PERIODIC TABLE OF ELEMENTS 4/23/14 Chapter 7: Chemical Reactions 1
CHAPTER 7: CHEMICAL REACTIONS 7.1 Describing Reactions 7.2 Types of Reactions 7.3 Energy Changes in Reactions 7.4 Reaction Rates 7.5 Equilibrium 4/23/14 Chapter 7: Chemical Reactions 2
7.1 DESCRIBING CHEMICAL REACTIONS n n Vocabulary: reactants, products, chemical equation, coefficients, mole, molar mass. Objectives: n Interpret chemical equations in terms of reactants, products, and conservation of mass. n Balance chemical equations by manipulating coefficients. n Convert between moles and mass of a substance using molar mass. n Calculate amounts of reactants or products by using molar mass, mole ratios, and balanced chemical equations. 4/23/14 Chapter 7: Chemical Reactions 3
DESCRIBING CHEMICAL REACTIONS Charcoal Burning Physical or Chemical Change? Why? 4/23/14 Chapter 7: Chemical Reactions 4
DESCRIBING CHEMICAL REACTIONS Charcoal Burning Physical or Chemical Change? Why? Chemical Change C reacts with O à carbon dioxide & heat 4/23/14 Chapter 7: Chemical Reactions 5
CHEMICAL EQUATIONS What substance is present before & after chemical reaction? Before: carbon (C) & oxygen (O 2 ) After: carbon dioxide gas 4/23/14 Chapter 7: Chemical Reactions 6
CHEMICAL EQUATIONS n reactants - substances that undergo a change in a chemical reaction n products - new substances formed as a result of a chemical reaction 4/23/14 Chapter 7: Chemical Reactions 7
USING EQUATIONS TO REPRESENT REACTIONS Reactants Products Burning of charcoal: What are the Reactants? Products? 4/23/14 Chapter 7: Chemical Reactions 8
USING EQUATIONS TO REPRESENT REACTIONS Reactants Products Burning of charcoal: What are the Reactants? Carbon + Oxygen Products? Carbon dioxide 4/23/14 Chapter 7: Chemical Reactions 9
USING EQUATIONS TO REPRESENT REACTIONS Reactants Products Burning of charcoal: What are the Reactants? Carbon + Oxygen Products? Carbon dioxide Simply by writing chemical formulas C + O 2 CO 2 4/23/14 Chapter 7: Chemical Reactions 10
USING EQUATIONS TO REPRESENT REACTIONS C + O 2 CO 2 carbon and oxygen react & form carbon dioxide Or reaction of carbon & oxygen yields carbon dioxide chemical equation - representation of a chemical reaction in which reactants and products are expressed as a formula 4/23/14 Chapter 7: Chemical Reactions 11
CONSERVATION OF MASS n mass of reactants = mass of products n i.e. mass of C & O 2 that reacted = mass of CO 2 produced n Law of conservation of mass states mass is neither created nor destroyed in a chemical reaction. Antoine Lavoisier 1743-1794 4/23/14 Chapter 7: Chemical Reactions 12
CONSERVATION OF MASS 1743-1794 1 atom of carbon reacts with 1 molecule of oxygen and form 1 molecule of carbon dioxide. 4/23/14 Chapter 7: Chemical Reactions 13
CONSERVATION OF MASS 6 6 6 1743-1794 # of atoms on left side = # of atoms on right mass is conserved 4/23/14 Chapter 7: Chemical Reactions 14
BALANCING EQUATIONS n Hydrazine, N 2 H 4 (compound found in rocket fuel) n Burned in presence of oxygen reaction produces nitrogen, water vapor, and heat. N 2 H 4 + O 2 à N 2 + H 2 O n Is this balanced (does # of atoms on left side = # on right side)? 4/23/14 Chapter 7: Chemical Reactions 15
BALANCING EQUATIONS n In order to show that mass is conserved during a reaction, a chemical equation must be balanced. n How can you balance a chemical equation? 4/23/14 Chapter 7: Chemical Reactions 16
BALANCING EQUATIONS n In order to show that mass is conserved during a reaction, a chemical equation must be balanced. n How can you balance a chemical equation? n Change coefficients n changes amount of reactant or product represented in chemical equation n Never change subscript!!! n Coefficient number that appears before a formula 4/23/14 Chapter 7: Chemical Reactions 17
BALANCING EQUATIONS N 2 H 4 + O 2 à N 2 + H 2 O n Step 1: Count # of atoms of each element on each side of equation. 4/23/14 Chapter 7: Chemical Reactions 18
BALANCING EQUATIONS 4/23/14 Chapter 7: Chemical Reactions 19
BALANCING EQUATIONS N 2 H 4 + O 2 à N 2 + H 2 O n Step 2: Change one or more coefficients until the equation is balanced. 4/23/14 Chapter 7: Chemical Reactions 20
BALANCING EQUATIONS N 2 H 4 + O 2 à N 2 + H 2 O n Step 2: Change one or more coefficients until the equation is balanced. N 2 H 4 + O 2 à N 2 + 2H 2 O Equation is now balanced! Tutorial on how to balance out equations: http://www.wfu.edu/~ylwong/balanceeq/balanceq.html 4/23/14 Chapter 7: Chemical Reactions 21
BALANCING EQUATIONS 4/23/14 Chapter 7: Chemical Reactions 22
BALANCING EQUATIONS 4/23/14 Chapter 7: Chemical Reactions 23
COUNTING WITH MOLES n units of atoms & molecules too small to be practical n chemical reactions involve large #s of small particles à use counting unit called the mole to measure amounts of a substance 4/23/14 Chapter 7: Chemical Reactions 24
COUNTING WITH MOLES n mole (mol) - amount of a substance that contains approximately 6.02 x 10 23 particles of that substance n i.e. mole of iron is 6.02x10 23 atoms of iron n 6.02 x 10 23 is known as Avogadro s number 4/23/14 Chapter 7: Chemical Reactions 25
MOLAR MASS n mole of 1 substance may have different mass than mole of different substance i.e. C molar mass = 12.0 g S molar mass = 32.1 g n molar mass - mass of one mole of a substance n molar mass = atomic mass of element (in grams) n i.e. carbon (C) atomic mass = 12.011 amu molar mass = 12.011 grams 6 C Carbon 12.011 4/23/14 Chapter 7: Chemical Reactions 26
MOLAR MASS n How do you calculate molar mass for a compound? 4/23/14 Chapter 7: Chemical Reactions 27
MOLAR MASS n How do you calculate molar mass for a compound? Answer: Add up atomic masses of component atoms & express in grams 4/23/14 Chapter 7: Chemical Reactions 28
MOLAR MASS n What is the molar mass of a carbon dioxide molecule? 4/23/14 Chapter 7: Chemical Reactions 29
MOLAR MASS n What is the molar mass of a carbon dioxide molecule? n C: 12.0 amu n O: 2 x 16.0 amu = 32 amu n 12.0 amu + 32.0 amu = 44.0 amu = 44.0g 4/23/14 Chapter 7: Chemical Reactions 30
MOLAR-MASS CONVERSIONS n Once you know molar mass, can covert moles into mass (or vice versa). Express as conversion factor. n Example: CO 2 Molar Mass = 44.0 grams 44.0 g CO 2 or 1 mol CO 2 1 mol CO 2 44.g CO 2 4/23/14 Chapter 7: Chemical Reactions 31
MOLAR-MASS CONVERSIONS n 55.0 grams of CO 2. To Calculate how many moles of CO 2 you have, multiply mass by conversion factor. 55.0 g CO 1 mol CO 2 x 2 = 1.25 mol CO 2 44.g CO 2 4/23/14 Chapter 7: Chemical Reactions 32
CHEMICAL CALCULATIONS n Chemical equation tells you how to relate amounts of reactants to amounts of products. n Molar masses let you convert amounts of reactants and amounts of product into masses. 4/23/14 Chapter 7: Chemical Reactions 33
CHEMICAL CALCULATIONS n Balanced equation for formation of water: 2H 2 + O 2 à 2H 2 O 2 molecules of hydrogen react with 1 molecule of oxygen and form 2 molecules of water 4/23/14 Chapter 7: Chemical Reactions 34
CHEMICAL CALCULATIONS: CONVERTING MASS TO MOLES n In terms of moles equation reads: 2 moles of hydrogen react with 1 mole of oxygen and form 2 moles of water 4/23/14 Chapter 7: Chemical Reactions 35
CHEMICAL CALCULATIONS: CONVERTING MASS TO MOLES n molar mass = atomic mass 4/23/14 Chapter 7: Chemical Reactions 36
CHEMICAL CALCULATIONS: CONVERTING MASS TO MOLES n Molar masses let you convert amounts of reactants & amounts of product into masses n To convert from moles to mass, use molar masses as conversion factors 4/23/14 Chapter 7: Chemical Reactions 37
CHEMICAL CALCULATIONS: CONVERTING MASS TO MOLES (2 mol x 2 g/mol) + (1 mol x 32.0 g/mol) à (2 x 18.0 g/mol) 4.0 grams of H 2 reacts with 32.0 grams of O 2 and forms 36.0 grams of H 2 O. 4/23/14 Chapter 7: Chemical Reactions 38