AGENDA TODAY

• Lesson on 7.3 Which Reagent Runs Out First?

• Mini Investigation p. 327

• 7.2 Practice for homework

7.3 WHICH REAGENT RUNS OUT FIRST?• When the exact stoichiometric amounts of reactant are combined, all the reactants are used up (no leftovers).

• However, many times measuring the exact stoichiometric amounts for all reactants is impractical

• Usually, one of the reactants is used up first and other reactants are present in excess.

Limiting Reagent: The reactant that is completely consumed in a chemical reaction; the reactant that determines how much product will be formed.

Excess Reagent: A reactant that is still present after the reaction is complete (not completely used up)

ANALOGY FOR LIMITING AND EXCESS REAGENTS

EXAMPLE 2: SANDWICH ANYONE?

What item(s) is/are the limiting reagent(s)? (What item is completely used up?)

EXAMPLE 3: SYNTHESIS OF CO2

5 molecules C 10 molecules O2 5 molecules CO2 excess molecules O2

What chemical is the limiting reagent? (What item determines amount of CO2 made?)

APPLICATIONS OF LIMITING REAGENTS Industry Applications about Limiting Reagents:

1. Reducing Costs – manufacturers choose most expensive reactant to be the limiting reactant to ensure it does not get wasted.

2. Reducing Environmental Impact – it is benefial for companies to reduce the amount of harmful reactants used or products made to make a “greener” process

3. Environmental Cleanup – controlling the amount of reactant or product used is important if products must be treated to make safer (I.e. acids/bases can be neutralized or dissolved toxic metal ions can be precipitated out)

4. Improving Fuel Efficiency – ensuring that our fuels undergo complete combustion vs incomplete combustion requires having a sufficient oxygen supply

DEMO: BALLOON STOICHIOMETRY Equipment and Materials: safety goggles, lab apron, 4 balloons, permanent marker, 4 x Erlenmeyer flasks, funnel, teaspoon, baking soda (sodium hydrogen carbonate), vinegar (acetic acid solution)

Procedure:

1. Put on personal protective equipment.

2. Label the flasks 1, 4, 7 and 10.

3. Using the funnel, add 1 level tsp of baking soda to each balloon.

4. Add 1 tsp, 4 tsp, 7 tsp, and 10 tsp to flasks #1, 4, 7, and 10 respectively.

5. Carefully stretch a balloon over the mouth of each bottle. Be careful not to allow the baking soda to fall into the flask.

6. Raise the balloon on each flask so that its contents fall into the flask. Swirl to mix. Record your observations.

BALLOON STOICHIOMETRY: OBSERVATIONS

Flask #1 Flask #4 Flask #7 Flask #10

Amount of baking soda (tsp)

1 1 1 1

Amount of vinegar (tsp)

1 4 7 10

Rank Size (1 is largest)

Limiting Reagent

Excess Reagent(s)A. If two or more of your balloons were equal in size, explain why. Refer to the quantities of reactants in your explanation.

B. Based on your observations, predict the limiting and excess reagents in each bottle.

CH3COOH(aq) + NaHCO3(s) H2O(l) + CO2(g) + NaCH3COO

7.3 PRACTICE

P. 329 #2, 5, 7