LIMITING REAGENT

Recipe for Chocolate Cake: 2 c flour 1 c sugar 2 eggs 1 c oil ½ c cocoa X 3 6 c flour 3 c sugar 6 eggs 3 c oil 1½ c cocoa

Recipe for Chocolate Cake: 2 c flour 1 c sugar 2 eggs 1 c oil ½ c cocoa 6 c flour 3 c sugar 6 eggs 3 c oil 1½ c cocoa I only have 4 cups of flour Flour is your limiting reagent!

In a chemical reaction, an insufficient quantity of any of the reactants will limit the amount of product that forms.

Ammonia, NH 3, is used in the production of fertilizers, ammonium nitrate. N 2 (g) + 3H 2 (g) 2NH 3 (g) Ammonia is produced industrially by the reaction of nitrogen gas with hydrogen gas. What would happen if two moles of N 2 reacted with three mol of H 2 ? 2 mol3 mol2 mol Limiting reagent Excess reagent

N 2 (g) + 3H 2 (g) 2NH 3 (g) 2 mol3 mol2 mol Limiting reagent Excess reagent Limiting reagent – the reactant that determines the amount of product that can be formed by a reaction. Excess reagent – the reactant that is not completely used up in a reaction.

Sometimes, the given quantities of reactants are expressed in units other than moles. In such cases, the first step is to convert the quantity of each reactant to moles. Then the limiting reagent can be identified. The amount of product formed in a reaction can be determined from the given amount of limiting reagent.

Why use an excess reagent? Some reactions do not continue until all the reactants are used up. Instead, they appear to stop while portions of the reactants are still present. Chemists have found that by using an excess of one reactant – often the least expensive one, reactions can be driven to continue until all of the limiting reactant is used up. Using an excess of one reactant can also speed up a reaction.

The equation for the complete combustion of ethene (C 2 H 4 ) is C 2 H 4 (g) + 3O 2 (g) 2CO 2 (g) + 2H 2 O(g) If 2.70 mol of ethene reacts with 6.30 mol of oxygen gas, identify the limiting reagent. What reactant is in excess?

Copper reacts with sulfur to form copper(I) sulfide according to the following unbalanced equation: Cu(s) + S(s) Cu 2 S(s) What is the limiting reagent when 80.0 g of copper reacts with 25.0 g of sulfur? What reactant is in excess? 2Cu(s) + S(s) Cu 2 S(s) Determining the Limiting Reagent in a Reaction

What is the maximum number of grams of copper(I) sulfide that can be formed when 80.0 g of copper reacts with 25.0 g of sulfur? 2Cu(s) + S(s) Cu 2 S(s) Recall, we found copper to be the limiting reagent, and there are 1.26 mol Copper. How much of the excess reagent remains after the reaction is complete? Using a Limiting Reagent to Find the Quantity of a Product

The equation for the incomplete combustion of ethene (C 2 H 4 ) is C 2 H 4 (g) + 2O 2 (g) 2CO(g) + 2H 2 O(g) If 2.70 mol of ethene reacts with 6.30 mol of oxygen gas, calculate the moles of water produced.

The heat from an acetylene torch is produced by burning acetylene (C 2 H 2 ) in oxygen. C 2 H 2 (g) + O 2 (g) CO 2 (g) + H 2 O(g) (unbal) How many grams of water can be produced by the reaction of 2.40 mol acetylene with 7.40 mol oxygen. 2C 2 H 2 (g) + 5O 2 (g) 4CO 2 (g) + 2H 2 O(g)

WHITEBOARD PRACTICE

Hydrogen gas can be produced by the reaction of magnesium metal with hydrochloric acid. Mg(s) + HCl(aq) MgCl 2 (aq) + H 2 (g) (unbal) Identify the limiting reagent when 6.00 g of HCl reacts with 5.00 g of Mg. Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g)

Iron is obtained commercially by the reaction of hematite (Fe 2 O 3 ) with carbon monoxide. Fe 2 O 3 (s) + CO( g ) Fe(s) + CO 2 ( s ) How many grams of iron are produced if 25.0 moles of hematite react with 30.0 moles of carbon monoxide? Fe 2 O 3 (s) + 3CO( g ) 2Fe(s) + 3CO 2 ( s )

The reaction between solid white phosphorus and oxygen produces solid tetraphosphorus decoxide. P 4 (s) + O 2 (g) P 4 O 10 (s) (unbal) 25.0 g of phosphorus reacts with 50.0 g of oxygen. a)What is the limiting reagent? b)How much tetraphosphorus decoxide is formed? P 4 (s) + 5O 2 (g) P 4 O 10 (s)

Disulfur chloride is used to vulcanize rubber, a process that makes rubber harder, stronger, and less likely to become soft when hot or brittle when cold. In the production of disulfur dichloride, molten sulfur reacts with chlorine gas according to this equation. S 8 ( l ) + Cl 2 ( g ) S 2 Cl 2 ( l ) If 200. g of sulfur reacts with 100. g of chlorine, what mass of disulfur dichloride is produced? What is the mass of excess reactant that remains after the reaction is complete? S 8 ( l ) + 4Cl 2 ( g ) 4S 2 Cl 2 ( l )

The reaction between solid sodium and iron(III) oxide is one in a series of reactions that inflates an automobile airbag. Na(s) + Fe 2 O 3 ( s ) Na 2 O( s ) + Fe(s) If 100. g of Na and 100. g of Fe 2 O 3 are used in the reaction, How much iron is formed? What is the mass of excess reactant that remains after the reaction is complete? 6Na(s) + Fe 2 O 3 ( s ) 3Na 2 O( s ) + 2Fe(s)

The equation below represents the production of tetraphosphorus trisulfide, a substance used in some match heads. P 4 (s) + S 8 ( s ) P 4 S 3 ( s ) If 125 g of phosphorus and 100. g of sulfur are used in the reaction, how much tetraphosphorus trisulfide is formed? 8P 4 (s) + 3S 8 ( s ) 8P 4 S 3 ( s )

PERCENT YIELD

A batting average is actually a percent yield. A batting average is calculated by dividing the number of hits a batter has had (actual yield) by the number of at-bats (theoretical yield).

When a balanced chemical equation is used to calculate the amount of product that will form during a reaction, the calculated value represents the theoretical yield. The theoretical yield is the maximum amount of product that could be formed from given amounts of reactants. In contrast, the amount of product that actually forms when the reaction is carried out in the laboratory is called the actual yield. The percent yield is the ratio of the actual yield to the theoretical yield expressed as a percent.

The percent yield is a measure of the efficiency of a reaction carried out in the laboratory.

Calculating the Theoretical Yield of a Reaction Calcium carbonate, which is found in seashells, is decomposed by heating. The balanced chemical equation for this reaction is CaCO 3 (s) CaO(s) + CO 2 (g) What is the theoretical yield of calcium oxide if 24.8 g of calcium carbonate is heated?

When 84.8 g of iron(II) oxide reacts with an excess of carbon monoxide, iron is produced. Fe 2 O 3 (s) + CO(g) Fe(s) + CO 2 (g) (unbal) What is the theoretical yield of iron? Fe 2 O 3 (s) + 3CO(g) 2Fe(s) + 3CO 2 (g)

When 5.00 g of copper reacts with excess silver nitrate, silver metal and copper(II) nitrate are produced. Cu(s) + AgNO 3 (aq) Ag(s) + Cu(NO 3 ) 2 What is the theoretical yield of silver in this reaction? Cu(s) + 2AgNO 3 (aq) 2Ag(s) + Cu(NO 3 ) 2

Calculating the Percent Yield of a Reaction What is the percent yield if 13.1 g CaO is actually produced when 24.8 g CaCO3 is heated? CaCO 3 (s) CaO(s) + CO 2 (g)

If 50.0 g of silicon dioxide is heated with an excess of carbon, 27.9 g of silicon carbide is produced. SiO 2 (s) + C(s) SiC(s) + CO(g) What is the percent yield of this reaction? SiO 2 (s) + 3C(s) SiC(s) + 2CO(g)

If 15.0 g of nitrogen reacts with 15.0 g of hydrogen, 10.5 g of ammonia is produced. N 2 (g) + H 2 (g) NH 3 (g) What is the percent yield of this reaction? N 2 (g) + 3H 2 (g) 2NH 3 (g)

WHITEBOARD PRACTICE

The compound calcium cyanamide (CaNCN) can be used as a fertilizer. To obtain this compound, calcium carbide is reacted with nitrogen at high temperatures. CaC 2 (s) + N 2 (g) CaNCN(s) + C(s) What is the theoretical yield of CaNCN of 7.50 mol calcium carbide reacts with 5.00 mol of nitrogen?

In a reaction chamber, 46.5 g of aluminum is mixed with 65.0 g of chlorine. Al(s) + Cl 2 (g) AlCl 3 (s) What is the theoretical yield of aluminum chloride? 2Al(s) + 3Cl 2 (g) 2AlCl 3 (s)

Heating an ore of antimony (Sb 2 S 3 ) in the presence of iron gives the element antimony and iron (II) sulfide. Sb 2 S 3 (s) + Fe(s) Sb(s) + FeS(s) When 15.0 g of Sb 2 S 3 reacts with an excess of iron, 9.84 g antimony are produced. What is the percent yield of this reaction? Sb 2 S 3 (s) + 3Fe(s) 2Sb(s) + 3FeS(s)

If 75.0 g of silderite ore (FeCO 3 ) is heated with 5.6 L of oxygen, 45.0 g of ferric oxide (Fe 2 O 3 ) is produced. FeCO 3 (s) + O 2 (g) Fe 2 O 3 (s) + CO 2 (g) What is the percent yield of ferric oxide? 4FeCO 3 (s) + O 2 (g) 2Fe 2 O 3 (s) + 4CO 2 (g)