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Química Geral · Solutions and equilibrium

Soluções e equilíbrio químico

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O que você vai aprender

  • Convert between molarity, molality, mole fraction and ppm, and plan dilutions with M₁V₁ = M₂V₂.
  • Calculate freezing-point depression, boiling-point elevation, osmotic pressure and vapour pressure using the van 't Hoff factor, Raoult's law and Henry's law.
  • Write K expressions, compare Q with K, solve ICE tables, convert Kc to Kp and predict Le Châtelier shifts, including the effect of temperature through ΔH°.
  • Relate Ksp to molar solubility, predict precipitation from Q, and explain the common-ion effect and complex-ion formation.
  • Draw the proton transfers and Lewis acid–base steps behind solution equilibria with curved arrows.

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Concentration units

Molarity, M, is moles of solute per litre of solution; it changes slightly with temperature because volume does. Molality, m, is moles of solute per kilogram of solvent and does not. Mole fraction, χ, is moles of one component over total moles. Parts per million is mass of solute over mass of solution × 10⁶, so 1 ppm is 1 mg per kg (about 1 mg/L for dilute water). Dilution keeps moles of solute constant: M₁V₁ = M₂V₂.

Colligative properties

Properties that depend only on the number of dissolved particles: ΔTf = i·Kf·m, ΔTb = i·Kb·m and Π = i·M·R·T (T in kelvin, R = 0.08206 L·atm/(mol·K)). The van 't Hoff factor i counts particles per formula unit in the ideal limit: 1 for glucose, 2 for NaCl, 3 for CaCl₂. For water Kf = 1.86 °C·kg/mol and Kb = 0.512 °C·kg/mol. Raoult's law gives the vapour pressure of a component as P = χ·P°, so a nonvolatile solute lowers the solvent's vapour pressure.

Gases in liquids

Henry's law: the solubility of a gas is proportional to its partial pressure above the liquid, S = kH·P. Dissolving a gas is exothermic, so gases such as O₂ and CO₂ are less soluble in warm water. Dissolved CO₂ reacts slightly with water to form carbonic acid, which is why soda water is mildly acidic.

K, Q and ICE tables

For aA + bB ⇌ cC + dD, K = [C]ᶜ[D]ᵈ/([A]ᵃ[B]ᵇ); pure solids and liquids are left out. Q has the same form with current concentrations: Q < K shifts toward products, Q > K toward reactants. An ICE table (initial, change, equilibrium) turns K into one equation in x; the small-x approximation is safe only when x is under about 5% of the initial amount. Reversing a reaction inverts K, multiplying the coefficients by n raises K to the power n, and Kp = Kc(RT)^Δn with Δn = moles of gaseous products − moles of gaseous reactants.

Le Châtelier and temperature

A stressed equilibrium shifts to relieve the stress: added reactant shifts it to products, compression shifts it toward fewer gas moles, and an inert gas at constant volume or a catalyst changes nothing. Only temperature changes K. The van 't Hoff equation, ln(K₂/K₁) = −(ΔH°/R)(1/T₂ − 1/T₁), shows that K rises with temperature for an endothermic reaction and falls for an exothermic one.

Solubility equilibria

A sparingly soluble salt sets up MₐXᵦ(s) ⇌ aMⁿ⁺ + bXᵐ⁻ with Ksp = [M]ᵃ[X]ᵇ. For AgCl, Ksp = s²; for CaF₂, Ksp = 4s³, so salts of different formula types must be compared through s, not Ksp. A common ion lowers solubility. A precipitate forms when Q > Ksp, using concentrations after mixing. Acid dissolves salts of basic anions (carbonate, hydroxide, fluoride), and ammonia dissolves AgCl by forming [Ag(NH₃)₂]⁺: K = Ksp·Kf.

What to carry forward

Count particles for colligative properties, keep track of what is a pure phase when writing K, compare Q with K (or Ksp) to predict direction, remember that only temperature changes K, and solve ICE tables honestly when x is not small.

Problemas de prática (40)

Praticar é grátis com uma conta.

Desenhe o mecanismo 6

Solution equilibria with curved arrows

  • Acetate ion accepts a proton from waterDraw the proton transfer that makes a solution of sodium acetate basic, and relate it to Kb = Kw/Ka.
  • Ammonia accepts a proton from waterDraw the proton transfer from water to ammonia and use Le Châtelier's principle to predict the effect of a common ion on this equilibrium.
  • Ammonium ion donates a proton to waterDraw the proton transfer that makes a solution of ammonium chloride acidic, and relate it to Ka = Kw/Kb.
  • Carbon dioxide reacts with water to form carbonic acidDraw the addition of water to dissolved CO₂ and the proton transfers through water that give carbonic acid, and relate the small hydration constant to the acidity of soda water.
  • Hydronium protonates carbonate as limestone dissolves in acidDraw the proton transfer from hydronium to carbonate and explain why removing a basic anion increases the solubility of its salt.
  • Ammonia coordinates to silver(I)Draw the Lewis acid–base step in which an ammonia lone pair forms a coordinate bond to Ag⁺, the step that lets ammonia dissolve silver chloride.

Preveja o produto 4

Concentration and colligative properties

  • Pressurising CO₂ over waterPredict how the solubility of a gas changes with its partial pressure using Henry's law.

Equilibrium constants and Le Châtelier

  • Making more NO₂Predict which stress shifts an endothermic gas-phase equilibrium toward products.
  • Which way will it shift?Compute a reaction quotient with stoichiometric exponents and compare it with K to predict the direction of net reaction.

Solubility and complex ions

  • Will CaF₂ precipitate?Predict whether a precipitate forms by comparing Q, calculated after dilution on mixing, with Ksp.

Classificar 3

Solubility and complex ions

  • Which salt dissolves in acid?Recognize that salts of basic anions become more soluble in acidic solution.
  • Ksp of calcium phosphateWrite a solubility-product expression with the correct exponents for a salt with unequal ion ratios.

Equilibrium constants and Le Châtelier

  • K for a heterogeneous equilibriumWrite an equilibrium-constant expression that omits pure solids.

Comparar 2

Concentration and colligative properties

  • Rank boiling pointsRank aqueous solutions by boiling point using the total molality of dissolved particles.

Solubility and complex ions

  • Rank salts by molar solubilityRank salts of different formula types by molar solubility rather than by Ksp.

Calcular 19

Solubility and complex ions

  • CaF₂ in a fluoride solutionCalculate the molar solubility of a salt in a solution that already contains one of its ions.
  • Solubility of AgCl in 1.0 M NH₃Combine Ksp and Kf into one equilibrium constant and solve for the solubility of a salt in a complexing solution.
  • Ksp from molar solubilityCalculate Ksp from the molar solubility of a 2:1 salt.
  • Molar solubility of CaF₂Calculate the molar solubility of a 1:2 salt from its Ksp.

Concentration and colligative properties

  • Freezing point of a CaCl₂ solutionCalculate a freezing point using ΔTf = i·Kf·m with the van 't Hoff factor of a 1:2 electrolyte.
  • Dissolved oxygen in waterUse Henry's law with a partial pressure to find the solubility of a gas in mg/L.
  • Molality of a glucose solutionCalculate molality from the mass of solute and the mass of solvent.
  • Molarity of a salt solutionCalculate molarity from the mass of solute and the volume of solution.
  • Mole fraction of ethanolCalculate the mole fraction of one component of a binary liquid mixture from masses.
  • Molar mass from osmotic pressureDetermine the molar mass of a macromolecule from the osmotic pressure of a dilute solution.
  • Osmotic pressure of salineCalculate osmotic pressure with Π = iMRT for a strong electrolyte at a temperature given in °C.
  • Nitrate in a water sampleExpress a trace concentration in parts per million by mass.
  • Vapour pressure of a sugar solutionApply Raoult's law to the solvent of a solution containing a nonvolatile solute.
  • Vapour above a benzene–toluene mixtureCombine Raoult's and Dalton's laws to find the composition of the vapour above an ideal liquid mixture.

Equilibrium constants and Le Châtelier

  • When x is not smallSolve an ICE table with the quadratic formula when the small-x approximation fails.
  • ICE table for HISolve an ICE table for a perfect-square equilibrium expression to find an equilibrium concentration.
  • Convert Kc to KpConvert Kc to Kp using Kp = Kc(RT)^Δn with the correct sign of Δn.
  • K for a rewritten equationFind K for a reaction that is reversed and multiplied by a fractional coefficient.
  • K at a higher temperatureUse the van 't Hoff equation to calculate K at a new temperature for an exothermic reaction.

Encontre o erro 1

  • Find the faulty claimIdentify the incorrect statement in a plan to raise an equilibrium yield, distinguishing rate effects from equilibrium effects.

Interprete as evidências 2

Equilibrium constants and Le Châtelier

  • Read ΔH° from K(T)Deduce the sign of ΔH° from how an equilibrium constant changes with temperature.

Solubility and complex ions

  • Separating Br⁻ from Cl⁻Use two Ksp values to judge how completely one ion is removed before a second begins to precipitate.

Planeje uma síntese 1

  • Plan a dilutionUse M₁V₁ = M₂V₂ to find the volume of stock solution needed for a dilution.

Explique por quê 2

Concentration and colligative properties

  • Warm soda goes flatExplain why gases such as CO₂ and O₂ are less soluble in warm water.

Solubility and complex ions

  • Why ammonia dissolves AgClExplain how complex-ion formation increases the solubility of a sparingly soluble salt.

Travou em soluções e equilíbrio químico? Escaneie o problema.

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