ChemSphere

Unit 4 · 7–9% of the exam

Chemical Reactions

Unit 4 is about writing reactions correctly and using them quantitatively. You will balance and write net ionic equations, use stoichiometry and titration to connect amounts, and classify reactions as precipitation, acid–base or redox. These are the working tools for every calculation later in the course.

Limiting reactant lab

Mix reactant amounts and see which runs out first, how much product forms and what is left over.

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Topics in this unit

  1. 4.1

    Introduction for Reactions

    A chemical reaction rearranges atoms into new substances; evidence includes gas production, a precipitate, a color change or a temperature change, but none of these alone proves a reaction occurred.

  2. 4.2

    Net Ionic Equations

    Net ionic equations show only the species that change; strong electrolytes in solution are written as ions and spectator ions are left out.

  3. 4.3

    Representations of Reactions

    Balanced equations, particle diagrams and net ionic equations must all conserve atoms and charge.

  4. 4.4

    Physical and Chemical Changes

    Chemical changes break and form chemical bonds to make new substances; physical changes alter only intermolecular interactions or state.

  5. 4.5

    Stoichiometry

    Mole ratios from the balanced equation connect amounts of reactants and products, and the limiting reactant determines how much product can form.

  6. 4.6

    Introduction to Titration

    In a titration, a solution of known concentration reacts with an analyte until the equivalence point, which reveals the analyte's amount.

  7. 4.7

    Types of Chemical Reactions

    Most reactions can be classified as acid–base (proton transfer), redox (electron transfer) or precipitation (formation of an insoluble solid).

  8. 4.8

    Introduction to Acid-Base Reactions

    Brønsted–Lowry acids donate protons and bases accept them, producing conjugate acid–base pairs.

  9. 4.9

    Oxidation-Reduction (Redox) Reactions

    Oxidation is loss of electrons (oxidation number increases) and reduction is gain of electrons (oxidation number decreases); they always occur together.

Must know

  • Moles from solution: n = M × V (V in liters)
  • Dilution: M1V1 = M2V2
  • Percent yield = (actual ÷ theoretical) × 100
  • Always soluble: Na+, K+, NH4+ and NO3− compounds (the solubility rules AP expects you to know)
  • Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO4 — written as ions in net ionic equations
  • Oxidation numbers: elements 0; O usually −2; H usually +1; the sum equals the charge of the species
  • OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
  • Equivalence point in a titration: moles of titrant added = stoichiometric amount of analyte

Common mix-ups

  • The reactant with the smaller mass is always the limiting reactant.

    You must compare moles, adjusted by the coefficients; a small mass of a light substance can be a large number of moles.

  • Weak acids such as CH3COOH should be written as ions in a net ionic equation.

    Weak acids and bases are mostly un-ionized in water, so they are written as molecules (CH3COOH(aq)).

  • The oxidizing agent is the substance that is oxidized.

    The oxidizing agent is reduced: it accepts the electrons that the substance being oxidized gives up.

Practice questions

AP-style multiple choice. Every answer comes with the reasoning, including why the tempting wrong answer is wrong.

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Aqueous silver nitrate is mixed with aqueous sodium chloride and a white precipitate forms. What is the net ionic equation?

Free-response practice

A student titrates a 0.500 g sample of impure potassium hydrogen phthalate (KHP, KHC8H4O4, M = 204.22 g/mol), a monoprotic acid, with 0.100 M NaOH. The equivalence point is reached after 18.50 mL of NaOH is added. (a) Calculate the moles of NaOH added. (b) Calculate the mass of KHP in the sample. (c) Calculate the mass percent of KHP in the sample. (d) The student did not rinse the buret with NaOH solution, so a few drops of water remained inside. Would the calculated percent KHP be too high, too low or unaffected? Explain.

Flashcards

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See it in real life

Everyday reactions that use Unit 4 ideas, animated atom by atom.