Limiting Reactants

Chapter 13

Plan of the day

• Intro to Limiting Reactants

• S’mores

• Example Problems

• Work through homework problems– Ch 13 HW Assignment: #1-9, 12,13

• Chapter 13 Test: 5/4Chapter 13 Test: 5/4

Limiting Reactants—you will be able to…. (objectives)

• Define a limiting reactant

• Articulate the importance of a limiting reactant

• Identify the limiting reactant in a chemical reaction

• Calculate limiting reactant problems.

Definitions• Reactant

– Is a substance that takes part in a chemical reaction to make a product

• Limiting Reactant– The reactant that is completely used up

in the reaction and limits the amount of product

• Excess Reactant– The reactant that is in excess, not all

used up.

Why do we care? (Don’t write… just listen)

• Scientific Reasons– Determine how much reactant you

need – Determine how much product will be

produced

• Economic (Show me the money!)– Keep production cost’s minimal– Maximize output in production

S’mores Stoichiometry

• Limiting Reactants• Complete S’mores wkst. #1-4

–We’ll get started together–When you are done, get a “S’mores Kit”

S’mores Kit Stoichiometry• Record on back of your worksheet:Record on back of your worksheet:1.1. Obtain a bag with s’more ingredientsObtain a bag with s’more ingredients

2.2. Write the balanced equation for Write the balanced equation for S’mores! S’mores!

3.3. Using Stoichiometry, determine how Using Stoichiometry, determine how many S’mores you can make with many S’mores you can make with each ingredient given.each ingredient given.

4.4. What is your limiting reactant? What is your limiting reactant?

5.5. Using stoichiometry, determine the Using stoichiometry, determine the amount of your excess reactants.amount of your excess reactants.

6.6. Wash your hands, and make yourself Wash your hands, and make yourself a S’more as a reward for a job well a S’more as a reward for a job well done done

Steps in Solving LR problems!

When two amounts of reactants are given in a problem, we need to identify the limiting reactant to solve for the amount of product possible!

1.1. Start with a Balanced EquationStart with a Balanced Equation

2.2. Change grams of reactants to moles – this is how Change grams of reactants to moles – this is how much you have. much you have.

3.3. Pick one of the reactants you just converted to Pick one of the reactants you just converted to mols (how much you have) & use Stoich from the mols (how much you have) & use Stoich from the balanced eqn to determine the number of mols balanced eqn to determine the number of mols of of other reactant neededother reactant needed..

4.4. Compare mols of Compare mols of other reactant neededother reactant needed to what to what you have of that reactant… do you have enough?you have of that reactant… do you have enough?

5.5. Decide which is the limiting reactantDecide which is the limiting reactant

Another Method

1. Always begin with a BALANCED EQUATION!!

2. Convert the amount (grams/volume) of each reactant to moles of reactants.

3. Convert the number of moles of reactant to moles of product (doesn’t matter which one as long as you use the same product for both reactants)

4. Which ever reactant produces the least product is the limiting reactant.

Example 1• The reaction begins with 2.51g of HF The reaction begins with 2.51g of HF

and 4.56g of SiOand 4.56g of SiO22. .

HF + SiOHF + SiO22 SiF SiF44 + H + H22OO• Balance EquationBalance Equation• Convert g Convert g mols of each (What you have) mols of each (What you have)• Pick one & convert mols Pick one & convert mols mols of other reactant. mols of other reactant.

This is what you need…Do you have enough? This is what you need…Do you have enough?• If “yes,” then the other is your LR… if no than that If “yes,” then the other is your LR… if no than that

one is your LRone is your LR

• How much excess reactant is left over?– Determine the amount of the excess

reactant used then subtract from the starting amount.

How much product can be formed?

• Start with the amount (g) of limiting reactant, use stoichiometry (balanced equation) to find out the amount (g) of product!

Example 2• Determine the limiting reactant, if an

80.0g solution of NaOH, which is 40.0% NaOH by mass, reacts with a 75.0g solution of H2SO4, which contains 45.0% water by mass.

NaOH + H2SO4 Na2SO4 + H2O

1. Balance Equation

2. Calculate grams of NaOH and H2SO4

3. g mols of each (What you have)

4. Pick one & compare (What you need)

5. Do you have enough?

• So… the reaction will go to completion until all the NaOH is used up.

• Calculate the amounts of each product that will be produced.

Reflection

• Looking at the objectives, give me a finger vote. 1—still feel clueless 5—makes perfect sense!

ExampleExample

• If 40.0 g of HIf 40.0 g of H33POPO44 react with 60.0 g react with 60.0 g of MgCOof MgCO33 calculate: calculate:

a.a. g of Mgg of Mg33(PO(PO44))22 produced produced

b.b. g of COg of CO22 produced produced

c.c. Volume of COVolume of CO22 at STP at STP

HH33POPO44 + MgCO + MgCO33 Mg Mg33(PO(PO44))22 + CO + CO22 + + HH22OO

IMPORTANT INFO…IMPORTANT INFO…

•Chapter 13 HW:Chapter 13 HW:– #1-9, 12,13#1-9, 12,13

•Chapter 13 Test DatesChapter 13 Test Dates– 5/4/15 (A-day)5/4/15 (A-day)