Unit 5 Percent Composition, Empirical Formulas, and Reactions

Transcription

1 Chemistry 1 West Linn High School Unit 5 Packet and Goals Name: Period: Unit 5 Percent Composition, Empirical Formulas, and Reactions Unit Goals: As you work through this unit, you should be able to: 1. Calculate the percent composition of a substance from its chemical formula or experimental data. (10.3) 2. Identify empirical and molecular formulas. (10.3) 3. Derive the empirical formula and molecular formula of a compound from experimental data. (10.3) 4. Calculate the empirical formula of a compound using experimental reaction data. (10.3) 5. Write formula equations from word equations using appropriate symbols. (11.1) 6. Balance chemical reactions. (11.1) 7. Classify the equations into the five basic reaction types. (11.2) Reading: Chapter 10: (pp ) Section 10.3 Chapter 11: (pp ) Sections Key Terms: percent composition, molecular formula, empirical formula, word equation, reactant, product, chemical change, physical change, formula equation, product, precipitate, symbols, catalyst, coefficient, conservation of atoms, conservation of mass, combination, decomposition, single-replacement, double-replacement, combustion Homework: HW 1 HW 2 Description Percent Composition and Empirical Formula Writing and Balancing Chemical Equations Classwork and Labs Goals 1-3 Unit 5 Worksheet Packet % Composition Activity 5-7 Reaction Types Activity Unit 4/5 Exam/Semester Final Exam Empirical Formula Formal Lab (To start 2 nd Semester)

2 Chapter Percent Composition and Chemical Formulas Percent Composition of a Compound: (page 325) Math Warm up: 1. What is 73% of 150? 2. What percentage of 6.5 is 3.1? Percent Composition: 1. Calculating % from Experimental Data Based on MASS (not moles or # of particles) For a compound it depends on the Formula % = 100% When a g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound? 2. Calculating % from Formulas and Molar Mass % = 100% What is the percent composition of the elements in Propane (C3H8)? 3. Percent composition as a conversion factor How many grams of Carbon and Hydrogen are in 82.0 g of C3H8? Hydrated Crystals CoCl2 6H2O Hydrated Crystal CoCl2 Anhydrous Crystal Example Problem: What is the percent of water in Cobalt (II) chloride hexahydrate?

3 Empirical Formulas: page 330 Lowest whole number ratio of elements in a compound Formula Units ARE Empirical Formulas Examples NaCl (NH4)2SO4 Molecular Formulas are not necessarily Empirical Formulas Examples C2H6 H2O2 H2O Identify each compound as Molecular or Ionic. Which ones are also Empirical Formulas? Mg3(PO4)2 C6H12O6 CO CaCl2 C6H6 H2O (NH4)2CO3 Calculating Empirical Formulas 1. Find mass of each element in a compound 2. Convert masses to moles 3. Using the element with the smallest number of moles Calculate the whole number mole ratio of each element (Divide each mole value by the smallest) Example What is the empirical formula for a compound which contains 41.1% iron, 23.6% sulfur and 35.3% oxygen? First assume each % is a mass in g s (corresponding to a total substance mass of g). Find the atomic mass of each element from the periodic table and use it to convert the mass in grams to moles. Next, convert the mole values to lowest whole number mole ratios. Do this by dividing each mole value by the smallest mole value of the group. These whole numbers tell the empirical formula ratios.

4 Calculating Molecular formula Divide the molar mass of the molecular formula by the molar mass of the empirical formula. The answer represents the factor the empirical formula needs to multiplied by. Molecular Formula Example: 15. Determine the molecular formula for each compound: (Hint: you will need to calculate the empirical formula first) a. 94.1% O and 5.9% H: Molar mass = 34 g/mol Describing Chemical Reactions Introduction to Chemical Equations: (page 346) ALL chemical reactions follow 2 laws! Law of Conservation of mass: Chapter 11 Law of Conservation of Atoms: Chemical Reactions are about the arrangement of Atoms, Not the changing of Atoms Reaction Vocab: Reactants: Starting Atoms, Molecules, or Compounds Products: Ending Atoms, Molecules, or Compounds Yields: Progress of reaction Gives Catalyst: Substance that speeds up chemical reactions Reactants Catalyst Products Additional Information States of Matter Shown as (s), (l), (g), (aq) after formulas Catalyst Shown above Yield arrow

5 Example: Aqueous hydrogen peroxide will decompose into gaseous forms of oxygen and water in the presence of manganese dioxide as a catalyst Balancing Chemical Equations: Page 349 Following the law of conservation of ATOMS Example: Hydrogen gas combusts with Oxygen gas to form water. 1. Get Formulas correct 2. Organize REACTANTS vs. PRODUCTS 3. BALANCE Balancing Reaction HINTS Look for Polyatomic Ions! Save elements O2 and H2 for the end. If you assume the coefficient of a large particle is 1 and come to a dead end, change it to a 2. Mg3(PO4)2 + NaCl MgCl2 + Na3PO4 C2H6 + O2 CO2 + H2O

6 11.2. Types of Chemical Reactions Classifying Reactions: Page Combination Reaction: 2 or more substances as Reactants combine to form 1 Product Also Called Synthesis 2. Decomposition Reaction: 1 Reactant breaks down into 2 or more Reactants Most require energy to take place (take in Energy) 3. Single-Replacement Reaction: 1 element replaces another element in a compound Reactivity of Metals and Non-Metals is important: Use activity series (Page 361). 4. Double-Replacement Reaction: An Exchange of positive ions between 2 Ionic Compounds Both Reactants are Ionic Compounds Often takes place in solution with water. Sometimes a Product is a solid that will not dissolve in water and forms a PRECIPITATE 5. Combustion Reactions A Reactant combines with Oxygen (other Reactant) Usually produce energy (give off energy) Usually involves a Molecular Compounds with (C,H, & or O) and Oxygen Often have Water and Carbon dioxide as Products o Burning Hydrocarbons o Wood, Gas, Fuel, Sugars (Remember Cellular Respiration?) May also be a Combination (synthesis) Reaction if only one Product forms o Burning Elements Predicting Reaction Types Examples 1. A single Reactant: H 2O 2 2. A metal (Na) and an Ionic Compound (MgCl 2) React: 3. C 2H 6 reacts with O 2: 4. Two Ionic compounds react [KCl & Mg(NO 3) 2] 5. A single product: H 2O