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Solutions & Reactions

Chapter 4

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Solutions are made up of

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  1. Solute - substance dissolved

2) Solvent - substance that is the dissolving medium - what the solute is dissolved in (usually water)

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Water

Water is a good solvent because it’s polar.

O – H

І

H

-

+

+

Negative ions are attracted to the positive hydrogen

Positive ions are attracted to the negative oxygen

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Aqueous (aq) means dissolved in water.

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Nonelectrolyte

  • A nonelectrolyte may dissolve in water, but it does not dissociate into ions

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  • Does not conduct electricity

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Nonelectrolytes

Molecular compounds tend to be nonelectrolytes, (except for acids and bases)

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  • C12H22O11 (sugar)

C12H22O11(s) 🡪 C12H22O11 (aq)

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Electrolytes

  • An electrolyte is a substance that becomes ions when dissolved in water.

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  • Conducts electricity

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  • Soluble ionic compounds, acids, and bases are electrolytes.

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1. Soluble Ionic Compounds

  • Ionic compounds dissociate into ions when dissolved in water.
  • Any compound that contains sodium, nitrate, ammonium, or potassium (SNAP) is soluble.
    • There are others, but AP just wants you to know these (Don’t argue with the test!)

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NaCl(s) 🡪 Na+ (aq) + Cl- (aq)

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2. Acids

  • Substance that increases H+ in a solution (Arrhenius)
  • An acid is usually a compound of hydrogen attached to something that looks like an anion
  • Acids are molecular compounds.
  • HCl
  • H2SO4

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Acids

  • Acids are molecular, therefore they do not dissociate into ions

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  • However, they do react with water to form ions. The acid is said to be ionized (not dissociated)

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HCl + H2O 🡪 H3O+ + Cl-

H3O+ is a water molecule with an extra proton (H+)

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HCl 🡪 H+ + Cl-

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Bases:

  • Substances that increases OH− when dissolved in water (Arrhenius).

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  • Compound with a metal and the hydroxide ion.
    • NaOH, Ca(OH)2

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  • NH compounds:

NH3 + H2O ⇋ NH4+ + OH-

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Strong Electrolytes

  • A strong electrolyte dissociates/ionizes completely when dissolved in water.

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HCl + H2O 🡪 H3O+ +Cl-

The aqueous solution contains H3O+ and Cl- ions

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Strong Electrolytes Are…

1. Soluble Ionic compounds

sodium, nitrate, ammonium, potassium (SNAP)

Na+ NO3- NH4+ K+

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2. Strong Acids

There are seven common strong acids:

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    • Hydrochloric (HCl)
    • Hydrobromic (HBr)
    • Hydroiodic (HI)
    • Nitric (HNO3)
    • Sulfuric (H2SO4)
    • Chloric (HClO3)
    • Perchloric (HClO4)

(All other acids are weak)

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3. Strong Bases

The strong bases are the soluble salts of the hydroxide ion

    • Alkali metals
      • LiOH, NaOH, KOH, RbOH, CsOH, FrOH
    • Ca(OH)2
    • Sr(OH)2
    • Ba(OH)2

Most other -OH compounds are weak bases

NH3 is a common weak base

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Weak Electrolytes

A weak electrolyte only ionizes/dissociates partially when dissolved in water

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HF + H2O ⇋ H3O+ + F-

HF ⇋ H+ + F-

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The aqueous solution contains HF, H3O+, and F-

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Sulfuric Acid

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Sulfuric acid is diprotic and it ionizes in two steps:

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H2SO4 + H2O 🡪 H3O+ + HSO4-

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HSO4- + H2O ⇋ H3O+ + SO4-2

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A solution of H2SO4 contains H3O+, HSO4-, and SO4-2

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Big Chart

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AP Equations – Part 2��Reactions that happen in solution!

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Single Replacement�

  • More active metals replace less active metals from their compounds

Cu + 2 AgNO3 🡪 Cu(NO3)2 + 2 Ag

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  • More active nonmetals replace less active nonmetals from their compounds.

Cl2 + 2 NaI 🡪 2 NaCl + I2

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  • Very active metals can replace one of the hydrogens from the stable compound water.

2 Na + 2 H2O 🡪 2 NaOH + H2

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The Activity Series

  • You will not be required to memorize it!
  • It lists elements based on ease of oxidation.
  • The metals at the top of the activity series are called active metals and are easily oxidized.
  • The metals at the bottom of the activity series are called noble metals and NOT easily oxidized.

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Double Replacement (Ionic)

  • You must produce a precipitate or molecule that is NOT a strong acid

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AgNO3 + HCl 🡪 AgCl + HNO3

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HCl + NaOH 🡪 NaCl + H2O

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Special Notes:

  • H2S is a gas (rotten egg smell!)

2HCl(aq) + Na2S(aq) 🡪 H2S(g) + 2NaCl(aq)

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  • The following are unstable in solutions…
    • H2CO3 write as CO2 + H2O
    • H2SO3 write as SO2 + H2O
    • NH4OH write as NH3 + H2O

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*AP typically accepts either way, but you may see it in the more stable form! (Don’t argue with the test ☺)

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Net Ionic Reactions

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Consider the following reaction:

potassium chromate + barium nitrate 🡪

barium chromate + potassium nitrate

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K2CrO4 + Ba(NO3)2 🡪 BaCrO4 + 2KNO3

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The barium chromate is a precipitate, but all other compounds are soluble strong electrolytes. (SNAP)

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K2CrO4(aq) + Ba(NO3)2(aq)🡪BaCrO4(s) + 2KNO3(aq)

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K2CrO4(aq) + Ba(NO3)2(aq)🡪BaCrO4(s) + 2KNO3(aq)

The above equation is called the molecular equation.

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To write a complete ionic equations, write all strong electrolytes as ions:

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2K+1(aq) + CrO4-2(aq)+ Ba+2(aq) + 2NO3-1 (aq)🡪 BaCrO4(s) + 2K+1(aq)+ 2NO3-1(aq)

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  • Notice that there are K+1 and NO3-1 ions on both sides of the reaction. Thus, they are not actually part of the reaction and are called spectator ions.

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The ions that actually participate in the chemical reaction are the following:

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CrO4-2 (aq) + Ba+2 (aq) 🡪 BaCrO4 (s)

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  • This last equation is called the net ionic equation and includes only the participating ions of the reaction. (No spectator ions)

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An example forming a molecule

Molecular equation:

KOH (aq) + HCl(aq) 🡪 H2O (l) + KCl (aq)

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Complete ionic equation:

K+1(aq) + OH-1(aq) + H+1(aq) + Cl-(aq) 🡪

H2O(l) + K+1(aq) + Cl-(aq)

Net ionic equation:

OH-1(aq) + H+1(aq) 🡪 H2O(l)

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Replacement Reaction (From your Dime Lab)

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Cu(s) + 2AgNO3(aq) 🡪

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Cu(s) + 2Ag+(aq)+2NO3-(aq)🡪

Cu2+(aq) + 2NO3-(aq) + 2Ag(s)

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Net Ionic

Cu(s) + 2Ag+(aq) 🡪 Cu2+(aq) + 2Ag(s)

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Cu(NO3)2(aq) + 2Ag(s)

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Example:

Fe(s) + Ni(NO3)2(aq) 🡪 Fe(NO3)2(aq) + Ni(s)

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Net ionic:

Fe(s) + Ni+2(aq) 🡪 Fe+2(aq) + Ni(s)

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Net ionic equations can make it easier to identify redox reactions!

Fe is oxidized to Fe+2

Ni+2 is reduced to Ni.

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A little lie I told….

1st year – acids are molecular so the reaction happens.

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All strong electrolytes are written as ions!

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Ex: NaCl (aq) + HNO3(aq) 🡪

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Na+ + Cl- + H+ + NO3- 🡪 Na+ + NO3- + H+ + Cl-

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No new substances are formed = no reaction

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NaNO3 (aq) + HCl(aq)

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