Chem 115 lec 15

Double Displacement Reactions

• Double displacement reactions occur when two ionic compounds exchange partners, typically producing a solid, gas, or water.

• For a double displacement reaction to happen, one of the following must occur:

  • Formation of a precipitate (insoluble compound)

  • Formation of a molecular compound (like water)

  • Formation of a weak electrolyte (weak acids or bases)

Predicting Products

• When two ionic compounds do not have common ions, their positive ions and negative ions can be mixed to predict product formation.

  • Example: NaCl + Ca(NO3)2 → CaCl2 + NaNO3

• The reaction will not occur if all products are soluble strong electrolytes.

  • In this case, all ions remain as spectator ions with no net change.

Key Criteria for Chemical Reactions

• To ensure a reaction occurs, you must produce:

  • An insoluble precipitate

  • A stable molecular compound

  • A weak electrolyte that cannot dissociate completely.

Special Cases of Weak Electrolytes

• When potassium sulfate reacts with hydrochloric acid:

  • Products include potassium chloride and sulfurous acid (H2SO3).

  • H2SO3 is unstable and partially dissociates into water (H2O) and sulfur dioxide (SO2), which is responsible for the characteristic smell.

Examples of Unstable Weak Electrolytes

• Carbonic Acid (H2CO3):

  • Breaks down to water and carbon dioxide. Found in soda.

• Ammonium Hydroxide (NH4OH):

  • Breaks down to ammonia and water, responsible for strong odors in cleaning products.

Mixing Multiple Solutions

• Mixing three soluble salts can complicate product prediction as each positive ion can combine with multiple negative ions.

• Example with AgNO3, Ba(ClO3)2, and K2SO4

  • All possible combinations need to be considered for precipitate prediction.

  • Three likely precipitates: AgCl, Ag2SO4, and BaSO4.

Concentration of Solutions

• Concentration is the amount of solute dissolved in a given volume of solution.

• Common units include percent concentration, grams per liter, and molarity (M).

• Molarity is defined as moles of solute per liter of solution (M = moles/L).

Molar Concentration Calculations

• Example calculation:

  • 0.25 moles NaCl in 500 mL (0.5 L):Molarity of NaCl = 0.25 moles / 0.5 L = 0.50 M.

• Concentration of individual ions:

  • E.g., 0.60 M Al2(SO4)3 results in 1.2 M aluminum ions and 1.8 M sulfate ions.

Dilution of Solutions

• Dilution occurs when solvent (water) is added to reduce solute concentration.

• Equation for dilution: M1V1 = M2V2.

  • M1 = initial molarity, V1 = initial volume;

  • M2 = diluted molarity, V2 = diluted volume.

• Example: From 12.0 M NaOH diluted to 250 mL, find new concentration:

  • M2 = (12.0 M * 50 mL) / 250 mL = 2.4 M.