Chemical Equations› Balancing, Types of Equations

Compound Composition› Atom mass; Atomic and formula weight› % Composition› Empirical Formulas› Mole Amounts; Molar Mass› Mole Calculations

Amounts in chemical reactions

Chemical equations help us to describe chemical reactions.

BEFORE AFTER

Combustion (of a hydrocarbon with O2) Combination Reactions Decomposition Reactions

Single Displacement Reactions (Ch.4) Double Displacement Reactions (Ch.4)

Hydrocarbons react with oxygen to give carbon dioxide and water:

CH3CH2CH3(g) + O2(g) CO2(g) + H2O(g)

Balance it!

Hydrocarbons react with oxygen to give carbon dioxide and water:

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Balance it!

Combination Reactions: A + B C

Mg(s) + O2(g) ?

Combination Reactions: A + B C

Mg(s) + O2(g) MgO(s)

Combination Reactions: A + B C

2Mg(s) + O2(g) 2MgO(s)

Fig 3.5

Decomposition Reactions: C A + B

Sodium azide decomposes to its elements:

NaN3(s) ?

(N3- is a polyatomic ion)

Decomposition Reactions: C A + B

Sodium azide decomposes to its elements:

NaN3(s) Na(s) + N2(g)

(N3- is a polyatomic ion)

Balance it!

Decomposition Reactions: C A + B

Sodium azide decomposes to its elements:

2NaN3(s) 2Na(s) + 3N2(g)

(N3- is a polyatomic ion)

Another example:

2HgO(s) 2Hg(l) + O2(g)

Rxn

Some compounds decompose to more stable compounds:

2H2O2(l) 2H2O(l) + O2(g)

CaCO3(s) CaO(s) + CO2(g)

Atomic Weight Molecular Weight / Formula Weight Percent Composition Empirical Formulas

When we describe the mass of atoms of an element, we must consider all of the naturally occurring isotopes.

Carbon is composed of › 98.8925 % C-12 (12.00000 amu)› 1.1080 % C-13 (13.00335 amu)

Its weighted average is 12.011 amu. Its atomic weight is 12.011 amu.

Atomic weight is different than mass number.

Isotope Mass

NumberAtomicMass

Abundance

Cu-63 63 62.94 amu 69.17 %

Cu-65 65 64.93 amu 30.83 %

(62.94 amu)(0.6917) = 43.5356(64.93 amu)(0.3083) = 20.0179

63.55 amu (4 sig figs)

The formula weight of a substance is the sum of the atomic weights of the atoms that make up the formula.

Mg(OH)2 2(16.00) amu

2(1.008) amu + 24.31 amu

58.33 amu = formula wt. Mg(OH)2

The formula weight of a molecular compound is also called its molecular weight.

H2O 1.008 amu

1.008 amu + 16.00 amu

18.02 amu = molecular wt. of H2O

Mass Percent Composition =

(#Atoms of Element)(Atomic Weight) x100 Formula Weight of Compound

%46.74100amu21.95

amu90.70x

What is the mass %Cl in MgClWhat is the mass %Cl in MgCl22??Formula weight = (2 x 35.45amu) + 24.31 amuFormula weight = (2 x 35.45amu) + 24.31 amu

95.21 amu95.21 amu

Mass of Cl: 2 x 35.45amu = 70.90amuMass of Cl: 2 x 35.45amu = 70.90amu

Mass %Cl = Mass %Cl =

What is the mass %Mg in MgCl2?

74.47% Cl 25.53% Mg

How would you go about determining the mass of carbon in 5.0 g CCl4?

How would you go about determining the mass of carbon in 5.0 g CCl4?

%C = (12.011amu/153.811 amu)x100= 7.8089% C

g Carbon = (0.078089)(5.0g) = 0.39 g

Which of the following has the greatest %O? Explain.

MgO CaO Na2O

K2O

What is the percent carbon in C2H4?

What is the percent carbon in C3H6?

What is the percent carbon in C2H4? % C = 85.63%

What is the percent carbon in C3H6? % C = 85.63%

Different compounds with the same percent composition have the same empirical formula.

The empirical formula shows the ratio of atoms of each element as the smallest whole-number ratio.

Which of the following are empirical formulas?

CH4

C2H2

N2O5

C2H6

Al2O3

Which of the following are empirical formulas?

CH4

C2H2

N2O5

C2H6

Al2O3