PowerPoint to accompany Chapter 2 Part 1 Stoichiometry: Calculations with Chemical Formulae and Equations

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Law of Conservation of Mass Figure 2.1 “We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal amount of matter exists both before and after the experiment. Upon this principle, the whole art of performing chemical experiments depends.” --Antoine Lavoisier, 1789

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Chemical Equations Concise representations of chemical reactions. They mainly give proportions phases They do not imply mechanism

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Figure 2.4 *

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Reactants appear on the left side of the equation. Note that the equation contains molecules or elements and the phases they exist in during the reaction. e.g. Methane (gas)

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Products appear on the right side of the equation. The Arrow indicates which direction the reaction tends to go. If it points both ways it signifiies equilibrium.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Anatomy of a Chemical Equation The states of the reactants and products are written in parentheses to the right of each compound. In this example all are gases. * CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g)

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Anatomy of a Chemical Equation CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) Coefficients are inserted to balance the equation. Balanced Equations have the same elements and numbers of atoms of each element on both sides of the equation. Numbers of Molecules are proportionate to a complete reaction, not absolute.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Subscripts and Coefficients Give Different Information  Subscripts indicate the number of atoms of each element in a molecule  Coefficients indicate the number of molecules Figure 2.2

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Reaction Types: * Combination: A + B  C Decomposition: A  B + C, often with a gas Combustion: Fuel + Oxidant  Ash, Gases

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

 Examples N 2 (g) + 3 H 2 (g) 2 NH 3 (g) C 3 H 6 (g) + Br 2 (l) C 3 H 6 Br 2 (l) 2 Mg (s) + O 2 (g) 2 MgO (s) Combination Reactions  Two or more substances react to form one product:  A+B  C Figure 2.5 *

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia  Examples N 2 (g) + 3 H 2 (g) 2 NH 3 (g) C 3 H 6 (g) + Br 2 (l) C 3 H 6 Br 2 (l) 2 Mg (s) + O 2 (g) 2 MgO (s) Combination Reactions  Nitrogen gas reacts with hydrogen gas to form ammonia gas (Nitrogen fixation).  Cyclopropane gas reacts with liquid bromine to form 1,5 dibromopropane  Magnesium metal reacts with oxygen gas to form solid magnesium oxide. Figure 2.5

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia 2 Mg (s) + O 2 (g) 2 MgO (s) Combination Reactions Figure 2.5 * g Mg g O 2 = g MgO What happens if you only have 16 g of O 2 ?

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Decomposition Reactions One substance breaks down into two or more substances  Examples  CaCO3 (s) CaO (s) + CO2 (g)  2 KClO3 (s) 2 KCl (s) + 3 O2 (g)  2 NaN3 (s) 2 Na (s) + 3 N2 (g) 2 NaN 3 (s) 2 Na (s) + 3 N 2 (g) Figure 2.6 *

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Decomposition Reactions One substance breaks down into two or more substances  Sodium azide solid reacts to form sodium metal and nitrogen gas.  (So what does it mean if somebody text’s you: “ U NaN 3 “ ?) 2 NaN 3 (s) 2 Na (s) + 3 N 2 (g) Figure 2.6

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia You Airbag!

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Decomposition Reactions One substance breaks down into two or more substances  Calcium carbonate (limestone) decomposes to form solid unslaked lime and carbon dioxide gas.  This is the 1 st step to make portland cement. CaCO 3 (s) CaO (s) + CO 2 (g)

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Decomposition Reactions One substance breaks down into two or more substances 2 KClO 3 (s) 2 KCl (s) + 3 O 2 (g) POP QUIZ! 2 moles of solid _______decomposes to form 2 moles of solid _ _ and 3 moles of gas.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Decomposition Reactions One substance breaks down into two or more substances 2 KClO 3 (s) 2 KCl (s) + 3 O 2 (g) POP QUIZ! 2 moles of solid Potassium Chlorate decomposes to form 2 moles of solid Potassium Chloride and 3 moles of Oxygen gas.

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Combustion Reactions  Rapid reactions that produce a flame  Most often involve hydrocarbons reacting with oxygen in the air  Examples CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) Figure 2.7

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Combustion Reactions Most often involve hydrocarbons reacting with oxygen in the air Which hydrocarbon fuel generates the least amount of greenhouse gas?  Examples CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (g) C 3 H 8 (g) + 5 O 2 (g) 3 CO 2 (g) + 4 H 2 O (g) Figure 2.7

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Formula Mass

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Formula Mass ( or Weight)  Sum of the atomic weights of the atoms in a chemical formula  The formula mass of calcium chloride, CaCl 2, would be Ca: 1 x 40.1 g = 40.1 g + Cl: 2 x 35.5 g = 71.0 g g According to Dalton’s Law what would the decomposition of 1 mole of Calcium Chloride generate?

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Molecular Mass  Sum of the atomic weights of the atoms in a molecule  For the molecule ethane, C 2 H 6, the molecular weight would be C: 2 x 12.0 g = 24.1 g + H: 6 x 1.0 g = 6.0 g 30.1 g For the complete combustion of 1 mole of ethane, what weight of water would be produced?

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Mass of Water from Ethane For the complete combustion of 1 mole of ethane, what weight of water would be produced? C 2 H 6 (g)  2 CO 2 (g) + 3 H 2 O (g). 6 g H makes 3 moles of H 2 O, each mole of water is 1 mole of Oxygen for 16 g and 2 moles of Hydrogen for 2 g, so 18 g. But there are 3 moles of water so: 3 moles water X 18 g/mole = 54g water

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Percent Composition One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation: % element = (number of atoms of that element)x(atomic mass of that element) (formula mass of the compound) x 100

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia Percent Composition For example, the percentage of carbon in ethane, C 2 H 6, is: % C = 2 x 12.0 g 30.0 g 24.0 g 30.0 g = x 100 = 80.0 % If there are 3X as many Hydrogens, why is the percentage of carbon so high?