2.c – conserving matter. when a car’s gas empties, where do the atoms in the gasoline go?

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 O2 CO2

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) + O2 (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 H2 + O2 2 H2O

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?

CH4 + 2 O2 CO2 + 2 H2O

Total:1 C4 H4 O

Total:1 C4 H4 O

The equation is balanced.

H

C

H

HH

O O

O OCO O H

OH

HO

H

H

CC

H

HH

O O

O OCCO O H

OH

HO

H

Taken from Kelly Deters

Make a table of elements

_____ __________

How to Balance By Inspection:

1

Reactants Products

H

O

C

CH4 + O2 H2 O CO2+_____


2 Count the number of each element or ion on the reactants and products side.

_____ __________


Reactants Products

H

O

4

2 3

2

C 1 1

CH4 + O2 H2 O CO2+_____

Don’t forget to add all the atoms of the same element together—even if it appears in more than one compound!


3Each time you add a coefficient, update your table with the new quantities of each atom.

Add coefficients to balance the numbers

_____ __________2


Reactants Products

H

O

4

2 3

2

C 1 1

2

4

44

CH4 + O2 _____ H2 O CO2+


4Filling 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


Reactants Products

H

O

4

2 3

2

C 1 1

2

4

44

1 1CH4 + O2 _____ H2 O CO2+


Choosing the Order of Balancing

Save for laterElements that are uncombined

Save for laterElements that appear more than 1 time per side

StartElements that appear

only 1 time per side

StartElements in most

complicated molecules

How do you know what order to balance in?

Pb + PbO2 + H+ Pb2+ + H2O

To balance this equation, use the order: O, H, Pb


_____

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 Pb+ O2 H2 O Pb2++H+_____+

O, H, Pb


Let’s Practice #1

Example:Balance the

following equation

__ HCl + __ Ca(OH)2 __ CaCl2 + __ H2O


Let’s Practice #2

Example:Balance the

following equation

__ H2 + __ O2 __ H2O


Let’s Practice #3

Example:Balance the

following equation

__ Fe + __ O2 ___ Fe2O3

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

C.5 The Mole Concept

Definition:

Mole – SI unit for counting


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


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.


A Mole of ParticlesA Mole of Particles Contains 6.02 x 1023 particles

1 mole C = 6.02 x 1023 C atoms

1 mole H2O = 6.02 x 1023 H2O molecules

1 mole NaCl= 6.02 x 1023 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


What does a “mole” count in?

A mole = 6.02 1023 (called Avogadro’s number)

“mole” 6.02 1023

1 mole of doughnuts 6.02 1023 doughnuts

1 mole of atoms

1 mole of molecules

6.02 1023 atoms

6.02 1023 molecules

6.02 1023 = 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


Definition

Molar Mass – The mass for one mole of an atom or molecule.

Other terms commonly used for the same meaning:Molecular WeightMolecular MassFormula WeightFormula Mass


Mass for 1 mole of atomsThe average atomic mass = grams for 1 mole

Element Mass

1 mole of carbon atoms 12.01 g

1 mole of oxygen atoms

1 mole of hydrogen atoms

16.00 g

1.01 g

Unit for molar mass: g/mole or g/mol

Average atomic mass is found on the periodic table


Molar mass for molecules

The molar mass for a molecule = the sum of the molar masses of all the atoms


Calculating a Molecule’s Mass

Count the number of each type of atom

Find the molar mass of each atom on the periodic tableMultiple the # of atoms molar mass for each atom

Find the sum of all the masses

1

2

3

4

To find the molar mass of a molecule:


Example: Molar Mass

Example:Find the

molar mass for

CaBr2


Example: Molar Mass

Count the number of each type of atom1

Ca

Br

1

2

Example:Find the

molar mass for

CaBr2


Example: Molar MassFind the molar mass of each atom on the periodic table2

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

Example:Find the

molar mass for

CaBr2


Example: Molar MassMultiple the # of atoms molar mass for each atom3

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

Example:Find the

molar mass for

CaBr2

= 40.08 g/mole

= 159.82 g/mole


Example: Molar MassFind the sum of all the masses4

Ca

Br

1

2

40.08 g/mole

79.91 g/mole

= 40.08 g/mole

= 159.82 g/mole+

199.90 g/mole

1 mole of CaBr2 molecules would have a mass of 199.90 g

Example:Find the

molar mass for

CaBr2


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(NO3)2


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


Example: Moles to Grams

Example:How many grams are

in 1.25 moles of water?


Example: Moles to Grams

1.25 mol H2O = _______ g H2Omol H2O

g H2O18.021

22.53

When converting between grams and moles, the molar mass is needed

1 mole H2O molecules = 18.02 g

HO

21

1.01 g/mole16.00 g/mole

= 2.02 g/mole= 16.00 g/mole+

18.02 g/mole

Example:How many grams are

in 1.25 moles of water?


Let’s Practice #3

Example:How many moles are in 25.5 g NaCl?


Let’s Practice #3

25.5 g NaCl = _______ mole NaClg NaCl

mole NaCl158.44

0.44

1 mole NaCl molecules = 58.44 g

NaCl

11


= 22.99 g/mole= 35.45 g/mole+

58.44 g/mole

Example:How many moles are in 25.5 g NaCl?


Example: Grams to Molecules

Example:How many molecules

are in 25.5 g NaCl?


25.5 g NaCl

Example: Grams to Molecules

= _________ molecules NaCl

g NaCl

mol NaCl158.44

2.63 1023

1 mol = 6.021023 molecules

1 moles NaCl molecules = 58.44 g

NaCl

11


= 22.99 g/mole= 35.45 g/mole+

58.44 g/mole

mol NaCl

molecules NaCl6.021023

1

Example:How many molecules

are in 25.5 g NaCl?


Let’s Practice #4

Example:How many grams is a sample of 2.75 × 1024

molecules of SrCl2?

2.c – conserving matter. when a car’s gas empties, where do the atoms in the gasoline go?

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