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Unit 4
7–9% of examChemical reactions make new substances by rearranging atoms. In this unit you learn to write balanced, net ionic and redox equations, sort reactions into three main types, and use stoichiometry to predict how much of each substance reacts or forms. These skills come up again in almost every later unit.
Longer videos that cover the whole unit. Good for a first pass or a final review.
A physical change alters a substance's form or phase without changing what it's made of, while a chemical change makes new substances. Possible clues that a chemical change happened include a gas forming, a solid (precipitate) appearing, a color change, or heat or light being given off.
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A balanced equation has the same number of each kind of atom, and the same total charge, on both sides. You can write a reaction as a molecular equation, a complete ionic equation (dissolved ionic compounds shown as separate ions), or a net ionic equation that leaves out the spectator ions, such as Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
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You should be able to turn a balanced equation into a particle diagram and back again. A correct diagram keeps every atom, matches the ratios set by the coefficients, and shows any reactant left over.
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Chemical changes break and form chemical bonds, while physical changes such as melting or boiling only change the attractions between particles (intermolecular forces). Some cases are borderline: dissolving a salt breaks ionic bonds but forms ion-dipole attractions with water, so it can be argued either way.
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The coefficients in a balanced equation give mole ratios, so you can calculate how much product forms from a known amount of reactant, or the reverse. You can link these ratios to grams, to gases with PV = nRT, and to solutions with molarity (moles per liter).
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In a titration you add a solution of known concentration (the titrant) to a sample (the analyte) until you've added exactly enough to react with all of it: that's the equivalence point. An indicator's color change, called the endpoint, signals it, and the volume of titrant used tells you how much analyte there was.
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Most reactions you'll meet are acid-base (a proton, H⁺, moves between particles), redox (electrons move, shown by changes in oxidation numbers) or precipitation (ions combine into an insoluble solid). Combustion is a redox reaction with O₂, and burning a hydrocarbon completely gives CO₂ and H₂O. The only solubility rule you must memorize: compounds containing Na⁺, K⁺, NH₄⁺ or NO₃⁻ dissolve in water.
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In the Brønsted-Lowry model, an acid donates a proton (H⁺) and a base accepts one. After an acid gives up its proton it becomes its conjugate base, and a base that gains one becomes its conjugate acid; water can act as either. In NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, water is the acid: it gives a proton to ammonia.
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Oxidation is losing electrons and reduction is gaining them, and the two always happen together. To balance a redox equation, split it into an oxidation half-reaction and a reduction half-reaction, balance atoms and charge in each, make the electrons lost equal the electrons gained, then add the halves back together.
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