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Química Geral · Reactions in water and gases

Reações em solução aquosa

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O que você vai aprender

  • Classify solutes as strong, weak or nonelectrolytes and predict which ions they release in water.
  • Use the solubility rules to predict precipitates and choose reagents that separate or identify ions.
  • Write molecular, complete ionic and net ionic equations and identify spectator ions.
  • Write neutralization and gas-forming reactions, draw proton transfer to hydroxide as curved arrows, and do the stoichiometry of precipitation and gas-forming reactions.
  • Predict single-replacement reactions from the activity series and classify reactions as combination, decomposition, combustion, single replacement, precipitation, acid–base or redox.

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Electrolytes

A solute that gives ions in water makes the solution conduct electricity. A strong electrolyte is essentially completely ionized: soluble ionic compounds such as NaCl, the strong acids (HCl, HBr, HI, HNO₃, HClO₄, and H₂SO₄ for its first proton) and the strong bases (the Group 1 hydroxides and Ca(OH)₂, Sr(OH)₂, Ba(OH)₂). A weak electrolyte ionizes only partly: weak acids such as acetic acid (drawn with its ions; about 1% ionized at 0.1 M), HF and HNO₂, and weak bases such as NH₃. A nonelectrolyte, such as glucose, methanol or ethanol, dissolves as neutral molecules. Ionic compounds keep their polyatomic ions intact: (NH₄)₂SO₄ gives 2 NH₄⁺ + SO₄²⁻. Molecular acids such as HCl make ions only by reacting with water, HCl + H₂O → H₃O⁺ + Cl⁻; in this topic H⁺(aq) is shorthand for H₃O⁺. Electrolyte strength is not the same as solubility: AgCl is completely ionized once dissolved, but only about 1.3 × 10⁻⁵ mol/L dissolves.

Solubility rules

Always soluble: compounds of Group 1 cations and NH₄⁺, and all nitrates and perchlorates. Usually soluble: chlorides, bromides and iodides, except those of Ag⁺, Pb²⁺ and Hg₂²⁺; sulfates, except those of Ba²⁺, Pb²⁺, Sr²⁺ and Hg₂²⁺ (CaSO₄ and Ag₂SO₄ are only slightly soluble). Usually insoluble: carbonates, phosphates, sulfides and hydroxides, except with Group 1 cations and NH₄⁺; Sr(OH)₂ and Ba(OH)₂ are soluble and Ca(OH)₂ slightly soluble. A rule from the 'always soluble' group wins: (NH₄)₂CO₃ dissolves although most carbonates do not. Precipitates identify ions: BaSO₄ (drawn as its ions) is a white solid that does not dissolve in dilute HNO₃, whereas BaCO₃ dissolves in acid with fizzing because carbonate is protonated to H₂CO₃, which releases CO₂.

Molecular, complete ionic and net ionic equations

The molecular equation shows whole formulas: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). The complete ionic equation splits every strong electrolyte in solution into its ions and leaves solids, liquids, gases, weak electrolytes and nonelectrolytes as formulas: Ag⁺ + NO₃⁻ + Na⁺ + Cl⁻ → AgCl(s) + Na⁺ + NO₃⁻. Ions that appear unchanged on both sides are spectator ions (here Na⁺ and NO₃⁻); cancelling them leaves the net ionic equation, Ag⁺(aq) + Cl⁻(aq) → AgCl(s). A net ionic equation must balance atoms and charge. Watch for two traps: a solid reactant such as CaCO₃(s) is never split into ions, and a single mixture can form two products, as in Ba(OH)₂ + H₂SO₄, which makes both BaSO₄(s) and water and so has no spectator ions. In precipitation stoichiometry, find the limiting ion first: 5.00 mmol Ag⁺ and 4.50 mmol Cl⁻ give 4.50 mmol AgCl, 0.645 g at 143.32 g/mol.

Neutralization and gas-forming reactions

An acid and a hydroxide base give a salt and water. For any strong acid with any strong base the net ionic equation is H₃O⁺ + OH⁻ → 2 H₂O, a proton transfer drawn with two curved arrows (K = 1/Kw = 1.0 × 10¹⁴). A weak acid or weak base is kept as a molecule: HF + OH⁻ → F⁻ + H₂O and NH₃ + H⁺ → NH₄⁺. Gas-forming reactions make an unstable or volatile product. Carbonates and hydrogen carbonates with acid give H₂CO₃, which decomposes to CO₂ and H₂O; sulfites give SO₂ the same way; sulfides give H₂S gas; ammonium salts warmed with hydroxide release NH₃ (NH₄⁺ + OH⁻ → NH₃ + H₂O, drawn). Carbonate takes protons one at a time: acid added slowly to Na₂CO₃ first makes HCO₃⁻, and fizzing starts only after about one H⁺ per carbonate. Example: 2.65 g Na₂CO₃ (105.99 g/mol) is 0.02500 mol and releases 0.02500 mol CO₂, 1.10 g.

Single replacement and the activity series

A more active element displaces a less active one from its compound. The activity series of metals, most active first, is K > Na > Ca > Mg > Al > Zn > Fe > Ni > Sn > Pb > H₂ > Cu > Ag > Au. A metal reduces the cation of any metal below it: Fe(s) + Cu²⁺ → Fe²⁺ + Cu(s), but Cu(s) does nothing in Fe²⁺ solution. Metals above hydrogen dissolve in dilute strong acid with H₂ evolution; the most active (K, Na, Ca) also react with cold water, Ca + 2 H₂O → Ca(OH)₂ + H₂. Balance charge as well as atoms: 2 Al + 3 Cu²⁺ → 2 Al³⁺ + 3 Cu moves six electrons. Halogens follow F₂ > Cl₂ > Br₂ > I₂: bromine oxidizes iodide to iodine but cannot oxidize chloride. You can build a series from experiments: a metal that coats itself with a second metal in that metal's salt solution is the more active of the two.

Classifying reactions

Combination: two or more substances form one product (CaO + H₂O → Ca(OH)₂). Decomposition: one substance breaks into two or more (2 KClO₃ → 2 KCl + 3 O₂ on heating). Combustion: rapid reaction with O₂; a hydrocarbon burns to CO₂ and H₂O (C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O). Single replacement, precipitation and acid–base reactions are described above. These labels overlap with a second question: is it redox? A reaction is redox when any oxidation state changes. Every combustion and single replacement is redox (in CH₄ combustion carbon goes from −4 to +4 and oxygen from 0 to −2), whereas precipitation and acid–base reactions are not, and a decomposition such as CaCO₃ → CaO + CO₂ is not either.

What to carry forward

Decide what is really in the beaker: strong electrolytes as ions, weak electrolytes, nonelectrolytes and solids as formulas. Then ask what drives the reaction: an insoluble solid (solubility rules), water from H⁺ + OH⁻, a gas (CO₂, SO₂, NH₃), or electron transfer to a less active element (activity series). Cancel the spectator ions, balance atoms and charge, and do the stoichiometry from the limiting reactant.

Problemas de prática (40)

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Desenhe o mecanismo 2

Neutralization and gas-forming reactions

  • Ammonium and hydroxide release ammoniaDraw the proton transfer from ammonium to hydroxide behind the ammonia gas-forming reaction and write its net ionic equation.
  • Hydronium neutralizes hydroxideDraw the proton transfer from hydronium to hydroxide that is the net ionic equation of every strong acid–strong base neutralization, and identify the spectator ions.

Preveja o produto 13

Displacement and reaction types

  • Use the activity seriesPredict whether a metal displaces another metal or hydrogen from solution using the activity series.
  • Aluminium in copper(II) chlorideWrite a balanced net ionic equation for a single-replacement reaction in which the metals have different ion charges.
  • Bromine water with two halide saltsPredict which halide a halogen can displace from the halogen activity series, and write the net ionic equation.
  • Calcium meets waterPredict the products when a very active metal reacts with water.

Neutralization and gas-forming reactions

  • Net ionic equation for a weak baseWrite the net ionic equation for neutralization of a weak base by a strong acid.
  • Products of a neutralizationWrite the balanced molecular equation for neutralization of a strong base by a strong acid, with the correct salt formula.
  • Which mixture gives off a gas?Recognize the anion that releases a gas when treated with a strong acid.
  • Net ionic equation for a weak acidWrite the net ionic equation for neutralization of a weak acid by a strong base, keeping the weak acid as a molecule.

Electrolytes and solubility

  • A carbonate that dissolvesRecognize that an always-soluble cation overrides the rule that carbonates, hydroxides and sulfides are usually insoluble.
  • Which compound is insoluble?Apply the solubility rules to pick the one insoluble compound from a list of salts.

Precipitation and net ionic equations

  • Net ionic equation for calcium phosphateWrite a balanced net ionic equation for a precipitation that needs unequal ion coefficients.
  • The yellow precipitatePredict the precipitate formed when two soluble salt solutions exchange partners.
  • Which pair stays clear?Use the solubility rules on both possible new pairings to find a mixture in which no precipitate forms.

Classificar 8

Electrolytes and solubility

  • Ions from ammonium sulfateWrite the ions released when a salt of two polyatomic ions dissolves, keeping each polyatomic ion intact.
  • Classify three solutesClassify a strong acid, a weak acid and an alcohol as strong, weak or nonelectrolytes.
  • Spot the weak electrolyteIdentify a weak electrolyte among a strong acid, a soluble salt, a weak acid and a nonelectrolyte.

Displacement and reaction types

  • Two labels for one reactionClassify the combustion of methane both by reaction type and as a redox reaction, using oxidation states.
  • Which one is combustion?Identify a combustion reaction among decomposition, combination and single-replacement equations.
  • Heating potassium chlorateClassify a reaction in which one compound breaks into simpler substances.
  • Which reaction is redox?Recognize a redox reaction by checking for changes in oxidation state across precipitation, acid–base, decomposition and single-replacement reactions.

Precipitation and net ionic equations

  • Find the spectator ionsIdentify the spectator ions in a precipitation reaction from its complete ionic equation.

Comparar 1

  • Rank solutions by conductivityRank equal-concentration solutions by electrical conductivity from the number of ions each solute releases.

Calcular 6

Neutralization and gas-forming reactions

  • Acid neutralized by an antacid tabletUse the mole ratio of a neutralization with a dibasic hydroxide to find how much acid a given mass of base consumes.
  • Which reactant runs out?Identify the limiting reactant in a gas-forming reaction and calculate the mass of gas released.
  • Carbon dioxide from washing sodaCalculate the mass of gas released in a carbonate gas-forming reaction from the mass of carbonate.

Precipitation and net ionic equations

  • Barium ion left in solutionCalculate the concentration of the excess ion that remains after a precipitation, using the combined volume.
  • Mass of silver chloride precipitatedFind the limiting ion in a precipitation and calculate the mass of precipitate formed.

Displacement and reaction types

  • Copper deposited by ironCalculate the mass of metal deposited in a single-replacement reaction from the mass of the more active metal.

Encontre o erro 3

Precipitation and net ionic equations

  • A neutralization that also precipitatesDiagnose a net ionic equation that misses a precipitate formed alongside water in a neutralization.

Neutralization and gas-forming reactions

  • Find the error: marble in acidDiagnose a net ionic equation that wrongly splits a solid reactant into ions.

Electrolytes and solubility

  • Strong electrolyte, poor conductorDiagnose the confusion between electrolyte strength and solubility for a sparingly soluble salt.

Interprete as evidências 2

Precipitation and net ionic equations

  • Read two precipitate testsInterpret the behaviour of barium precipitates in acid to tell carbonate from sulfate.

Displacement and reaction types

  • Build an activity series from experimentsCombine displacement and acid observations to rank four unknown metals by activity.

Trabalhe de trás para frente 2

Electrolytes and solubility

  • Identify the cationWork back from two precipitation tests to the identity of an unknown cation.

Precipitation and net ionic equations

  • Which reaction has this net ionic equation?Work backwards from a net ionic equation to a molecular equation whose other ions are all spectators.

Planeje uma síntese 1

  • Separate Ag⁺ from Ba²⁺Choose a precipitating reagent that removes one cation from a mixture and leaves the other in solution.

Explique por quê 2

Neutralization and gas-forming reactions

  • Why the fizzing starts lateExplain the delayed release of CO₂ when acid is added slowly to carbonate, using the stepwise protonation of carbonate.

Electrolytes and solubility

  • Why aqueous HCl conductsExplain why a molecular acid that does not conduct as a pure liquid becomes a strong electrolyte in water.

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