Chapter 3 Stoichiometry Calculations with Chemical Formulas and

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 H 2(g) + O 2(g) 2 H 2 O(l) Reactants Products

Chemical Equations �Ex) 2 H 2(g) + O 2(g) Phase Symbols 2 H 2 O(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) C 6 H 12 O 6 �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) + O 2(g) 2 Mg. O(s) 2. Decomposition – one substance decomposes to several new ones. Generic: A B + C Ex) 2 Na. N 3(s) 2 Na(s) + 3 N 2(g)

Patterns of Reactivity 3. Single Replacement – one element replaces the other. Generic: A + BC AC + B Ex) 2 Ag. NO 3(aq) + Cu(s) Cu(NO 3)2(aq) + 2 Ag(s) 4. Double Replacement (aka “Metathesis”) – trading partners. Generic: AB + CD AD + CD Ex) Hg(NO 3)2(aq) + 2 Na. I(aq) Hg. I 2(s) + 2 Na. NO 3(aq) 5. Combustion – a rapid reaction with O 2(g) producing a flame. Ex) CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O(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: �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 CO 2 is 44. 0 grams. �Thus, one mole of CO 2 = 44. 0 grams. �One mole of anything will contain Avogadro’s Number of particles. � 1 mol C = 6. 02 x 1023 atoms � 1 mol CO 2 = 6. 02 x 1023 molecules � 1 mol Na. Cl = 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: 1. 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.

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

Interpreting a Reaction �A simple reaction like: N 2(g) + 3 H 2(g) 2 NH 3(g), can be interpreted on many levels. �Molecular Level: one molecule of N 2 plus three molecules of H 2 react to form two molecules of NH 3

Interpreting a Reaction �For this reaction, we can establish that: 1 molecule N 2 = 3 molecules H 2 1 molecule N 2 = 2 molecules NH 3 3 molecules H 2 = 2 molecules NH 3 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 N 2 plus three moles of H 2 react to produce two moles of NH 3. �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 H 2 + O 2 2 H 2 O �Suppose a vessel contained 10 molecules of H 2 and 7 molecules of O 2. 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. �Where the Theoretical Mass is the maximum amount possible based on your limiting reactant. �LEP #9 and #10