Chapter 9 Stoichiometry

Recall--in your notes Predict the products for the following reaction and balance the equation. Aluminum metal reacts with hydrochloric acid

Stoichiometry—What is it?? 2 types Composition stoichiometry—deals with the mass relationships of elements in compounds. Reaction stoichiometry—involves mass relationships between the reactants and products in a reaction. A reaction will happen until one of the reactants is completely used up.

Law of Conservation of Mass All balanced reactions obey the law of conservation of mass. We can prove it through a balanced equation. Calculate the masses of reactants and the masses of the products for the given chemical equation: 4 Fe(s) + 3 O2(g) --> 2 Fe2O3(s)

Mole Ratios 2Al2O3(l)  4Al(s) + 3O2(g) What are they? A conversion factor that relates the amount of moles of one substance to the amount of moles of another substance. Obtained directly from the balanced equation. 2Al2O3(l)  4Al(s) + 3O2(g) How many moles of Al are formed from 27 moles of Al2O3? Mole ratios are exact and do not limit the number of significant figures.

For each equation, write all possible mole ratios. 1.) 2HgO  2Hg + O2 2.) 4NH3 + 6 NO  5N2 + 6H2O Each reflects the ratio of one substance to the other substances in the equation.

Importance of Mole Ratios Calculations of the amount of reactants Can determine how much to use to get a certain amount of product Can determine how much can be produced from the amount of reactants present

On a separate piece of paper Write the balanced chemical equation for the reaction between magnesium sulfate and sodium hydroxide.

Solving stoichiometry problems 1.) All stoichiometric calculations begin with a balanced chemical equation. 2.) Mole ratios are then found to set up conversion factors 3.) Then mass to mole conversions are made

Practice Predict the products and find all possible mole ratios for the following reactions: 1.) Na + O2  2.) Pb(NO3)2 + LiI

Moles to Moles Example: 1. How many moles of hydrogen are produced when 0.040 moles of potassium react with water? ( the other product is potassium hydroxide) 2. How many moles of oxygen are produced from the decomposition of 6 moles of KClO3 into potassium chloride and pure oxygen?

Mole to Mass Example: What mass of sodium chloride will be produced when 1.25 moles of chlorine gas reacts with sodium?

Mass to Mass Example: What mass of water is produced from the decomposition of 25.00g of solid ammonium hydroxide (by-product is ammonia, NH3)

On a half-sheet of paper… What mass of carbon dioxide is needed to react 3.00 mol of water in a photosynthesis reaction? CO2 + H2O  C6H12O6 + O2

Limiting Reagents Limiting Reagent—the reactant that is used up completely in a reaction and dictates the amount of products formed. Excess Reagent—The substance that is not used up completely in a reaction.

Method for finding limiting reagent 1.) Calculate moles of each reactant. 2.) Calculate moles of product that will form with given amount of reactant (pick 1 product only) 3.) The reactant that yields the lowest amount of product is the limiting reagent.

Practice Silicon dioxide (quartz) reacts readily with hydrofluoric acid to produce silicon tetrafluoride and water. If 6.0 mol of hydrofluoric acid is added to 4.5 mol of silicon dioxide, what is the limiting reagent?

Practice Rocket engines use a mixture of hydrazine (N2H4) and hydrogen peroxide, H2O2,as the propellant. The mixture combines to produce nitrogen gas and water. A. Which is the limiting reagent in this reaction when 0.750 mol N2H4 is mixed with 0.500 mol of H2O2? B. How much excess reactant, in moles, remains unchanged? C. How much of each product, in grams, is formed?

Practice cont. Zn + S8  ZnS For the following reaction: A.) If 2.00 mol of Zn are heated with 1.00 mol of S8, identify the limiting reagent. B.) How many moles of product are formed? C.) How many moles of excess reagent remain?

More practice Aspirin, C9H8O4, is produced from salicylic acid, C7H6O3 and acetic anhydride, C4H6O3 (acetic acid, CH3COOH, is a by-product). 1.) Write a complete, balanced equation for the reaction. 2.) What mass of aspirin can be produced from 75.0 mol of salicylic acid? 3.) How many liters of acetic acid would be formed? (Density of acetic acid is 1.05 g/mL)

Percent Yield Terms to know Theoretical Yield—the maximum amount of product that can be formed from a given amount of reactants. Actual Yield—The measured amount of product obtained from a reaction. Percent Yield—the ratio of actual yield to the theoretical yield, multiplied by 100. 𝐴𝑐𝑡𝑢𝑎𝑙 𝑌𝑖𝑒𝑙𝑑 𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑌𝑖𝑒𝑙𝑑 ×100 Shows efficiency of rxn—how much product the chemist isolates.

Calculating percent yield… For the following reaction, 36.8 g of benzene, C6H6, reacts with an excess of Cl2. The actual yield of C6H5Cl is 38.8 g (by-product is hydrochloric acid). 1.) What is the percent yield of C6H5Cl? Steps: 1.) Calculate theoretical yield first. 2.) Plug actual and theoretical yield into percent yield equation.

How many grams of fluorine gas are needed to produce 120 How many grams of fluorine gas are needed to produce 120.00 g of phosphorous trifluoride if the reaction has a 78.10% yield? P4 + F2  PF3