Calculations and Chemical Equations. Example: Hydrogen atomic weight = amu Carbon atomic weight = amu

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1 Calculations and Chemical Equations Atomic mass: Mass of an atom of an element, expressed in atomic mass units Atomic mass unit (amu): x g Atomic weight: Average mass of all isotopes of a given element; listed on the periodic table Example: Hydrogen atomic weight = amu Carbon atomic weight = amu Carbon atoms are 12 times as massive as hydrogen atoms Ca atomic mass = amu Ne atomic mass = amu How many Neon atoms are required to give the same mass as one calcium atom? 1 Dozen = 12 1 Gross = Mole = 6.02 x Avogadro s Number: The Mole 1 mole of atoms = x atoms 1 mole of molecules = x molecules Molar Mass: the mass (in grams) of one mole a particular substance There is a unique relationship between molar mass and atomic weight: Oxygen s atomic weight is amu. 1 mole of oxygen is 6.02 x atoms of oxygen 1 amu = x g What is the molar mass (g/mole) of oxygen? amu/oxygen atom x x g/amu = x g/atom x 6.02 x atoms/mole =16.00 g/mole Molar mass (in grams) is always equal to the atomic weight of the atom! Molar mass (in grams) is always equal to the atomic weight of the atom! Atomic weight of carbon is 12.01amu Molar mass of carbon is g/mol Atomic weight of helium is 4.00 amu Molar mass of helium is 4.00 g/mol Mole (mol): The number of particles (atoms or molecules) in a sample of element or compound with a mass in grams equal to the atomic (or molecular) weight Example: Atomic weight of sodium = g of sodium contains 1 mole (6.02 x ) of atoms 1

2 Calculations Using the Mole 1 mole represents the mass of a sample that contains Avogadro s number of particles Example: Atomic wt. of Potassium (K)= 39 amu 1 mol K atoms = 6.02 x atoms = 39 g K Example: Atomic wt. of Sulfur (S) = 32 u = 32 g S What is the mass in grams of 1 atom of Sulfur? 1 S atom = g 1 S atom x 32 g S/6.02 x atoms = g 1 S atom = 5.32 x10-23 g How many moles of Sulfur in 98.6 grams? 98.6g = moles Sulfur 98.6g x 1 mole/32g = moles sulfur = 3.07 moles sulfur How many atoms in this sample of 98.6g of S? 98.6g = atoms of S 98.6g x 6.02x10 23 atoms/32g = atoms of S = 1.85x10 24 atoms S What is the mass of 1 atom of Sulfur? 1 atom = g 1 atom x 32g/6.02x10 23 atoms = g The mole concept applies to molecules, as well as to atoms Chemical formulas indicate relative quantities of atoms within a compound Example: H 2 O has 2 H atoms for every 1 Oxygen atom C 6 H 12 O 6 has how many atoms of each element? = 5.33 x10-23 g S 2

3 Formula Weight (F.W.): sum of all atomic weights of all atoms in a compound; expressed in amu A mole of a molecule will have a mass in grams equal to its formula weight Example: F.W. of CH 4 N 2 O (urea) =? Atomic weights: C = 12 amu H = 1 amu N = 14 amu O = 16 amu F.W. CH 4 N 2 O = (1) + 2 (14) + 16 = 60 amu Molar mass of CH 4 N 2 O is 60 g/mol Formula weight of H 2 O =? F.W. of H 2 O = atomic weight of H atoms + atomic weight of O atom Atomic weight of H =1amu, O =16 amu F.W. of H 2 O = 18 amu Molar mass of H 2 O = 18 g/mol More Prozac, C 17 H 18 F 3 NO, is a widely used antidepressant that inhibits the uptake of serotonin by the brain. What is the molar mass of Prozac? 17C (12.0) + 18H (1.0) + 3F (19.0) + 1N (14.0) + 1 O (16.0) = = g/mole Chemical Equations Chemical Reaction: Interaction between substances that results in one or more new substances being produced Example: hydrogen + oxygen water Reactants of a Reaction: Starting materials that undergo chemical change; written on the left side of the equation representing the reaction Products of a Reaction: Substances formed as a result of the reaction; written on the right side of the equation representing the reaction The arrow points towards the products formed by the reaction Individual products and reactants are separated by a plus sign Chemical Equation: A written statement using symbols and formulas to describe the changes that occur in a reaction Example: 2H 2 (g) + O 2 (g) 2H 2 O (l) Letter in parentheses indicates the state of the substance: gas (g), liquid (l), solid (s), dissolved in water (aq) If heat is required for the reaction to take place, the symbol Δ is written over the reaction arrow Types of Chemical Reactions Combination Reactions: Reaction in which 2 or more substances react to form a single substance Decomposition Reactions: Reaction in which a single substance reacts to form 2 or more simpler substances Single-Replacement Reactions: Reaction in which an element reacts with a compound and displaces another element from the compound Double-Replacement Reactions: Reaction in which 2 compounds react and exchange partners to form 2 new compounds 3

4 Precipitation Reactions: Reaction in which chemical change results in formation of one or more insoluble product Acid-Base Reactions: Reactions involving the transfer of a hydrogen ion, H +, from one reactant to another Redox Reactions: Reaction in which substances undergo oxidation and reduction; involve the transfer of electrons from one reactant to another Oxidation: loss of electrons Combination reaction: two or more elements or compounds combine to form one product. A + B AB H 2 + Cl 2 2HCl 2S + 3O 2 2SO 3 4Fe + 3O 2 2Fe 2 O 3 Reduction: gain of electrons Decomposition reaction: one substance is broken down (split) into two or more simpler substances. Single replacement reaction: one element takes the place of an element in a reacting compound AB A + B A + BC AC + B Example: Zn(s) + HCl(aq) ZnCl 2 (aq) + H 2 (g) 2HgO 2Hg + O 2 2KClO 3 2KCl + 3 O 2 H 2 Zn HCl ZnCl 2 Double replacement reaction: two elements swap places among the reactants (the positive ions in the reacting compounds) AB + CD AD + CB Combustion reaction: a reactant (often containing carbon) reacts with oxygen O 2. AgNO 3 + NaCl AgCl + NaNO 3 C + O 2 CO 2 CH 4 + 2O 2 CO 2 + 2H 2 O ZnS + 2HCl ZnCl 2 + H 2 S C 3 H 8 + 5O 2 3CO 2 + 4H 2 O 4

5 Oxidation and Reduction Reactions An oxidation-reduction reaction involves the transfer of electrons from one reactant to another. In oxidation, electrons are lost Zn Zn e - (loss of electrons--leo) In reduction, electrons are gained. Cu e - Cu (gain of electrons--ger) Identify each of the following as an oxidation or a reduction reaction: A. Sn Sn e- Oxidation B. Fe e - Fe 2+ Reduction Balanced Equation: Equation in which the number of atoms of each element in the reactants is the same as the number of atoms of that element in the products Law of Conservation of Mass: Atoms are neither created nor destroyed in chemical reactions C. Cl 2 + 2e - 2Cl - Reduction Example: CaS + H 2 O CaO + H 2 S Reactants Products Example: NO(g) + O 2 (g) NO 2 (g) SO 2 + O 2 SO 3 2SO 2 + O 2 2SO 3 Adjust the coefficient of the reactants and products to balance the equation 2NO + O 2 2NO 2 Reactants Product H 2 + Cl 2 HCl H 2 + Cl 2 2HCl 5

6 Chemical Equations and the Mole Stoichiometry: Study of mass relationships in chemical reactions; ratios of different molecules Example: CH 4 + 2O 2 CO 2 + 2H 2 O 10 CH O 2 10 CO H 2 O 6.02 x10 23 CH x10 23 O x10 23 CO x10 23 H 2 O 1 mol CH mol O 2 1 mol CO mol H 2 O 16.0g CH g O g CO gH 2 O 6.02 x10 23 CH x10 23 O x10 23 CO x10 23 H 2 O 1 mol CH mol O 2 1 mol CO mol H 2 O 16.0g CH g O g CO gH 2 O How many moles of O2 would be required to react with 1.72 mol CH4? 1.72 mol CH 4 = mol O mol CH 4 x 2 mol O 2 /1 mol CH 4 = 3.44 mol O 2 How many grams of H 2 O will be produced from 1.09 mol of CH 4? 2-part problem: first find moles of H 2 Othen convert moles to grams Use this equation to obtain conversion factor for moles of CH 4 to moles of H 2 O: 1 mol CH mol O 2 1 mol CO mol H 2 O 1.09 mol CH 4 = mol H 2 O 1.09 mol CH 4 x 2 mol H 2 O/1 mol CH 4 = 2.18 mol H 2 O 2.18 mol H 2 0 = grams H 2 O 2.18 mol H 2 0 x 18.0g H 2 O/1 mol H 2 O = 39.2grams H 2 O Theoretical and Percent Yield Theoretical yield: the maximum amount of product that would be formed from a particular reaction in an ideal world Actual yield: the amount of product formed from a particular reaction in the real world (usually less than the theoretical yield) Percent yield: ratio of actual yield to theoretical yield, times 100 % % yield = actual yield/theoretical yield x 100% Example: theoretical yield = 39.2g water actual yield = 35.5 g water % yield = 35.5g/39.2g x 100% = 90.6 % yield 6

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