Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107	Fundamentals of Chemistry	Fall 2008
Lecture Notes: 28 October	© R. Paselk 2005
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H₂ funnel demo

Balancing Redox Equations

There are two common methods for balancing redox reactions: the oxidation number method and the half-reaction method. The half-reaction method works very well for ionic reactions, it is relatively easy to give partial credit, and it is the only method I will use in this class. If you know how to do the other method you are welcome to do so, but be careful to make sure you show your work or I won't be able to give partial credit!

The Half-Reaction Method

In the half-reaction method what we do is first break an equation into two parts and then balance the parts individually. Presented stepwise:

• Separate the reaction into two half-reactions.
• Balance each half-reaction separately:
  1. Balance atoms other than O & H by inspection.
  2. Balance O by adding H₂O to the opposite side.
  3. Balance H:
     • In acid solution add H⁺ as appropriate.
     • In base solution add H₂O to the side needing H, balance O by adding equal numbers of OH^- to the opposite side.
  4. Balance the charge by adding electrons (e^-) - add to same side as excess of positive charge, or opposite side if excess negative charge.
• Balance the charges of the two half-reactions by multiplying appropriately, then add to two equations together and cancel items appearing on both sides.

Example. Balance the following equation as it occurs in acid solution:

MnO₄^- + Cl^- Mn²⁺ + Cl₂

Stoichiometry

Determination of Empirical Formulae

Want to determine the ratios, in moles, of elements in an analysis.

• Example: A compound of mercury, oxygen, and nitrogen gave an analysis of 61.80% mercury and 8.63% nitrogen by weight. What is the empirical formula for this compound?

When should you round off? One of the problems in finding the simplest formula is determining how much error is legitimate in rounding off. Ultimately this is a decision determined by the error of the experimental data - how many significant figures do we have. For this course we generally have at least three sig figs, but I promise not to get too subtle, so as a rule of thumb for this class only values of x.33x and x.5xx should be assumed to be not in error, and so must be multiplied to get the correct formula. (e.g. XY_2.331 gives X₃Y₇)

Molecular Formulae

Notice that for molecular compounds that the empirical formula is not necessarily the molecular formula! That is, the actual molecular formula could be a multiple of the simplest formula. Thus, to find molecular formulae we need two kinds of information, the empirical formula (from percentage composition) and the molecular weight (from physical characterization):

• Example: A molecule is found to have an empirical formula of C₃H₅ by elemental analysis and a MW of 85 by freezing point depression. What is its molecular formula?

Theoretical Yield Problems

Another frequent question arising in chemical processes is the percent yield. This deals with the question of how effective was a given process in producing a product. Its an important consideration because chemical reactions rarely go completely to products. The maximum possible yield for a reaction is known as the Theoretical Yield.

• Example: How many g of water will be produced during the complete combustion of 475.5 g of natural gas (methane)?

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Last modified 27 October 2008