Chemical Reactions

Chapter 7Pg. 192-219

Describing ReactionsChapter 7 Section 1

Pg. 192-198

Chemical Equations

• Reactants- the substances that undergo change in a chemical reaction

• Products- the new substances formed as a result of that change

• Chemical equation- a representation of a chemical reaction in which the reactants and products are expressed as formulas.

Chemical Equations

• Example:

Conservation of Mass

• The law of conservation of mass states that mass in neither created nor destroyed in a chemical reaction.

• During chemical reactions, the mass of the products is always equal to the mass of the reactants.

• Number of atoms on the left side of the equation equals the number of atoms on the right.

Balancing Equations

• In order to show that mass is conserved during a reaction, a chemical equation must be balanced.

• Balance a chemical equation by changing the coefficients (the number that appears before the formulas)

• Never change the subscripts in a formula when balancing equations.

Balancing Equations• Step 1: count the number of atoms of each element on

each side of the equation• Step 2: change one or more coefficients until the equation

is balancedCH4 + O2 CO2 + H2O

unbalancedC = 1 C = 1H = 4 H = 2O = 2 O = 3

balanced

Balancing Equations

• H2 + O2 H2O

• H2 + N2 NH3

• Al2O3 Al + O2

Counting with Moles

• Because chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.

• A mole (mol) is an amount of a substance that contains approximately 6.02 x 1023 particles (atoms, molecules, or ions) of that substance. This number is known as Avogadro’s number.

Counting with Moles

• Molar Mass– The mass of one mole of a substance– The molar mass is the same as its atomic mass

expressed in grams. (Ex: C is 12amu, so molar mass of C is 12 g)

– A CO2 molecules is composed of one C atom (12 amu) and 2 O atoms (2 x 16amu = 32amu). So CO2 has a molar mass of 44 grams. (add the two amu totals together)

Counting with Moles

• Mole-Mass Conversions– Once you know the molar mass, you can convert

moles of that substance into mass or vice versa.– Example: the molar mass of CO2 is 44 grams,

which means that one mole of CO2 has a mass of grams. This relationship yields the following conversion factors.

44 g CO2 1mol CO2

1mol CO2 44g CO2

Counting with Moles

• Mole-Mass Conversion (continued)– Suppose you have 55 grams of CO2. To calculate

how many moles of CO2 you have, multiply the mass by the conversion factor.

55g CO2 X 1mol CO2 = 1.25 mol CO2

44g CO2

You can check your answer by using the other conversion factor

1.25mol CO2 X 44g CO2 = 55g CO2

1mol CO2