Solution Chemistry
In the first week of class we learned that a homogeneous mixture of two or more substances is called a solution. If one of the substances is present in much greater quantities than all the other substances then it is called the solvent. The other substances in solution are known as solutes. For example, when a small amount of NH4Cl is dissolved in a large quantity of water we refer to water as the solvent and NH4Cl as the solute. Another example is Napthalene (used in mothballs) can be dissolved in benzene. In this example benzene is the solvent and napthalene is the solute.
Solutes dissolved in water (solvent) are called aqueous solutions. Not all substances are soluble in water. Why do some substances dissolve in water and others don't? It has to do with the structure of the water molecule.
Oxygen has a greater attraction for electrons, so the shared electrons (bonding electrons) spend more time close to oxygen then to either of the hydrogens. This gives oxygen a slightly excess negative charge and hydrogen a slightly more positive charge.
This unequal charge distribution makes water a polar molecule, and gives water its ability to dissolve compounds. When an ionic solid dissolves in water, the positive ends of the water molecule are attracted to the negatively charged anions and the negative ends of the water molecule are attracted to the positively charged cations. For example, when NaCl is dissolved in water we find
So when an ionic substance (salt) dissolves in water, it is broken up into individual cations and anions which are surrounded by water molecules. For example, when NH4 NO3 is dissolved in water it breaks up into separate ions.
NH4+ and NO3- ions are floating around in H2O essentially independent of each other.
Water also dissolves non-ionic substances. For example, C2H5OH (ethanol) is very soluble in H2O. This is because C2H5OH has a polar OH bond that the water molecules like to hang around.
Many substances do not dissolve in water and that is because they are non-polar and do not interact well with water molecules. A common example is oil and water. Oil contains molecules that are non-polar, thus they do not dissolve in water.
How do we know that ionic solids dissolve in water and form cations and anions that float around separately? One clue comes from conductivity experiments. Anions and Cations should act as charge carriers in solution. Therefore a solution with dissolved ions should conduct electricity. Let's look at a few examples. Pure (distilled) water contains no dissolved ions. Therefore pure water will not conduct electricity. In a simple conductivity experiment as shown below we would not expect the light to be on.
An aqueous NaCl solution, however, will have dissolved ions present and therefore will conduct electricity. Therefore the light in our conductivity experiment will be on if dipped in an aqueous NaCl solution.
NaCl ionizes completely when dissolved in water. It's helpful to think of this process as two steps:
Substances that exist in solution almost completely as ions are called strong electrolytes.
Substances that do not form ions when they dissolve in water are called non-electrolytes. And example of a non-electrolyte is sugar. Sugar will readily dissolve in water but doesn't form cations and anions in solution. That is, there are no charge carriers formed.
Substances that only partially ionize into ions when dissolved in water are called weak electrolytes. For example, Acetic Acid (HC2H3O2) dissolves in water, but only partially dissociates into ions.
Be careful not to confuse how soluble a substance is in water with whether it is a weak, strong, or non-electrolyte. For example, sugar dissolves completely in water but it is a non-electrolyte. Another example are salts that can be very insoluble in water but the small amount of salt that does dissolve in water is a strong electrolyte.
Acids and Bases
With our understanding of strong, weak, and non-electrolytes we can now examine the Arrhenius Definition of Acids and Bases:
- Acids:
- Substance that produces H+ ions when it is dissolved in H2O.
- Bases:
- Substance that produces OH- ions when it is dissolved in H2O.
For example, HCl is an acid,
and NaOH is a base,
Acids and Bases that are strong electrolytes are called Strong Acids and Strong Bases, respectively. Acids and Bases that are weak electrolytes are called Weak Acids and Weak Bases, respectively.
Strong Acids
HCl is an example of a strong acid:
HCl(aq) → H+(aq) + Cl-(aq)
Other examples include HBr, HI, HClO4, HClO3, H2SO4, and HNO3.
Weak Acids
HF is an example of a weak acid:
Other examples include HC2H3O2, H2CO3, H2SO3, H3PO3, and H3PO4. The last four are examples of polyprotic acids. These are acids that can produce more than one H+ ions when dissolved in water. H2CO3 and H2SO3 are called diprotic acids, and H3PO3 and H3PO4 are called triprotic acids. HF, HCl, HBr, and HC2H3O2 are examples of monoprotic acids. The dissociation of polyprotic acids usually occurs in steps. For example, only after H3PO4 loses its first H+ ion will it lose its second H+ ion, and then it will lose its third.
In this example, all three species H3PO4, H2PO4-, and HPO42- are weak electrolytes so H3PO4 is considered to be a weak acid. H2SO4 is another example of a diprotic acid. In the case of H2SO4 the first H+ ion is produced readily so the species H2SO4 is considered to be a strong electrolyte (i.e. strong acid). The species HSO4-, however, is a weak electrolyte.
Strong Bases
NaOH is an example of a strong Base:
Other examples include LiOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, and Ba(OH)2.
Weak Bases
Ammonia, NH3, is a good example of a weak base.
Other examples include C6H5N (pyridine) and C6H5NH2 (aniline).
Homework from Chemisty, The Central Science, 10th
Ed.
4.1, 4.3, 4.5, 4.7, 4.9, 4.11, 4.13, 4.15, 4.17, 4.33, 4.35, 4.37,