CHEM 1060 Lecture Notes - Lead, Silver Iodide, Magnesium Chloride

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26 Jan 2013
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Chemistry Chapter Five
The Nature of Aqueous Solutions
Reactions in aqueous (water)solution are important because: (1) water is
inexpensive and is able to dissolve a vast number of substances; (2) in such
solutions, many substances are dissociated into ions which can participate
chemical reactions; and (3) these solutions are found everywhere, from seawater to
living systems.
Unlike metallic conductors in which electrons carry the electric charge, the
electricity conducted in aqueous solutions is carried by the ions. When a solute
dissociates into ions in an aqueous solution and becomes and electric conductor, it
is known as an electrolyte. ***Pure water contains so few ions that it does not
conduct an electric current. *** Based on how well a solution conducts electricity,
we can deduce the strength of the presence of ions. We can label a solute as a non-
electrolyte, strong electrolyte, or weak electrolyte. A non-electrolyte is a
substance that is not ionized and does not conduct electric current (e.g. the lamp
fails to light up). Therefore, there are no ions or extremely low concentration of
ions. A strong electrolyte is a substance that is essentially completely ionized in
aqueous solution, and the solution is a good electrical conductor (e.g. the lamp
lights up brightly) and thus, has a high concentration of ions. A weak electrolyte is
partially ionized in aqueous solution and the solution is only a fair conductor of
electricity, thus, the concentration o f ions in the solution is low (e.g. the lamp lights
up only dimly). When determining if a solution is more likely to be a strong
electrolyte, weak electrolyte or non-electrolyte, it is best to remember this
generalization:
Essentially all soluble ionic compounds and only a relatively few molecular
compounds are strong electrolytes.
Most molecular compounds are either non-electrolytes or weak electrolytes.
Some examples of a strong electrolyte are: HCl, NaOH and KBr. Some examples of a
weak electrolyte are: HF, CH3COOH. Some examples of non-electrolytes are: H2O
and CH3OH.
If a solution contains strong electrolytes, the equation is written with the arrow of
the reaction going in one direction, usually right. This indicates that the ionization in
water is complete.
MgCl2(s) (H20) Mg 2+ (aq) + 2Cl-(aq)
In a situation where the solution is characterized as a weak electrolyte is best
described as a reaction that does not go to completion. In these cases, only a
portion of the solute molecules in the solution are ionized. The double arrows
indicate that the process is reversible. This means that while the forward reaction is
taking place, the reverse action is also occurring and its products are the reactants
of the forward reaction.
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