Ch 3. Chemical Reactions and Reaction Stoichiometry

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1 Ch 3 Chemical Reactions and Reaction Stoichiometry

2 { { Chemical Reactions involve the rearrangement of atoms. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Reactant Products s Law of Conservation of Mass states that during a chemical reaction, atoms are neither created or destroyed. They are rearranged. Mass of reactants = Mass of products CH 4 Subscript say how many of atom before the number Coefficient say how many of molecule, atom, or moles 2 O 2

3 A g sample of Mg is completely burned in g of O 2 (g). The only product is MgO. After the reaction the mass of unreacted O 2 (g) is g. What mass of MgO is formed?

4 CH 4 (g) Physical state of substance g = gas l = liquid s = solid aq = aqueous (dissolved in water)

5 Balancing chemical equations Know reactants, products, and physical state Know correct formulas, write as an unbalanced equation Conserve atoms by placing coefficients in front of the chemicals until there is the same number of each kind of atom on opposite sides of

6 C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O (g) (NH 4 ) 2 Cr 2 O 7 (s) Cr 2 O 3 (s) + N 2 (g) + H 2 O (g) Ag 2 O (s) Ag (s) + O 2 (g) NH 3 (g) + O 2 (g) NO (g) + H 2 O (g)

7 Simple Patterns of Chemical Reactivity Combination (synthesis) A + B C Decomposition Combustion C A + B C x H y O z + O 2 CO 2 + H 2 O Single Replacement (single displacement) A + XY X + AY Double Replacement (double displacement) AB + XY AY + XB

8 Formula weight (FW) is the sum of atomic weights in amu of the atoms in the chemical formula. Molecular weight (MW) is the same as the formula weight for molecules. Molar mass is the sum of atomic weights in grams for all atoms in the chemical formula.

9 Mole is the number of atoms in exactly g of carbon. A mole of any substance is the amount that contains the same number of elementary entities as there are in 12 C atoms in exactly g 12 C. That number is Avogadro s Constant = 6.02 x The mass of 1 mole of atoms is the molar mass. It is the number of atoms (6.02 x atoms) and is equal to the mass in amu expressed in grams. A mass of iron is described as being 2.35 mol Fe. How many Fe atoms are present? How many moles of Mg are present in a sample containing 1.00 x Mg atoms? What is the mass of the sample?

10 Potassium 40 K is one of the few naturally occurring radioactive isotopes of low atomic number. It s % abundance is 0.012%. How many 40 K atoms do you ingest drinking one cup of whole milk containing 371 mg K?

11 Gold (Au) occurs in seawater to the extent of 0.15 mg / ton of seawater. How many Au atoms are present in a glassful (300 g) of seawater? (1 ton = 2000 lbs) (1 lb = g)

12 How many ions form when 0.1 mg MgCl 2 dissolves? Halothane, C 2 HBrClF 3, is an anesthetic. What is the amount of F in moles, in 75 ml sample of halothane? (d = g / ml )

13 Chloral hydrate (C 2 H 3 Cl 3 O 2 ), is a drug formerly used as a sedative and hypnotic. It is the compound used to make Mickey Finns in detective stories. a. Calculate u. b. What amount (in moles) of C 2 H 3 Cl 3 O 2 molecules are in 500.0g? c. What is the mass in grams of 2.0 x 10-2 mol? d. What number of Cl atoms are in 5.0 g? e. What mass of C 2 H 3 Cl 3 O 2 would contain 1.0 g Cl? f. What is the mass of exactly 500 molecules?

14 For the compound Ge[S(CH 2 ) 4 CH 3 ] 4 1. Total number of atoms in 1 molecule? 2. Ratio by number of C atoms to H atoms? 3. Ratio by mass of Ge to S? 4. Number of S in 1 mol of compound? 5. Number of C atoms in g of compound?

15 % Composition by element (m) % comp of element by m Total mass of element = ( Mass of compound ( 100 The formula for penicillin is C 14 H 20 N 2 SO 4. Find the % composition for each element.

16 A hydrate compound is one which has water attached to the molecule. An anhydrous compound is one which has no water attached. It is dry.

17 % H 2 O = ( Total mass of water Mass of hydrate compound ( 100 Find the % H 2 O in CuSO 4. 5H 2 O.

18 Formula is the smallest unit of a compound. Empirical formula is the simplest formula you can write with the types and number of atoms reflected in the formula. Molecular formula is the actual molecule formula and is equal to or some multiple of the empirical formula. (formula and molecular formula are basically the same)

19 Steps for establishing formula from % composition. 1. List mass of elements (as a of %, use mass in 100 grams) 2. Convert mass of element to moles 3. Write a tentative formula using mole values as subscripts 4. Attempt to convert subscripts to smallest whole numbers by dividing mole values by the smallest mole value 5. If decimal numbers, multiple by a small whole number to obtain empirical formula 6. To find molecular formula, compare empirical formula mass to molar mass of compound. Multiple subscripts by multiplication factor.

20 Dibutyl Succinate is an insect repellant used against household ants and roaches. It s composition is % C, 9.63 % H, and % O. It s experimentally determined molar mass is 230 u. What is the empirical formula and molecular formula?

21 Caffeine, a stimulant found in coffee, contains 49.5 % C, 5.15 % H, 28.9 % N, and 16.5 % O by mass and has a molar mass of 195 g/ mol. Determine the empirical formula and molecular formula.

22 Adipic acid is an organic compound composed of % C, % O and the rest hydrogen. If the molar mass of adipic acid is g/ mol,what are the empirical and molecular formulas.

23 Monosodium glutamate (MSG), a flavor enhancer in certain foods, contains % C, 4.77 % H, % O, 8.29 % N, and % Na and has a molar mass of 169 g/ mol Determine the empirical formula and the molecular formula.

24 C x H y O z + _ O 2 _ CO 2 + _ H 2 O Combustion analysis is used to find the empirical formula of a CH or CHO molecule. It is allowed to go through combustion to produce CO 2 + H 2 O. Analyze the amount of C + H then compare the mass to the original mass to determine the amount of O that was originally present.

25 Vitamin C is essential for prevention of Scurvy. Combustion analysis of a g sample of this C, H, O compound yields g CO 2, and g H 2 O. What is the empirical formula?

26 Stoichiometry Making mass relations in chemical reactions Balance equation Work in moles Set up mole ratio X mole Y mole X = Y 2 A + B 3 C + 4 D 2 3 Use moles and coefficients of the known to determine moles of the unknown! Use limiting reagent (reactant) to determine the moles of products. This is a chemical that is used up, thus limiting the amount of product that can be made. Convert back to grams if needed moles = grams Molar mass

27 Fire in a jug. Determine the exact amount of ethanol required to combust with the amount of oxygen in the jug. C 2 H 6 O (g) + O 2 (g) CO 2 (g) + H 2 O (g) Knowns: 20 L jug Density of air 1.15 g / L Air is ~ 20% O 2 Density of C 2 H 6 O 0.80 g / ml

28 Determine the mass of sodium bicarbonate needed to react exactly with 1 g of citric acid, and the mass of CO 2 (g) released. + NaHCO 3 C 6 H 5 O 7 Na 3 + H 2 CO 3? grams citric acid H 2 CO 3 H 2 O + CO m 1 gram? grams

29 An alloy used in fabricating aircraft structures consists of 93.7 % Al and 6.3 % Cu by mass. The alloy has a density of 2.85 g / cm 3. A cm 3 piece of alloy reacts with an excess of HCl. If one assumes that all of the Al reacts, but none of the Cu reacts, what mass of H 2 (g) is obtained?

30 Solid LiOH is used in space vehicles to remove exhaled CO 2 (g) from the living environment. What mass of CO 2 (g) can be removed by 1.00 Kg of LiOH?

31 How many moles of H 2 O (g) are produced by burning 2.72 mole of H 2 (g) in an excess of O 2 (g)?

32 Limiting Reagent is the reactant which is completely consumed during a chemical reaction, therefore limits the amount of product formed. What mass of PCl 3 (L) is formed when 125 g of P 4 (L) reacts with 325 g Cl 2 (g)? P 4 (L) + Cl 2 (g) PCl 3 (L)

33 NaHCO 3 (s) + HC 2 H 3 O 2 (aq) NaC 2 H 3 O 2 (aq) + CO 2 (g) + H 2 O (L) Stoichiometry Balloon Race Flask Amount of 2M HC 2 H 3 O 2 10 ml 10 ml 10 ml 10 ml 10 ml 10 ml Moles of HC 2 H 3 O 2 Mass of NaHCO g 1.00 g 1.50 g 2.00 g 2.50 g 3.00 g Moles of NaHCO 3 Moles of CO 2 Limiting reactant

34 Theoretical Yield is the amount of product formed when all of the limiting reactant is used up. Actual Yield is the amount of product actually formed. Rarely does a reaction go to completion. Percent Yield = ( Actual Yield ) 100 Theoretical Yield Billions of pounds of urea CO(NH 2 ) 2 are produced annually for fertilizer. 1 mol 2 NH 3 + CO 2 CO(NH 2 ) 2 + H 2 O If 47.7 g urea forms per mol of CO 2 that reacts, what is the theoretical yield, actual yield, and percent yield?

35 Parts per million (ppm) Measured in mg / L

Chapter 3. Mass Relationships in Chemical Reactions

Chapter 3. Mass Relationships in Chemical Reactions Chapter 3 Mass Relationships in Chemical Reactions In this chapter, Chemical structure and formulas in studying the mass relationships of atoms and molecules. To explain the composition of compounds and