Thursday, November 26, 2009

STOICHIOMETRY:REAL WORLD REACTION


Summary (or Welcome to the Real World)

We don't live in an ideal world. Humans aren't able to measure exact quantities of substances; not even specially designed machines are completely reliable. In any reaction, there will always be one reactant that you should have added more of. However, even if you were a superhuman and could measure perfectly, you still would be unsure of the results of your reaction.

Chemical reactions never completely consume reactants. You will always only get a percentage of the product you think you will. Chemical reactions are not just equations that you write down on paper; they are living works of nature that produce heat and energy.

This lesson provides you with all the real-life details you need to know about Stoichiometry. You will learn about limiting reagents, percent yield,heat of reaction, and energy changes.

Terms

Actual yield - The amount of product that is actually produced in a chemical reaction.
Endothermic reaction - A reaction that requires heat energy to be carried to completion.
Enthalpy - The amount of heat a substance has at a given temperature and pressure.
Excess reagent - The reactant that is in excess in a chemical reaction. There is more than enough of it to react with the other reactant(s).
Exothermic reaction - A reaction that releases energy in the form of heat.
Heat of reaction - The change in enthalpy in a chemical reaction.
Limiting reagent - The reactant that limits or determines the amount of product that can be formed in a chemical reaction.
Percent yield - The decimal percentage resulting from dividing actual yield by theoretical yield.
Standard heat of formation - Unique to every substance, it is a measure of the change in enthalpy in the reaction that produces 1 mole of the substance from its constituent elements.
Theoretical yield - The calculated or expected amount of product to be formed in a chemical reaction.
Thermochemical reaction - Chemical reactions that include the amount of heat energy produced or absorbed.

Formulas

Formula for Percent Yield

Percent Yield =

δH

δH = δH f (products) - δH f (reactants)

Conversion Factor between kJ and kcal



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