Stoichiometry


In this unit we move up from writing words (chemical formulas) to writing sentences and maybe even paragraphs (chemical reactions).  Of course writing sentences means that one must be able to write words so in the study of chemistry one must be able to write formulas before attempting to write equations for chemical reactions.

Stoichiometry is simply a fancy word for calculating amounts (grams, moles, liters) in chemical reactions.  The key part of this process is understanding the the mole relationships in the chemical equation.  This involves balancing a chemical equation as the first step.

The process of balancing a chemical equation is a subset of a basic scientific principal of the Law of Conservation of Matter.  This law states that matter is neither created or destroyed (or matter is conserved in all non nuclear reactions).  In chemistry we demonstrate this by writing balanced equations.  A balanced equation is simply one where the number of each kind particle on each side of the equation is the same.  The first step in writing a balanced equation is to write a skeleton equation, one with correct formulas for reactants (on the left side of the arrow) and products (on the right side of the arrow).  Then one needs to change the number of particles on each side until they are all equal.  We do this by putting coefficients in front of the correct formulas (we do NOT change the formulas).

Once the equation is balanced, the stoichiometry part is possible.  This is because the coefficients of the balanced equation give the mole ratios of the compounds.  This is why we spent so much time in the last unit on moles.  The mole is the "super secret unit" of chemistry because almost all of the calculations that are done in chemistry involve moles.  If one can find the number of moles of a reactant or product then it is only one simple additional step of using the coefficients of the balanced equation to calculate the moles of anything else in the equation.  Using this process one can for example, calculate the volume of oxygen necessary to "burn" 120 grams of sugar when it is eaten.


Unit VII  Chemical Reactions
Be able to predict synthesis, single replacement, double replacement, decomposition and combustion reactions.
Be able to balance chemical reactions, including redox reactions
Be able to calculate mass, mole, and volume relationships in chemical reactions (Stoichiometry).

Terms:  chemical reaction, reactant, product, single displacement, double displacement, decomposition, synthesis, combustion, stoichiometry, mass-mass problem, mass-mole problem, mass-volume problem,  oxidation, reduction, oxidizing agent, reducing agent.

Suggested Problems:  Read Chapter 9 and Chapter 25
    pgs 243-247 (43-46, 48-55, 58-61) Review (1-12)
    pgs 646-647 (39, 43, 45-47, 49, 50)


Return to the HHS Science Home Page

Links

The Chemical Reaction Page
ChemTeam: Stoichiometry
Fundamentals from UNC, Chapel Hill
Problem solving strategies

On the ChemTeam site there is a Stoichiometry Worksheet (#3 under Lab Experiments & Worksheets).  There are 10 Stoichiometry problems here. I will give you 1 extra credit point for each correctly worked problem.