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Copyright © Houghton Mifflin Company. All rights reserved. 4–1

5.6 Chemical Equations

A chemical equation uses symbols and for-mulas to describe the changes that take place in a chemical reaction.

2 H2 + O2 2 H2O

Reactants Products

There must be the same number of each type of atom on both sides of the equation.



A skeleton equation has the same type, but not the same number, of each type of atom on each side of the equation. It must be balanced before it is useful in chemical calculations.

Fe2O3 + CO Fe + CO2



More equations to balance:

C3H8 + O2 CO2 + H2O

C4H10 + O2 CO2 + H2O

Al2O3 + HCl AlCl3 + H2O

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5.6 Chemical EquationsWe can read equations as referring to

individual atoms and molecules, but in the real world, we work with very large groups of atoms and molecules.


5.2 The Mole: A Counting Unit for Chemists

We often use words that refer to a specific number of items:

a pair of socks = 2 socks

a dozen eggs = 12 eggs

fourscore years = 4 x 20 years

It would be convenient to work with a specific number of atoms in chemical calculations.


5.2 The Mole

In 12.0 g of C-12, there are 6.022 x 1023 atoms.

6.022 x 1023 objects is one mole of objects

6.022 x 1023 is “Avogadro’s number”

A mole is a huge number, but we can use it like we use dozens and scores.


5.3 Molar Masses

6.022 x 1023 atoms of C-12 have a mass of 12.0 grams.

The mass of a mole of atoms of all other elements is defined relative to the mass of 1 mole of C-12.

Atomic mass of Mg = 24.305 amu

Molar mass of Mg = 24.305 g/mol


5.3 Molar Masses

We can obtain a mole of an element by weigh-ing it. If we need more or less than a mole, we can calculate the desired amount. We can also determine how many atoms are present.

Atoms Moles Molecules

Avogadro’s # Molar mass

6.022 x 1023 grams mole mole


5.3 Molar Masses

Use the molar mass of magnesium and Avogadro’s number as needed to make these calculations:

How many moles of Mg in 12.8 g of Mg?

How many grams of Mg in 2.50 mol of Mg?

How many atoms in 2.50 mol of Mg?

How many atoms in 42.8 g of Mg?


5.4 Moles and Chemical Formulas

1 Molecule of H2O 2 H atoms

1 O atom

1 Mole of H2O 2 moles H atoms

1 mole O atom

We work in moles, rather than molecules, because we can’t see individual molecules!



In a reaction equation:

2 H2 + O2 2 H2O

2 H2 molecules and 1 O2 molecule produce 2 H2O molecules.

2 moles of H2 molecules and 1 mole of O2 molecule produce 2 moles of H2O molecules.



What is the mass of 1.00 mole of H2O?

How many H2O molecules are present in one mole of water?


5.5 Calculations with Molar Mass

A can of soda contains 340 g of water. How many moles of water is this?

How many grams of hydrogen are present in this amount of water?

How many hydrogen atoms are present in this amount of water?


5.7 Chemical Equations and Moles

N2 + 3 H2 2 NH3

3 moles of H2 produce 2 moles of NH3

3 mol H2 2 mol NH3

2 mol NH3 3 mol H2


5.8 Mass Calculations for Chemical Equations

A person eats a candy bar that contains 14.2 g of sucrose (C12H22O11). How much water will be produced by the sucrose? It is metabolized according to the equation shown below:

C12H22O11 + 12 O2 12 CO2 + 11 H2O

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5.8 Mass Calculations

How much Fe2O3 must I use if I want 500 g of Fe metal?

Fe2O3 + 3 CO 2 Fe + 3 CO2

NaI and CH4O react accordingly:

NaI + CH4O CH3I + NaOH

If one starts with 750 g each of NaI and CH4O, how much CH3I will be produced?

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mole of water

chemical calculations

moles of h2 molecules

moles of water

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