Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2013

Lecture Notes 9: 7 February

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Reaction Stoichiometry: Chemical Equations, cont.

Notice that a chemical equation gives both qualitative information (what things react to give what products) and quantitative information (how much stuff is produced if a particular amount of stuff reacts). This gives rise to various practical applications.

• Example: How many grams of carbon dioxide are produced during the complete combustion of 475.5 g of natural gas (methane, CH₄)

First need to find moles of CH₄:

MW = 12.01 + 4 (1.008) = 16.04

Moles of CH₄ = 475.5 g / 16.04 g/mol = 29.64 mole.

Next need balanced equation:

CH₄ + 2 O₂ CO₂ + 2 H₂O

Note from the equation we have a 1:1 ratio of moles methane to mole carbon dioxide

So we have 29.64 moles CO₂

& MW of CO₂ = 12.01 + 2 (16.00) = 44.01

and (29.64 mol)(44.01 g/mol) = 1.304 kg

Let's look at a slightly more complicated reaction.

• Example: How many moles of oxygen will be needed for the complete combustion of 275.5 g of butane (C₄H₁₀).

2 C₄H₁₀ + 13 O₂ 8 CO₂ + 10 H₂O

Limiting Problems

Asking question of what is the maximum amount of something which can be produced from a given mixture of stuff. This is a fairly straight-forward sort of problem in the day-to-day world, but seems to cause a great deal of difficulty for lots of folks in chemistry. Let's start by looking at a non-chemical problem:

Consider you have to make a bunch of sandwichs for a party. The equation for the sandwichs (in slices) is:

2 Bread + 1 Cheese + 1 Meat 1 Sandwich

You have a 32 oz loaf of bread, 22 oz of sliced cheese and 32 oz of sliced meat. If bread slices weigh 1/2 oz, cheese slices 3/4 oz and meat slices 1 oz, how many sandwichs can you make?

Look at how many sandwichs can be made from each ingredient:

• If bread limits: (32 oz bread)(2 slices bread/oz bread)(1 sandwich/2slices bread) = 32 sandwichs
• If cheese limits: (22 oz cheese)(1 slice cheese/3/4 oz cheese)(1 sandwich/slice cheese) = 29.3 sandwiches
• If meat limits: (32 oz meat)(1 slice meat/oz meat)(1 sandwich/slice meat) = 32 sandwiches

Cheese limits and we can make 29 sandwichs.

• Example: Consider the reaction:

Fe₃O₄ + 4 C 3 Fe + 4 CO

The trick here is to find the maximum amount of iron which could be made from each reactant.

• Example: Consider the reaction of zinc metal with acid:

Zn + 2H⁺ Zn²⁺ + H_2(g)

What is the maximum amount (moles) of hydrogen gas which may be produced by reacting 0.50 g of Zinc with 0.800 mole hydrogen ion? Show work!

Percent Yield

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 completley to products. The maximum possible yield for a reaction is known as the Theoretical Yield.

• Example: Looking at the Zn limiting case above, the amount of hydrogen generated would be the theoretical yield = 7.6 x 10^-3 mol. Now if the actual yield turned out for a particular experiment to be 6.75 x 10^-3 mol, the percent yield would be calculated to be:

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© R A Paselk

Last modified 11 February 2013