2.C – Conserving Matter

When a car’s gas empties, where do the atoms in the gasoline go?

Keeping Track of Atoms Law of Conservation of Matter: Matter is neither created nor destroyed. Since chemical reactions cannot create or destroy atoms, chemical equations representing the reactions must always be BALANCED.

Atomic Perspective: C O 2  CO 2 1 Carbon atom 1 oxygen molecule  1 carbon dioxide molecule 1.What are the reactants in this chemical equation? 2.What are the products in this chemical equation? 3.Are there the same number of atoms on both sides of the equation? a.Where any atoms destroyed or created? b.Was the Law of Conservation of Matter maintained?

2 Cu (s) + O 2 (g)  2 CuO (s) COEFFICIENTS - indicates the number of units of each substance involved. 1.Does the oxygen molecule have a coefficient? 2.What do the subscripts represent? 3.Can subscripts be removed from chemical equations?

How do we Balance Equations? 2 H 2 + O 2  2 H 2 O Subscripts Coefficients # of atoms in a compound Number of compounds in the reaction Subscripts balance charges within a compound. Coefficients balance atoms in an equation

What do Coefficients Really Mean? CH O 2  CO H 2 O Total: 1 C 4 H 4 O Total: 1 C 4 H 4 O The equation is balanced.

Taken from Kelly Deters Make a table of elements _____ How to Balance By Inspection: 1 Reactants Products H O C CH4H4 + O2O2  H2H2 OCO2O2 +_____

Taken from Kelly Deters 2 Count the number of each element or ion on the reactants and products side. _____ How to Balance By Inspection: Reactants Products H O C11 CH4H4 + O2O2  H2H2 OCO2O2 +_____ Don’t forget to add all the atoms of the same element together— even if it appears in more than one compound!

Taken from Kelly Deters 3 Each time you add a coefficient, update your table with the new quantities of each atom. Add coefficients to balance the numbers _____ 2 How to Balance By Inspection: Reactants Products H O C CH4H4 + O2O2  _____H2H2 OCO2O2 +

Taken from Kelly Deters 4 Filling each coefficient location lets you and the grader know that you finished the problem rather than you left some blank because you weren’t done! Place a “1” in any empty coefficient location _____ 2 How to Balance By Inspection: Reactants Products H O C CH4H4 + O2O2  _____H2H2 OCO2O2 +

Taken from Kelly Deters Choosing the Order of Balancing Save for later Elements that are uncombined Save for later Elements that appear more than 1 time per side Start Elements that appear only 1 time per side Start Elements in most complicated molecules How do you know what order to balance in? Pb + PbO 2 + H +  Pb 2+ + H 2 O To balance this equation, use the order: O, H, Pb

Taken from Kelly Deters _____ How is Balancing Affected by Order? What happens if we balance in the order determined in the last slide? Reactants Products H Pb O _____ Pb +O2O2  H2H2 OPb 2+ + H+H+ _____ + O, H, Pb

Taken from Kelly Deters Let’s Practice #1 Example: Balance the following equation __ HCl + __ Ca(OH) 2  __ CaCl 2 + __ H 2 O

Taken from Kelly Deters Let’s Practice #2 Example: Balance the following equation __ H 2 + __ O 2  __ H 2 O

Taken from Kelly Deters Let’s Practice #3 Example: Balance the following equation __ Fe + __ O 2  ___ Fe 2 O 3

Adopted from "Chemistry You Need to Know" by Kelly Deters C.5 The Mole Concept Definition: Mole – SI unit for counting

Adopted from "Chemistry You Need to Know" by Kelly Deters What is a counting unit? You’re already familiar with one counting unit…a “dozen” “Dozen”12 A dozen doughnuts 12 doughnuts A dozen books A dozen cars A dozen people 12 books 12 cars 12 people A dozen = 12

Adopted from "Chemistry You Need to Know" by Kelly Deters How big is a mole? Enough soft drink cans to cover the surface of the earth to a depth of over 200 miles. If you had Avogadro's number of unpopped popcorn kernels, and spread them across the United States of America, the country would be covered in popcorn to a depth of over 9 miles. If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

Adopted from "Chemistry You Need to Know" by Kelly Deters A Mole of Particles A Mole of Particles Contains 6.02 x particles 1 mole C = 6.02 x C atoms 1 mole H 2 O = 6.02 x H 2 O molecules 1 mole NaCl= 6.02 x NaCl “molecules” (technically, ionics are compounds not molecules so they are called formula units) 6.02 x 1023 Na+ ions and 6.02 x 1023 Cl– ions

Adopted from "Chemistry You Need to Know" by Kelly Deters What does a “mole” count in? A mole = 6.02  (called Avogadro’s number) “mole” 6.02  mole of doughnuts 6.02  doughnuts 1 mole of atoms 1 mole of molecules 6.02  atoms 6.02  molecules 6.02  = 602,000,000,000,000,000,000,000 This means a 12 ounce bottle of water would have 19.7 “moles” of water…a much easier-to-work-with number!

Adopted from "Chemistry You Need to Know" by Kelly Deters Molar Mass

Adopted from "Chemistry You Need to Know" by Kelly Deters Definition Molar Mass – The mass for one mole of an atom or molecule. Other terms commonly used for the same meaning: Molecular Weight Molecular Mass Formula Weight Formula Mass

Adopted from "Chemistry You Need to Know" by Kelly Deters Mass for 1 mole of atoms The average atomic mass = grams for 1 mole ElementMass 1 mole of carbon atoms g 1 mole of oxygen atoms 1 mole of hydrogen atoms g 1.01 g Unit for molar mass: g/mole or g/mol Average atomic mass is found on the periodic table

Adopted from "Chemistry You Need to Know" by Kelly Deters Molar mass for molecules The molar mass for a molecule = the sum of the molar masses of all the atoms

Adopted from "Chemistry You Need to Know" by Kelly Deters Calculating a Molecule’s Mass Count the number of each type of atom Find the molar mass of each atom on the periodic table Multiple the # of atoms  molar mass for each atom Find the sum of all the masses To find the molar mass of a molecule:

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass Example: Find the molar mass for CaBr 2

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass Count the number of each type of atom 1 Ca Br 1 2 Example: Find the molar mass for CaBr 2

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass Find the molar mass of each atom on the periodic table 2 Ca Br g/mole g/mole Example: Find the molar mass for CaBr 2

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass Multiple the # of atoms  molar mass for each atom 3 Ca Br g/mole g/mole   Example: Find the molar mass for CaBr 2 = g/mole = g/mole

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass Find the sum of all the masses 4 Ca Br g/mole g/mole = g/mole = g/mole g/mole 1 mole of CaBr 2 molecules would have a mass of g Example: Find the molar mass for CaBr 2  

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Molar Mass & Parenthesis Be sure to distribute the subscript outside the parenthesis to each element inside the parenthesis. Example: Find the molar mass for Sr(NO 3 ) 2

Adopted from "Chemistry You Need to Know" by Kelly Deters Let’s Practice #2 Example: Find the molar mass for Al(OH) 3

Adopted from "Chemistry You Need to Know" by Kelly Deters Using Molar Mass in Conversions

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Moles to Grams Example: How many grams are in 1.25 moles of water?

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Moles to Grams 1.25 mol H 2 O = _______ g H 2 O mol H 2 O g H 2 O When converting between grams and moles, the molar mass is needed 1 mole H 2 O molecules = g H O g/mole g/mole  = 2.02 g/mole = g/mole g/mole  Example: How many grams are in 1.25 moles of water?

Adopted from "Chemistry You Need to Know" by Kelly Deters Let’s Practice #3 Example: How many moles are in 25.5 g NaCl?

Adopted from "Chemistry You Need to Know" by Kelly Deters Let’s Practice # g NaCl = _______ mole NaCl g NaCl mole NaCl mole NaCl molecules = g Na Cl g/mole g/mole  = g/mole = g/mole g/mole  Example: How many moles are in 25.5 g NaCl?

Adopted from "Chemistry You Need to Know" by Kelly Deters Example: Grams to Molecules Example: How many molecules are in 25.5 g NaCl?

Adopted from "Chemistry You Need to Know" by Kelly Deters 25.5 g NaCl Example: Grams to Molecules = _________ molecules NaCl g NaCl mol NaCl  mol = 6.02  molecules 1 moles NaCl molecules = g Na Cl g/mole g/mole  = g/mole = g/mole g/mole  mol NaCl molecules NaCl 6.02  Example: How many molecules are in 25.5 g NaCl?

Adopted from "Chemistry You Need to Know" by Kelly Deters Let’s Practice #4 Example: How many grams is a sample of 2.75 × molecules of SrCl 2 ?