You
work for a company that assembles bicycles.
Your job is to attach two wheels to a single frame. When the supply truck came in to drop
off inventory, your boss took delivery of 250 frames and 400 tires. You assemble bicycles all week long and on
Friday, the boss comes to inspect your work.
He is upset because you have not assembled 250 bicycles. Can you explain to him why you haven’t used
all the frames from the last delivery?
Of
course, you would say that you ran out of tires. With two tires per bicycle and 400 tires in
inventory, the best you could do was to assemble 200 complete bicycles before
running out of tires. The tires limited
how many complete bicycles could be built.
The extra, or excess, frames could not be used.
Chemistry
works the same way.
Reactants
combine in specific ratios. Sometimes,
we run out of one reactant and the reaction stops working. The reactant that runs out is called the limiting reactant. It limits how much of the products can be
made (like the tires in the above scenario).
The
reactant that doesn’t run out is called the excess reactant because it doesn’t get used up in the reaction
(like the frames in the above scenario).
Let’s
see this in action. Check out the videoand follow along:
ex. In a reaction, 15.8 g of aluminum is
reacted with 55.6 g of bromine.
(a)
Determine the identity of the limiting reactant.
(b)
Determine the mass of aluminum bromide produced.
As with all the other calculations we have done, format is vital, so be sure you use it. In the photo above you can see how I grade a question like this, based on the check marks.
I need to see:
- data (knowns/unknowns), equation, substitution, answer, in neat columns under each substance
- include well-laid out mole ratios where appropriate
- include units with every number
- round to the correct sig digs at, & only at, the end of a section of the question
- box your final answer
Correct answers are nice, but technique and format is everything! Never leave me to guess how you worked through a question. The more you give me, the more I can give you (in terms of part marks) if you mess up.
Homework (SCH 3U): Stoichiometry Problem Set # 7-9
Go over what you have learned about stoichiometry. Be sure you understand how to set up (FORMAT!) and solve stoichiometry problems (mole ratios, mass-to-mass stoich and limiting/excess reactant problems).
Student Questions:
1. For question 9 for the homework today for the answers you have it with 2 significant digits. I was wondering if you could explain why it isn't 1 significant digit (Since the coefficients have 1 significant digit so 'n' would have one and so on, right?)
Careful, the coefficients would be exact numbers and so would have an infinite number of sig digs. The coefficients are considered exact because they refer to the molecules in the balanced equation (and molecules only come in whole numbers - can't have parts of a molecule). Thus, when you use the coefficients in a mole ratio, they (with their infinite sig digs) would never be used for rounding.
