Unformatted text preview: 10/6/11 Wri�ng Chemical Equa�ons Wri�ng Chemical Equa�ons All chemical rxns obey one inviolate law: The amounts of each element are always conserved. This is the law of conserva�on of ma�er that Dalton recognized as one postulate of atomic theory. An equa�on that represents a rxn while conserving amounts of all elements is called a balanced chemical equa�on. A balanced equa�on provides a list of ingredients for the rxn and it shows what species the rxn produces. It also tells the rela�ve amounts of each species involved in the rxn. Ammonia Synthesis Ammonia Synthesis Real-‐Life Chemistry Real-‐Life Chemistry BBQ 1 10/6/11 Real-‐Life Chemistry Photosynthesis Real-‐Life Chemistry Glycolysis pathway Real-‐Life Chemistry Lithium ba�ery Real-‐Life Chemistry DNA Wri�ng Chemical Equa�ons Wri�ng Chemical Equa�ons No�ce these essen�al features of the eq. 1. The reactants appear on the le� and the products appear on the right. The arrow joining them indicates the direc�on of rxn. 2. An integer is associated with each substance. These #s are known as stoichiometric coeﬃcients. When no # appears we understand that it is 1. 3. Stoichiometric coeﬃcients refer to the rela�ve # of molecules involved in the rxn. Because the # of molecules of any substance divided by NA gives the #s of moles of the substance, the coeﬃcients also describe the rela�ve #s of moles involved in the rxn. 4. The stoichiometric coeﬃcients in any chemical equa�on are the set of smallest integers that gives a balanced equa�on. A balanced chemical equa�on can be interpreted in two ways. The equa�on for the ammonia synthesis, for example, tells us that: One molecule of N reacts with 3 molecules of H to give 2 molecules of NH3. One mol of N reacts with 3 mol of H to give 2 mol of NH3. 2 10/6/11 Wri�ng Chemical Equa�ons Balancing Equa�ons Keep in mind that the key feature of balanced chemicals equa�ons is the conversa�on law: The number of atoms of each element is conserved in any chemical rxn. A balanced chemical equa�on o�en includes informa�on about the condi�ons for the rxn. For example, the physical states of the reagents involved in a rxn may be indicated in parentheses: (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous. The integers in a balanced equa�on must be chosen so that the atoms of each element are conserved. Let us ﬁrst balance them by inspec�on. Balancing by inspec�on means changing coeﬃcients un�l the # of atoms of each element is the same on each side of the arrow. C3H8(g) + O2(g) CO2(g) + H2O(g) Examples Examples
P4O10 + H2O H3PO4 Mg3N2 + H2O Mg(OH)2 + NH3 As + Cl2 AsCl2 Au2S3 + H2 H2S + Au KClO3 KCl + O2 C2H4 + O2 + HCl C2H4Cl + H2O NH3 + NO N2 + H2O
C5H5N + O2 H2O + CO2 + N2 NH3 + O2 N2 + H2O H2 + NO NH3 + H2O C6H12O6 C2H5OH + CO2 C3H6 + NH3 + O2 C3H3N + H2O SiO2 + C + Cl2 SiCl4 + CO B10H18 + O2 B2H3 + H2O Chemical Compounds Chemical Compounds Just as each element has a characteris�c molar mass, so does every chemical compound. Chemical compounds are composed of atoms bound together into molecules or ions clustered together in electrically neutral aggregates. In either case, a chemical formula describes the atomic composi�on of a compound. One more of any chemical compound is 1 mol of its chemical formula unit. For example: 1. One mole of O2 is 1 mol of diatomic O2 molecules. Each molecule contains two oxygen atoms, so 1 mol of O2 contains 2 mol of oxygen atoms. 2. One mole of methane (CH4) is 1 mol of CH4 molecules so it contains 1 mol of C atoms and 4 mol of H atoms. 3 10/6/11 Chemical Compounds 3. One mole of sodium chloride (NaCl) contains 1 mol each of Na+ ca�ons and Cl-‐ anions. 4. One mole of magnesium chloride (MgCl2) contains 1 mol of Mg2+ and 2 mol of Cl-‐. Example: One mole of O2 molecules contains 2 mol of O2 atoms so the molar mass of molecular O2 is as follows: (2 mol O/1 mol O2) (16.00 g/mol O) = 32.00 g/mol O2 MM of O2 = 32.00 g/mol Stoichiometry Problem #1 We have shown the rela�onships among atoms, moles and masses which has allowed us to answer many ques�ons about individual molecules. Combining these ideas with the concept of a balanced chemical equa�on lets us answer many ques�ons about chemical rxns. The study about the amounts of materials consumed and produced in a chemical rxn is called stoichiometry. How much molecular hydrogen is required to produce 2.0 metric tons of ammonia by the Haber process? Problem #2 Problem #3 A chemist needs to make some geranyl formate for a batch of perfume. How many grams of geranyl formate can a chemist make from 375 g of geraniol? The poisonous gas HCN is an important industrial chemical. It is produced from CH4, NH3 and O2. The rxn also produces H2O. An industrial manufacturer wants to convert 175 kg of CH4 into HCN. How much HCN can be produced in the rxn? What masses of NH3 and O2 will be required? Remember: Calcula�ons in chemistry are centered around the mole. 4 10/6/11 Yields of Chemical Reac�ons Yields of Chemical Reac�ons When chemical rxns are performed under prac�cal condi�ons, the amounts of products obtained are almost always less than the amounts predicted by stoichiome�c analysis. Why is that? 1. Incomplete rxn. 2. Compe�ng reagents may consume some of the star�ng materials. 3. The star�ng materials may form more than one product. 4. Loss of product during puriﬁca�on and isola�on. We describe amounts obtained from a rxn in terms of the rxn yield. The quan�ty of product predicted by stoichiometry is called the theore�cal yield, whereas, the amount actually obtained is called the actual yield. Usually, a rxn is reported as a percent yield which is the percentage of the theore�cal amount: Percent yield = ((Actual yield)/(Theore�cal yield)) 100% Problem #1 Problem #2 We just examined a problem involving the industrial produc�on of HCN. If the yield of this synthesis is 97.5%, how many kg of CH4 and NH3 should be used to produce 1.50 X 105 kg of HCN? An industrial plant is synthesizing phosphoric acid from ﬂuoroapa�te. If every kg of apa�te yields 400 g of phosphoric acid, what is the yield based on the ﬂuoroapa�te consumed? The Limi�ng Reagent The Limi�ng Reagent This part is very important. We have so far only covered rxns where all the star�ng materials were used up. Ah, but life is not always fair. Chemists frequently run rxns where excess is used of one or more components. This means that one reactant will “run out” before the others. The reactant that runs out is called the limi�ng reagent because it limits how much product can be made. 5 10/6/11 The Limi�ng Reagent Problem #1 Esters are pleasant-‐smelling substances responsible for the ﬂavor and fragrance of many fruits and ﬂowers. For example, an ester called banana oil gives bananas their characteris�c odor. Banana oil can be prepared by the following rxn: C5H12O + C2H4O2 C7H14O2 + H2O How many grams of banana oil can a chemist prepare from 38 g of 3-‐methylbutanol and 32 g of ace�c acid? Which star�ng material will be present in excess and how much of it will be le� over a�er the rxn is complete? Problem #2 Problem #3 Phosphoric acid is among the top 10 U.S. industrial chemicals. Approx. 20 billion lbs of it are produced annually for fer�lizers, detergents and agents for water treatment. Phosphoric acid can be prepared be trea�ng mineral ﬂuoroapa�te with sulfuric acid in the presence of excess water: When a sample of P burns in air, the compound P4H10 forms. Assume one experiment showed that 0.744 of P formed 1.704 g of P4H10. use this informa�on to determine the ra�o of the atomic masses of P and O. If the atomic mass of O is assumed to be 16.000 amu, calculate the atomic mass of P. Ca5(PO4)3F + 5H2SO4 + 10H2O 3H3PO4 + 5CaSO4.2H2O + HF If a chemical manufacturer has 2.00 metric tons of ﬂuoroapa�te and 1.50 metric tons of sulfuric acid, how much phosphoric acid can be produced? How much of the excess reactant will be le� over? Problem #4 Problem #5 Analysis of ibuprofen, the ac�ve ingredient in Advil, shows that it contains 75.7% C, 8.8% H and 15.5% O. The mass spectrum of ibuprofen shows that it has a molar mass of about 200 g/mol. Determine the chemical formula of this compound. A petroleum chemist isolated a major component of gasoline and found its molar mass to be 114.2 g/mol. When 1.55 g of this compound were completely burned in excess oxygen, 2.21 g of H2O and 4.80 g CO2 were produced. Find the empirical and molecular formulas of the compound. 6 10/6/11 Problem #6 Problem #7 A pure substance was known to contain only C, H and Cl. When a 4.00 g sample was burned, 4.34 g of CO2 was produced. In a separate experiment, the chlorine contained in 0.125 g of the compound was converted to 0.334 g of AgCl. Determine the empirical formula of the compound. Transi�on metals can combine with CO to form compounds such as Fe(CO)5 and Co2(CO)
8. Assume that you combine 0.125 g of Ni with CO and isolate 0.364 g of Ni(CO)x. What is the value of x? 7 ...
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- Fall '11
- Chemistry, NH3