Transcript Chapter 3
Stoichiometry: Calculations with Chemical Formulas and Equations Chemical Equations Chemical reactions are represented in a concise method by a chemical equation. Ex) 2 H2(g) + O2(g) 2 H2O(l) Reactants Products Chemical Equations Ex) 2 H2(g) + O2(g) Phase Symbols 2 H2O(l) Coefficient Balancing an Equation A subscript in a chemical formula tells us how many of each type of atom are in the compound. Ex) C6H12O6 Subscripts cannot be altered!!! Atoms can be created nor destroyed in a chemical reaction. Thus, we balance a reaction by adding coefficients in front of each substance. Balancing an Equation Balance by inspection. Use a tally sheet. Start with elements that occur once on each side. Combustion – do C, then H, then O. LEP #1. Patterns of Reactivity Five basic types of reactions. 1 . Combination – two substances combine to make one new one. Generic: A + B C Ex) 2 Mg(s) + O2(g) 2 MgO(s) 2. Decomposition – one substance decomposes to several new ones. Generic: A B + C Ex) 2 NaN3(s) 2 Na(s) + 3 N2(g) Patterns of Reactivity 3. Single Replacement – one element replaces the other. Generic: A + BC AC + B Ex) 2 AgNO3(aq) + Cu(s) Cu(NO3)2(aq) + 2 Ag(s) 4. Double Replacement (aka “Metathesis”) – trading partners. Generic: AB + CD AD + CD Ex) Hg(NO3)2(aq) + 2 NaI(aq) HgI2(s) + 2 NaNO3(aq) 5. Combustion – a rapid reaction with O2(g) producing a flame. Ex) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l) Quantitative Aspect A chemical formula also has a quantitative aspect. A Formula Weight for an element or compound is found using the periodic table. Formula weights can refer to a single element’s weight or an ionic compound. Molecular weight refers to a molecular compound’s weight. Weights from periodic table should be rounded to the nearest 0.1 amu at the bare minimum! LEP #2 Percent Composition A formula weight can be used to calculate the mass percentage of any element in the formula by: Mass % of A = # atoms of A atomic mass of A 100 formula weight of compound This is one place to also test nomenclature! LEP #3 Moles It is not practical to weigh things in amu or think of reactions in terms of atoms or molecules. The Mole is a quantity used in chemistry that is of the size that we can observe. Molar Mass The molar mass of any compound is equal to the sum of the atomic weights expressed in grams. Ex) The molar mass of CO2 is 44.0 grams. Thus, one mole of CO2 = 44.0 grams. One mole of anything will contain Avogadro’s Number of particles. 1 mol C = 6.02 x 1023 atoms 1 mol CO2 = 6.02 x 1023 molecules 1 mol NaCl = 6.02 x 1023 formula units Mole Relationships Mole Relationships These two concepts (molar mass, Avogadro’s number) allow us to convert between mass, moles, and molecules. Examples – LEP #4 Empirical Formulas We can use moles to find an empirical (simplest) formula from mass percentages by: Assume a 100 gram sample (% grams). 2. Convert grams of each element to moles use the formula weights. 3. Divide each mole amount by the smallest one. 4. Using a multiplier to eliminate fractions like: 0.25, 0.33, 0.50, 0.67, and 0.75. 1. Empirical Formulas LEP #5 An empirical formula may not be the actual formula since molecular formulas do not have to be the lowest whole number subscripts. The multiplier, n, can be found if we know the overall molecular weight of the compound. LEP #6 molecular weight n= empirical weight Interpreting a Reaction A simple reaction like: N2(g) + 3 H2(g) 2 NH3(g), can be interpreted on many levels. Molecular Level: one molecule of N2 plus three molecules of H2 react to form two molecules of NH3 Interpreting a Reaction For this reaction, we can establish that: 1 molecule N2 = 3 molecules H2 1 molecule N2 = 2 molecules NH3 3 molecules H2 = 2 molecules NH3 LEP #7 Interpreting a Reaction The molecular level is really not practical as we cannot do reaction on this scale. Rather, we can do them on a mole scale. Thus: one mole of N2 plus three moles of H2 react to produce two moles of NH3. This means our relations can be shortened to moles. LEP #7 Limiting Reactant If given amounts of both reactants, we may run out of one of them first. This reactant limits how much can be made. Analogy: Putting together a bicycle – parts on hand are 200 frames and 350 wheels. How many bicycles can you make? Ex) 2 H2 + O2 2 H2O Suppose a vessel contained 10 molecules of H2 and 7 molecules of O2. How many water molecules are possible? Limiting Reactant This also applies to mole amounts as well. LEP #8 Stoichiometry Pronounced: stoy-key-OM-uh-tree. Relating quantities in chemical reactions – in particular – masses. Cannot use mole-to-mole ratios to convert mass of one substance to mass of another by one single step. A mass-to-mass conversion must be done in three steps. Stoichiometry Stoichiometry Can be used to find a mass of another reactant or a product. Can be part of a limiting reactant where amounts of both reactants are given. Can also be asked to find a percent yield. Percent Yield = Actual Mass Obtained 100 Theoretical Mass Where the Theoretical Mass is the maximum amount possible based on your limiting reactant. LEP #9 and #10