Properties of Solutes in Aqueous Solutions

• Water has the ability to dissolve many different types of substances, resulting in a homogeneous mixture.
• In homogeneous mixtures involving water, water is considered to be the solvent:

• The typical molar concentrations of substances dissolved in water would be on the order of 10^-6 to 10¹ molar, thus, they are present at far lower molar concentration and are considered to be the solute.

The polar nature of the water molecule

• The Lewis Structure of water:

• The central Oxygen has a tetrahedral geometry for the valence electron pairs
• Thus the H₂O molecule will adopt a bent molecular geometry:

• The Oxygen (3.5) is more electronegative than the Hydrogen (2.1), thus the O-H bond is polar covalent
• The bent geometry results in an overall dipole for the water molecule:

• Thus, H₂O can participate in the following types of non-covalent interactions:
• London Dispersion Forces
• Dipole-dipole interactions
• Ion-dipole interactions (with ions)
• Hydrogen bonds (between other water molecules or with an appropriate solute)

Water can participate in ion-dipole interactions.

• Water molecules will organize around an ion to orient the appropriate opposite partial charge of the water dipole:

• Water molecules will separate, surround and disperse ions in an ionic solid:

• Although H₂O is a poor conductor of electricity, dissolved ions in an aqueous solution can conduct electricity. Thus, ionic aqueous solutions are known as electrolytes.

Distinctions between concentrations of ionic compounds and resulting concentrations of ions in an aqueous solution

• 1.0 mole of NaCl dissolved in 1 liter of H₂O results in a 1.0 molar solution of NaCl
• When 1.0 mole of NaCl is dissolved it produces 1.0 mole of Na⁺ ion and 1.0 mole of Cl^- ion

• 1.0 mole of CaCl₂ dissolved in 1 liter of H₂O results in a 1.0 molar solution of CaCl₂
• When 1.0 mole of CaCl₂ is dissolved it produces 1.0 mole of Ca²⁺ ion and 2.0 moles of Cl^- ion

Molecular compounds in an aqueous solution

• Generally speaking, interaction with H₂O will not break any covalent bonds. Thus, the ability of water to dissolve a molecular compound is based on non-covalent interactions of H₂O with the molecular compound

Example: Methanol (CH₃OH) mixed with water

• The alcohol (-OH) group of methanol is similar to water in that the oxygen valence electron geometry is tetrahedral with two non-bonding pairs of electrons
• There is a polar bond between the hydrogen and oxygen of the alcohol group in methanol, similar to that in water:

• Thus, water can form Hydrogen bonds with the alcohol group of methanol, and in this way the water molecules can separate, surround and disperse the molecules of methanol

• Some molecular compounds interact so strongly with H₂O that covalent bonds of the compound may be broken
• Although the molecular compound in question may be neutral, the molecular fragments produced by the bond breakage may be oppositely charged ions. An example of this would be the uncharged molecular compounds H-Cl. Interaction with water is so strong that it results in the breakage of the H-Cl bond. This produces an H⁺ ion, and a Cl^- ion.
• Thus, although these types of molecular compounds may be neutral, in water they result in the production of ions and therefore, are electrolytes
• Acids are one example of neutral compounds that ionize in aqueous solution (i.e. interact so strongly that a covalent bond is broken and ions are produced)

Strong and weak electrolytes

• Some compounds in aqueous solution dissociate completely into ions. This would include most ionic compounds, and some molecular compounds (like H-Cl)
• Other compounds only have a slight tendency to ionize in aqueous solutions. In other words, only a few of the molecules in solution will ionize, and most will remain as neutral compounds (although the compound is completely dissolved in water)
• Compounds that ionize completely are known as strong electrolytes
• Compounds that ionize only partially are known as weak electrolytes

For example, acetic acid only partially ionizes (i.e. is a weak electrolyte) when dissolved in H₂O. This ionization involves the breaking of a covalent bond between an oxygen and hydrogen atom in the acid:

• The double arrow means that the reaction is significant in both directions
• At any given moment, some of the anion form of acetic acid is combining with H⁺ cation to form a covalent bond and produce the neutral acetic acid
• Likewise, at any given moment, some of the neutral acetic acid in aqueous solution is dissociating (i.e. ionizing) to form the anion and H⁺ cation
• The balance (i.e. the chemical equilibrium) between these two processes determines the relative concentration of the neutral molecule and ion forms

Chemists use a double arrow to indicate the ionization of weak electrolytes, and a single arrow to indicate the ionization of strong electrolytes (i.e. in strong electrolytes, the ions have essentially no tendency to recombine to form the neutral compound)