutralisation between an acid and an alkali is typically strongly exothermic and gives a predictable heat per mole of acid-base reaction. Displacement reactions between metals in solution depend on relative reactivity; the energetics depend on bond energies and may be exothermic or only mildly exothermic.
Preparing pure, dry soluble salts from an insoluble oxide or carbonate
A named insoluble oxide or carbonate reacts with a specific acid to form a soluble salt and water (and carbon dioxide if carbonate). A sample preparation sequence produces pure crystals: the insoluble solid is added to the acid until no further mass dissolves (excess solid confirms complete reaction), the mixture is filtered to remove unreacted solid, the filtrate is evaporated to the point of crystallisation and left to crystallise, and the crystals are collected and dried. Purity depends on the completeness of reaction, filtration quality, extent of evaporation (avoid complete dryness), and washing of crystals with small amounts of cold solvent to remove soluble impurities. Controlled heating and careful drying prevent decomposition or loss of water of crystallisation.
Key notes
Important points to keep in mind