1. Quiz analysis
2. Section 1.3: Limiting & Excess Reactants
3. Percent yield
1. Identify the limiting and excess reactants in chemical reactions.
2. Calculate percent yield using mass data.
Links: 1.3: Reacting Masses & Volumes
Textbook Readings: Section 1.3; p. 28-32
Mission 1: This World Ain't Perfect. Chemical reactions should theoretically go to completion, but they do not because some reactants are used up faster than others. The reactant that runs out first (the limiting reactant or limiting reagent) determines the amount of product that is formed. For example, if you're making cheese sandwiches and you have 16 pieces of bread but five pieces of cheese, but the recipe calls for two pieces of bread and one piece of cheese...how many sandwiches can you make before you run out of ingredients? Which is the limiting ingredient? Which is the excess ingredient (the reactant that is left over)?
Think back to the lab we did before Eid break. I gave you your product mass results and you know that mass wasn't conserved. Which of the reactants do you think ran out first...the silver nitrate or the copper? Usually, the more expensive reactant is the limiting one.
In order to complete LR/ER problems, you need to remember how to read a balanced chemical equation and determine the correct mole ratios. Pages 28-29 cover this, but I won't spend time on it in class because we've already done it. Basically, the mole ratio is key in determining how much product is obtained.
Having said that, here's a quick-N-easy set of guidelines on solving LR problems.
Here's an IB-level video to give you an idea of the math that is involved in understanding the concept. Don't go crazy over doing the problems; we will work those out in class. Just try to understand the concept and get familiar with how to do the work.
Practice problems: p. 32 #36-38.
Mission 2: Well How Much Did We Actually Get??? Percent yield is an expression of the efficiency of a reaction. It is based on the theoretical yield (what you're supposed to get) versus experimental yield (what you actually got). The theoretical yield (TY) is usually expressed in grams or moles, and refers to the maximum amount of obtainable product, assuming 100% of the limiting reactant is converted to products. The information comes from the balanced chemical equation.
A summary of the steps are listed on page 31. Mr. Thornley provides a (mercifully) short video showing how to calculate theoretical yields.
10/13 Homework: Continue working on p. 32, #40-42.