Chapter Three: STOICHIOMETRY 講義
Assignment for Chapter 3 13,18,28,52,59,68,78,83,91,101,130
Core Materials • Stoichiometry • Mole • Molar mass • Percent composition • Empirical formula • Molecular formula • Chemical reactions • Characteristics of a chemical equation • Stoichiometry calculations • Yield
Chemical Stoichiometry • Stoichiometry – The study of quantities of materials consumed and produced in chemical reactions.
Need average mass of the object. Objects behave as though they were all identical. 3.1 Counting by Weighing
Elements occur in nature as mixtures of isotopes. Carbon = 98.89% 12C 1.11% 13C <0.01% 14C Carbon atomic mass = 12.01 amu 3.2 Atomic Masses
The number equal to the number of carbon atoms in exactly 12 grams of pure 12C 1 mole of anything = 6.022 x 1023 units of that thing (Avogadro’s number) 1 mole C = 6.022 x 1023 C atoms = 12.01 g C 3.3 The Mole
Mass in grams of one mole of the substance: Molar Mass of N = 14.01 g/mol Molar Mass of H2O = 18.02 g/mol (2 × 1.008) + 16.00 Molar Mass of Ba(NO3)2 = 261.35 g/mol 137.33 + (2 × 14.01) + (6 × 16.00) 3.4 Molar Mass
Concept Check Which of the following is closest to the average mass of one atomof copper? a) 63.55 g b) 52.00 g c) 58.93 g d) 65.38 g e) 1.055 x 10-22 g
Concept Check Calculate the number of copper atoms in a 63.55 g sample of copper.
Concept Check Which of the following 100.0 g samples contains the greatest number of atoms? • Magnesium • Zinc • Silver
Exercise Rank the following according to number of atoms (greatest to least): • 107.9 g of silver • 70.0 g of zinc • 21.0 g of magnesium
Exercise Consider separate 100.0 gram samples of each of the following: H2O, N2O, C3H6O2, CO2 • Rank them from greatest to least number of oxygen atoms.
Mass percent of an element: For iron in iron(III) oxide, (Fe2O3): 3.5 Percent Composition
Exercise Consider separate 100.0 gram samples of each of the following: H2O, N2O, C3H6O2, CO2 • Rank them from highest to lowest percent oxygen by mass.
Molecular formula = (empirical formula)n [n = integer] Molecular formula = C6H6 = (CH)6 Actual formula of the compound Empirical formula = CH Simplest whole-number ratio 3.6 Formulas
Device used to determine the mass percent of each element in a compound. 3.6 Analyzing for Carbon and Hydrogen
Balancing Chemical Equations 3.7 and 3.8
A representation of a chemical reaction: C2H5OH+3O22CO2 + 3H2O reactants products Reactants are only placed on the left side of the arrow, products are only placed on the right side of the arrow. 3.7 and 3.8 Chemical Equation
C2H5OH+3O22CO2 + 3H2O The equation is balanced. All atoms present in the reactants are accounted for in the products. 1 mole of ethanol reacts with 3 moles of oxygen to produce 2 moles of carbon dioxide and 3 moles of water. 3.7 and 3.8 Chemical Equation
The coefficients in the balanced equation have nothing to do with the amount of each reactant that is given in the problem. 3.7 and 3.8 Chemical Equations
The balanced equation represents a ratio of reactants and products, not what actually “happens” during a reaction. Use the coefficients in the balanced equation to decide the amount of each reactant that is used, and the amount of each product that is formed. 3.7 and 3.8 Chemical Equations
Exercise Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it. CaO + C CaC2 + CO2 I. CaO2 + 3C CaC2 + CO2 II. 2CaO + 5C 2CaC2 + CO2 III. CaO + (2.5)C CaC2 + (0.5)CO2 IV. 4CaO + 10C 4CaC2 + 2CO2
Concept Check Which of the following are true concerning balanced chemical equations? There may be more than one true statement. I. The number of molecules is conserved. II. The coefficients tell you how much of each substance you have. III. Atoms are neither created nor destroyed. IV. The coefficients indicate the mass ratios of the substances used. V. The sum of the coefficients on the reactant side equals the sum of the coefficients on the product side.
The number of atoms of each type of element must be the same on both sides of a balanced equation. Subscripts must not be changed to balance an equation. A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction. Coefficients can be fractions, although they are usually given as lowest integer multiples. 3.8 Notice
Chemical equations can be used to relate the masses of reacting chemicals. 3.9 Stoichiometric Calculations
Balance the equation. Convert mass to moles. Set up mole ratios from the balanced equation. Calculate number of moles of desired reactant or product. Convert back to grams. 3.9 Calculating Masses in Reactions
Exercise • Methane (CH4) reacts with the oxygen in the air to produce carbon dioxide and water. • Ammonia (NH3) reacts with the oxygen in the air to produce nitrogen monoxide and water. • What mass of ammonia would produce the same amount of water as 1.00 g of methane reacting with excess oxygen?
Let’s Think About It We need to know: • How much water is produced from 1.00 g of methane and excess oxygen. • How much ammonia is needed to produce the amount of water calculated above.
Limiting reactant – the reactant that is consumed first and therefore limits the amounts of products that can be formed. Determine which reactant is limiting to calculate correctly the amounts of products that will be formed. 3.10 Limiting Reactants
Limiting Reactants 3.10
Methane and water will react to form products according to the equation: CH4 + H2O 3H2 + CO 3.10 Limiting Reactants
The amount of products that can form is limited by the methane. Methane is the limiting reactant. Water is in excess. 3.10 Limiting Reactants
Concept Check Which of the following reaction mixtures could produce the greatest amount of product? Each involves the reaction symbolized by the equation: 2H2 + O2 2H2O a) 2 moles of H2 and 2 moles of O2 • 2 moles of H2 and 3 moles of O2 • 2 moles of H2 and 1 mole of O2 d) 3 moles of H2 and 1 mole of O2 e) Each produce the same amount of product
We cannot simply add the total moles of all the reactants to decide which reactant mixture makes the most product. We must always think about how much product can be formed by using what we are given, and the ratio in the balanced equation. 3.10 Notice
Concept Check • You know that chemical A reacts with chemical B. You react 10.0 g of A with 10.0 g of B. • What information do you need to know in order to determine the mass of product that will be produced?
Let’s Think About It We need to know: • The mole ratio between A, B, and the product they form. In other words, we need to know the balanced reaction equation. • The molar masses of A, B, and the product they form.
Exercise • You react 10.0 g of A with 10.0 g of B. What mass of product will be produced given that the molar mass of A is 10.0 g/mol, B is 20.0 g/mol, and C is 25.0 g/mol? They react according to the equation: A + 3B 2C
An important indicator of the efficiency of a particular laboratory or industrial reaction. 3.10 Percent Yield
For Review • Stoichiometry • ˙Deals with the amounts of substances consumed and/or produced in a chemical reaction. • ˙We count atoms by measuring the mass of the sample. • ˙To relate mass and the number of atoms, the average atomic mass is required. • Mole • ˙The amount of carbon atoms in exactly 12 g of pure 12C. • ˙6.022 x1023 units of a substance • ˙The mass of one mole of an element= the atomic mass in grams • Molar mass • ˙Mass(g) of one mole of a compound or element • ˙Obtained for a compound by finding the sum of the average masses of its constituent atoms • Percent composition • ˙The mass percent of each element in a compound • ˙ • Empirical formula • ˙The simplest whole-number ratio of the various types of atoms in a compound • ˙Can be obtained from the mass percent of elements in a compound • Molecular formula • ˙For molecular substances: • ˙The formula of the constituent molecules • ˙Always an integer multiple of the empirical formula • ˙For ionic substances: • ˙The same as the empirical formula
For Review • Chemical reactions • ˙Reactants are turned into products. • ˙Atoms are neither created nor destroyed. • ˙All of the atoms present in the reactants must also be present in the products. • Characteristics of a chemical equation • ˙Represents a chemical reaction • ˙Reactants on the left side of the arrow, products on the right side • ˙When balanced, gives the relative numbers of reactant and product molecules or ions • Stoichiometry calculations • ˙Amounts of reactants consumed and products formed can be determined from the balanced chemical equation. • ˙The limiting reactant is the one consumed first, thus limiting the amount of product that can form. • Yield • ˙The theoretical yield is the maximum amount that can be produced from a given amount of the limiting reactant. • ˙The actual yield, the amount of product actually obtained is always less than the theoretical yield. • ˙
Chapter Three Stoichiometry 案例/討論
Figure 3.1 (left) A Scientist Injecting a Sample into a Mass Spectrometer. (right) Schematic Diagram of a Mass Spectrometer