Chapter 12 notes Part 2

Limiting Reagents Rarely in nature are reactants in a chemical reaction present in the exact ratios specified by the balanced equation. One or more reactants are in excess and the reaction proceeds until all of one reactant is used up.

Limiting reactant  limits the extent of the reaction and determines the amount of the product Limiting reactant  limits the extent of the reaction and determines the amount of the product Excess reactants  a portion of all the other reactants remains after the reaction stops, these are the excess reactants Excess reactants  a portion of all the other reactants remains after the reaction stops, these are the excess reactants

1 banana + 5 s.berries + 2 scoops i.c. + 8oz milk  1 shake if you had: 8 bananas, 40 s.berries, 10 scoops i.c., and a gallon of milk 8 bananas, 40 s.berries, 10 scoops i.c., and a gallon of milk how many shakes could you make? what limits you? how many shakes could you make? what limits you? bananas berries ice cream milk 8/1 = 8 40/5 = 8 10/ 2 = 5 128oz/8 = 16

New shipment arrives 320 oz bananas, 80oz berries, 80 scoops i.c., 3 gal milk (1 banana weighs 8oz, 1 berry weighs 0.5oz, 1 gal=128oz) what will “limit” production of shakes now? what will “limit” production of shakes now? what will be “in excess” ? what will be “in excess” ?

Determining amt of product expected should be based on the reagent (reactant) not present in excess. Use the limiting reactant to solve the problem.

10.0g NaOH are reacted with 10.0g HCl to form ? Salt NaOH + HCl → NaCl + H2O 10.0g 10.0g ?g 1 st : change g of each reactant to moles and ÷ by coeffic. 2 nd :Which is smaller? That is the limiting reactant.

3 rd :Use the limiting reactant to solve the problem. 4 th : To find out how much is leftover of excess reactant use the limiting reactant to solve for the one in excess (as if it were unknown). This is theoretically how much was “used.” 5 th : Subtract the “used” g from the “given” g to find unused

Lime (CaO) is obtained by heating limestone CaCO3  CaO + CO2 Lime is important in the production of cement, glass, steel…heated w/ coke (type of carbon) forms calcium carbide 2CaO(s) + 5C(s)  2 CaC2(s) + CO2(g) CaC2(s) + 2H2O(ℓ) → C2H2(g) + Ca(OH)2(aq) acetylene – an important fuel for welding and a chemical base for many industrial chemicals like vinyl chloride

Suppose your company took delivery of kg of CaO and kg of coke. You supervised the mixing and heating of these. How much CaC2 would your boss expect you to produce? He has promised to sell kg of your product (CaC2) to the acetylene producer by next week. Do you have enough reagents on hand to meet the demand? If not, how much more do you need to order?

Examples 2NO + 5H2 → 2NH3 + 2H2O 150g 20g limiting _______ mol of H2O ____ 4mol 6mol CaO + 3C → CaC2 + CO ? g ? g limiting ________ CaC2 _______g

Examples: C2H4 + 3O2 → 2CO2 + 2H2O 280g 320g ?g limiting________ CO2 ______g Zn + 2HCl → ZnCl2 + H2 130g 100g ?g limiting _________ H2 _______g