Moles, Formulas, Reactions & Stoichiometry Lecture 3 Topics Brown, chapter 3 1. Moles 3.4 Molecular mass (aka molecular weight) 3.3 2. Molar converstions Percent composition 3.3 Empirical formulas 3.5 3. Stoichiometry: balancing chemical equations 3.1 4. Patterns of Chemical Reactivity 3.2 Combination Decomposition Combustion Exchange 5. Stoichiometry & Conversions 3.6 Limiting reactants 3.7 Theoretical & percent yield

Stoichiometry: chemical equations are molar Chemical equations are balanced with coefficients. Stoichiometric coefficients are molar. The general process of advancing scientific knowledge by making experimental observations and by formulating hypotheses, theories, and laws. It’s a systematic problems solving process AND it’s hands-on….. Experiments must be done, data generated, conclusions made. This method is “iterative”; it requires looping back and starting over if needed. [Why do you think they call it REsearch?] Often years, decades or more of experiments are required to prove a theory. While it’s possible to prove a hypothesis wrong, it’s actually NOT possible to absolutely prove a hypothesis correct as the outcome may have had a cause that the scientist hasn’t considered.

Stoichiometry: ratios in balanced equations In this chapter we begin to discuss one of the meatier topics in intro chemistry, chemical reactions and the equations we write to describe them. What’s the difference between these two equations? (Count atoms on both sides to find out.) Na(s) + H2O(l)  H2(g) + NaOH(aq) + heat  qualitative 2Na(s) + 2H2O(l)  H2(g) + 2NaOH(aq) + heat  BALANCED! & therefore quantitative Qualitative tells you what’s happening, but doesn’t provide enough details to allow you to replicate. Like being told that a cake is made of flour, butter, eggs and baking powder. But you don’t know how much of each ingredient to use. Balanced equations provide quantitative information and give you a recipe that is very clear about amounts of ingredients and about yield of “cake” or product. Only the second equation obeys the Law of Conservation of Mass! Chemical reactions don’t create or destroy atoms; they only rearrange atoms. In the process they change molecules & molecular identities. p.98 - 99

It’s all about the coefficients! Stoichiometric coefficients are the numbers in front of molecular formulas. In the absence of a stoichiometric coefficient, assume the coefficient is 1. p.98 - 99

How to balance a chemical equation Goal: To change stoichiometric coefficients so that both sides of a chemical reaction have the same types & numbers of atoms. Steps: 1. If necessary, predict the products to the right side of the arrow. 2. NEVER change subscripts within molecular formulas ….since that changes molecular identities. 3. Change COEFFICIENTS to balance the number of atoms on left & right. - Coeffiecients are multipliers for molecules (& for subscripts). - In general, start with more complicated molecules, & non-O, non-H atoms. 4. Coefficients can’t be fractions, so multiply through by the denominator. Fe2O3(s) + C(s)  Fe(s) + CO(g) 3 2 3 In this example, balance Fe first as it’s the first non-oxygen, non-hydrogen molecule you see. Next, balance C. Hmmm, that looks balanced with 1 atom on either side of the equation. Finally, balance O, oxygen. Now go back & be sure that the other atoms are STILL balanced. Whoops, C(s) has to be changed to balance CO! p.80 - 81

What’s really happening in reactions? Write & balance an equation for this chemical reaction: oxygen nitrogen Start by writing the molecular formulas of the reactants & products by identifying the atoms involved & describing their ratios using subscripts O2 + NO --> NO2 OK, now ask yourself if the equation is balanced. ………..NOPE! Notice that there are twice as many NO as O2 reactants molecules AND there are the same number of product molecules as NO molecules! USE THIS INFO TO HELP YOU BALANCE! O2 + 2NO --> 2NO2 Now that’s balanced; count atoms on each side. p.84 - 84