Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109
General Chemistry
Summer 2002

Lecture Notes:: 11 June
© R. Paselk 2002

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*Humboldt State University* ® *Department of Chemistry*

Richard A. Paselk
Chem 109	General Chemistry	Summer 2002
Lecture Notes:: 11 June	© R. Paselk 2002

*PREVIOUS*		*NEXT*

Stoichiometry

Reaction Stoichiometry, cont.

Reaction Stoichiometry: Hydrogen stoichiometry reaction demo.

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 Æ 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).

First we need the MW of butane: MW = 4 (12.01) + 10 (1.008) = 58.12 g/mol

and the number of moles of butane = (275.5 g) / (58.12 g/mol) = 4.740 mole

Then from the balanced equation:

2 C₄H₁₀ + 13 O₂ Æ 8 CO₂ + 10 H₂O

we can find the number of moles of oxygen required

O₂ = (4.740 moles butane)(13 moles O₂ / 2 moles butane) = 30.81 moles

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:

Pretend you are working at a bike shop and you are given the following inventory of parts:

4 frame assemblies; 6 handlebar assemblies, 7 seats, 11 wheel sets, and 15 tires. If the equation for a bicycle is:

Frame assembly + Handlebar + Seat + Wheel set + 2 Tires Æ Bicycle

How many bicycles can you make?

In this case the limiting factor is the frame, so only 4 can be assembled (lots of spare parts).

Example: Consider the reaction: Fe₃O₄ + 4 C Æ 3 Fe + 4 CO

What is the maximum mass of Fe which could be made from 115.0 g Fe₃O₄ and 24.00 g C?

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

The lesser amount will then be the max possible:

C: (3 mol Fe/ 4 mol C)(24.00 g C/ 12.01 g C/mol C) = 1.499 mole

Fe₃O₄: (3 mol Fe/ mol Fe₃O₄)(115.0 g Fe₃O₄/231.6 g Fe₃O₄/mol Fe₃O₄) = 1.490 mole

\ Fe₃O₄ limits, can only make 1.490 moles.

Example: Consider the reaction of zinc metal with acid:

Zn + 2H⁺ Æ 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!

For Zn limiting: (1 mole H₂/1 mole Zn)(0.50 g Zn) / (65.39 g Zn/mol) = 7.646 x 10^-3 mol H₂

For H₂ limiting: (1 mole H₂/2 mole H⁺)(0.800 mole H⁺) = 0.400 mol H₂

Much less with Zn \ 7.6 x 10^-3 mol H₂

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:

(6.75 x 10^-3 mol)(100 %) / (7.6 x 10^-3 mol) = 88.8% = 89%

Aqueous Solutions

Definitions:

Solution - a homogeneous mixture of substances.
- The major component is known as the solvent.
- The substances dissolved in it (by definition minor components) are solutes.
Solubility - a measure of how much solute will dissolve in a given solvent under given conditions (default = 20 °C, 1 atm).
- Liquids which dissolve in each other are called miscible.
- Liquids which do not dissolve in each other are considered immiscible.

Water

Water: water is so ubiquitous, and has so many important and even special properties, that we will talk a bit more about it.

Water is a very unusual, even incredible substance whose amazing properties are often unappreciated because of its ubiquitousness. Water's special properties include extremely high mp and bp (0 °C & 100 °C, compare to methane, -183 °C & -161 °C, with a MW of 16 vs. water's 18); a high heat capacity (18 cal/°C mol vs. 8 cal/°C mol for methane); it has a high viscosity; its solid form is less dense than the liquid form at the same temperature (ice floats on water - very rare), and it has a high dielectric constant (78.5 vs. 1.9 for hexane).

The high mp, bp, and heat capacity all predict relatively strong bonding between water molecules, H-bonding. Note environmental consequences - Earth's weather is much more pleasant because it is moderated by water, especially along coasts. Ice floating prevents "solid" seas, definitely a downer in environmental terms.

Syllabus / Schedule


C109 Home

C109 Lecture Notes

© R A Paselk

Last modified 11 June 2002

Humboldt State University ® Department of Chemistry

Richard A. Paselk

Stoichiometry

Reaction Stoichiometry, cont.

Limiting Problems

Aqueous Solutions

Water