Chemical Quantities

9.1 Information Given by Chemical Equations Recall from Dalton’s Atomic theory Atoms are just rearrange (not created or destroyed) in a chemical reaction Consider the unbalance equation 1CO(g) + 2H 2 (g)  1CH 3 OH(l)

9.2 Mole – Mole Relationships Express the moles ratios Relate the mole of one substance to another substance a balanced equation Also used as a conversion factor E.g 2H 2 (g) + O 2 (g)  2H 2 O(l) What is the mole ratio between H 2 (g) and O 2 (g) H 2 (g) and H 2 O(g) O 2 (g) and H 2 O(g)

Calculating Moles using Moles Ratio Methane burns in oxygen to form carbon dioxide and water according to the balanced equation CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g) What is the number mole of oxygen gas is required to react with 7.4 moles of methane?  Hydrogen sulfide gas reacts with oxygen gas to produce sulfur dioxide gas and water according to the balanced equation 2H2S(g) + 3 O 2 (g)  2 SO 2 (g) + 2 H 2 O(l) How many mole of oxygen gas are required to react with 5.6 moles of hydrogen sulfide?

9.3 Mass Calculation Steps for Calculating the Masses of Reactants and Product in Chemical Reactions Step 1: Balance the equation for the reaction Step 2: convert the masses of reactants or product to moles Step 3: use the balanced equation to set up the appropriate mole ratio(s) Step 4: Use the mole ratio(s) to calculate the number of moles of the desired reactant or product Step 5: Convert from moles back to masses (of the desired reactant or product)

Example Some sulfur is present in coal in the form of pyrite (FeS2; also known as fool’s gold). When it burns, it pollutes the air with the combustion product SO2, as shown by the following chemical equation 4 FeS 2 (s) + 11 O 2 (g)  2Fe 2 O 3 (s) + 8SO 2 (s) What mass of SO 2 is produced by the combustion of 38.8 g of FeS 2 ?

Example Hydrofluoric acid, an aqueous solution containing dissolved hydrogen fluoride, is used to etch glass by reacting the silica, SiO 2, in the glass to produce gaseous silicon tetrafluoride and liquid water. The unbalance equation is ….HF(aq) + ….SiO 2 (s)  …..SiF 4 (g) + ….H 2 O(l) Calculate the mass of hydrogen fluoride needed to react with 5.68 g of silica Calculate the mass of water produced in the reaction described in part a

9.4Calculations Involving a Limiting Reactant Limiting Reactant (limiting reagent): is the reactant that is completely consumed in a chemical reaction and limits the amount of product. Excess Reactant: Any of the other reactants still present after determination of the limiting reactant.

Steps in Determination Limiting Reagent i. Check to be sure you have a balanced equation aA + bB  c C ii. Convert the amount of reagent one that was given into the number moles product that you could form if that reagent was completely consumed. gA  moles A  moles C iii.Convert the amount of reagent two that was given into the number moles of product that you could form if that reagent was completely consumed. gB  moles B  moles C

Steps in Determination Limiting Reagent iv. The reactant that produced the LEAST amount of product in step 2 or 3 will the limiting reagent. v. Convert the LEAST moles into the number grams product and that will be your theoretical yield. Theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reagent

Copyright © 2008 Pearson Prentice Hall, Inc.Chapter 3/11 Reactions with Limiting Amounts of Reactants At a high temperature, ethylene oxide reacts with water to form ethylene glycol which is an automobile antifreeze and a starting material in the preparation of polyester polymers: C 2 H 4 O(aq) + H 2 O(l) C2H6O2(l)C2H6O2(l)

Practice Problem If we have 42.5 g Mg and 33.8 g O 2, what is the limiting reactant? 2 Mg(s) + O 2 (g)  2 MgO(s)

Practice Problems Ammonia, NH 3, can be synthesis by the following reaction: 2 NO(g) + 5 H 2 (g)  2 NH 3 (g) + 2H 2 O(g) Starting with 86.8 g NO and 25.6 g H 2, find the theoretical yield of ammonia in grams

9.5 Percent Yield Recall Theoretical yield is the amount of product that can be made in a chemical reaction based on the amount of limiting reagent

Examples In a given experiment, a 4.70 g quantity of H 2 is allowed to react with N 2 ; a 12.5 g quantity of NH 3 is formed. What is the percent yield on the H 2 ? The equation for the reaction is N 2 (g) + 3 H 2 (g)  2 NH 3 (g)

Example Zinc and silver undergo a single replacement reaction according to the equation Zn(s) + 2AgNO 3 (aq)  Zn(NO 3 ) 2 (aq) + 2Ag(s) When 25.0 g of zinc is added to the silver nitrate solution, the percent yield is 72.3%. What is the mass of silver is formed?