CHEM UNIT 9: Chemical Reactions and Stoichiometry

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Vernon Ellis
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1 CHEM UNIT 9: Chemical Reactions and Stoichiometry This is a good website that has videos of Mr. Cummings (our Chem. Dept. Head) lecturing on topics. For this unit, we recommend videos #34, #35, #36, and #37. Another useful video on Limiting Reagents and %Yield is at: Review of Key Old Material and Skills: Make Sure You Know All the Words in Bold!! Compounds: Chemicals that are composed of two (or more) different types of Elements. Ionic Compounds: Compounds composed of at least one Cation (+) and one Anion (-), which are bonded by Ionic Bonds. Almost all of these contain metal ions (some exceptions, e.g. NH4Cl) Molecules: Chemicals whose atoms are attached by Covalent Bonds. Molecular Elements: Only have one type of element (e.g. H2, N2, Cl2, S8) Molecular Compounds: Have two or more different kinds of elements. (e.g. H2O, C6H12O6) Atomic Mass: The Mass of one atom of an element in amu (Atomic Mass Units). Atomic Masses are found on the Periodic Table (P.T.) Molar Mass: Molar Mass (g/mol.) is the mass of one mole of something. Its units are grams per mole (g/mol.). Molar Mass (g/mol.) for single Atoms is just the P.T. Atomic Mass, but with the units changed to g/mol. Molar Mass (g/mol.) for Compounds or Molecules is just the sum of all of the atoms in the Chemical Formula. Scientific Notation: An easy way to express very big or very small numbers (or anything in between). Percent: Calculating Percentages (%) Four Key Math Tools for using Moles in Chemistry: A. (#Moles) x (A.N.) = #Particles A.N. = Avogadro s Number = x B. (#Particles) / (A.N) = #Moles C. (#Moles) x (Molar Mass) = #Grams D. (#Grams / Molar Mass) = #Moles 1

2 CHEMICAL REACTIONS Chemical Reactions: When chemicals change into other chemicals. Reactants: In a Chemical Reaction, the chemicals that are going to be changed. Products: In a Chemical Reaction, the new chemicals that are left after a reaction is finished. CHEMICAL EQUATIONS: Chemical Equations are a way to describe and show chemical reactions. They are not math equations, but are like them in that there is a left and a right side, connected by a symbol. They may also contain numbers like math equations do, but these have special meanings. Here s one for reacting Glucose with Oxygen: C6H12O6 + O2 H2O + CO2 In Chemical Equations, Reactants always go on the left side, and Products go on the right side. We use plus signs (+) between reactants to indicate they are reacting with each other. We use an arrow ( ) to point to the products on the right side. The (+) between the products is just used to separate them, the products are not reacting with each other! Be careful, symbols may have more than one meaning! For example, what do the two (+) signs in this equation mean? Ag +1 + Cl -1 AgCl Do they mean the same thing? NO! In Ag +1, the (+) in the superscript means a positive charge. But, the (+) between the two reactants means they are reacting with each other. DIATOMIC ELEMENTS: Recall Diatomic Molecules are molecules made of just two atoms. (e.g. HCl, CO, HF, H 2, O 2 ) Diatomic Elements are elements that occur naturally as diatomic molecules. There are seven Diatomic Elements: H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2. Note that F 2, Cl 2, Br 2, and I 2 are all halogens, are diatomic. (So all halogens are diatomic!) BALANCED CHEMICAL EQUATIONS: (The Law of Conservation of Matter / Mass) Balanced Chemical Equations have the same number of atoms of each type of element on both the reactant side and the product side. Not Balanced: N2 + 3H2 2NH3 Balanced: N2 + H2 NH3 BALANCING CHEMICAL EQUATIONS: How do I do it? List each element that is found on the reactant side (left side) of the chemical equation. Duplicate that list beneath the product side (right side) Count the number of atoms of each type of element on the reactant side, and record this. Count the number of atoms of each type of element on the product side, and record this. If the numbers for each element are the same for each side, it s balanced already!! No more work needs to be done! 2

3 BALANCING CHEMICAL EQUATIONS: (continued) If the numbers are not the same, you must balance the equation by using Coefficients. Coefficients are integer numbers that are put in front of chemical formulas to show how many moles of each reactant or product chemical are needed in a balanced chemical equation. Example: Here s an unbalanced equation: C6H12O6 + O2 H2O + CO2 Here s the Balanced version: C6H12O6 + 6O2 6H2O + 6CO2 The Coefficients are 1, 6, 6, and 6. (If a Coefficient is 1, we don t write it down, we assume it s there.) Do not confuse Coefficients with Subscripts!!! Subscripts are used in Formulas for Chemicals to shown how many atoms of an element are in a chemical. Changing subscripts will indicate a new, different chemical. We cannot change the reactant or product chemicals when balancing!! Example: H2O is water, one water has two H and one O. H2O2 is hydrogen peroxide, one molecule has two H and two O, it is not water! However, 2H2O means we have two H2O, or two moles of H2O. 2H2O is not a new chemical! MOLAR RATIOS: Molar Ratios are the integer Coefficients from a Balanced equation, arranged starting with the reactant coefficients, and ending with the product ones. Example: For C6H12O6 + 6O2 6H2O + 6CO2, the Molar Ratios are 1 : 6 : 6 : 6 Again, don t confuse Coefficients with Subscripts!!! Only Coefficients are used for Molar Ratios. STOICHIOMETRY: Stoichiometry is the study of mole mass relationships in chemical reactions. The figure below shows the main relationships between reactant mass, reactant moles, product moles, and product mass. Theoretical Yield Actual Yield %Yield Stoichiometry requires balanced chemical equations, the use of molar ratios, understanding of percentages, the use of the four key math tools for using moles in chemistry, and, most importantly, applied common sense. In doing Stoichiometry, you must be able to calculate Molar Masses, convert from grams to moles (g moles) and from moles to grams (moles g). You must also be able to go from moles reactant to moles product using molar ratios. 3

4 LIMITING REACTANTS: Limiting Reactant: In a chemical reaction, the one reactant whose amount limits (determines) the amount of product that can be made. All Reactants that are not the Limiting Reactant are in excess, and some of these will be left over after the reaction has finished. THEORETICAL YIELD: Theoretical Yield is the calculated amount of product that can be made. (If everything goes perfectly!) You must know what the Limiting Reactant is in order to calculate Theoretical Yield. ACTUAL YIELD: Actual Yield is the actual, measured amount of product that is made. Actual Yield must be measured, not just calculated. Actual Yield can never be larger than the Theoretical Yield! (This would not obey the Law of Conservation of Matter (or Mass). PERCENT YIELD (%YIELD): Percent Yield is a measure of the quality of how well a reaction was performed! Calculation of %Yield: %Yield = 100 x (Actual Yield / Theoretical Yield) %Yield can never be greater than 100%! (This would not obey the Law of Conservation of Matter (or Mass). We cannot create matter from nothing!! THE BIG PICTURE: The figure below is the BIG PICTURE for Stoichiometry. You must understand it! Note the four boxes connected to each other. From any one box, we can go to any other box, PROVIDED we have a balanced equation AND, we know how to calculate Molar Mass and how to use the key math tools for using moles. Note we hardly every use Avogadro s Number!! Never use Avogadro s Number unless you are very sure you need it!! 4

5 Key Vocabulary Words and Skills: (The ones inside the box are review from previous units. Compounds Ionic Compounds Molecules Atomic Mass Molar Mass Scientific Notation Percent Four Key Math tools for using Moles Chemical Reactions Reactants Products Chemical Equations Diatomic Molecules Diatomic Elements: H2, N2, O2, halogens Balanced Chemical Equations Coefficients Molar Ratios Stoichiometry Limiting Reactants / Reagents Theoretical Yield Actual Yield Percent Yield (%Yield) Stoichiometry The Big Picture Limiting Reactants / Reagents Theoretical Yield Molar Ratios 14 Key Things to know for the Test: 1. How to use Scientific Notation using a calculator. 2. What is Molar Mass, how to calculate it, and how to use it in calculations. 3. Chemical Reactions and how to read them. 4. What are the Diatomic Elements? 5. How to Balance Chemical Equations using Coefficients. 6. How to determine and use Molar Ratios. 7. The Big Picture: Converting g reactant to moles reactant. (gr molesr) Converting moles reactant to moles product. (molesr molesp) Converting moles product to g product. (molesp gp) Doing these in reverse! 8. What are Limiting Reactants and how to identify them. 9. How to calculate Theoretical Yield. 10. What is Actual Yield. 11. How to calculate Percent Yield (%Yield). 5

6 NOTES: 6

Name AP CHEM / / Chapter 3 Outline Stoichiometry

Name AP CHEM / / Chapter 3 Outline Stoichiometry Atomic Masses The modern system of atomic masses, instituted in 1961, is based on carbon-12. Carbon-12 is assigned the mass of exactly 12 atomic mass units