powerpoint to accompany chapter 2 part 1 stoichiometry: calculations with chemical formulae and...

PowerPoint to accompany

Chapter 2Part 1

Stoichiometry:Calculations with

Chemical Formulae and Equations

Brown, LeMay, Bursten, Murphy, Langford, Sagatys: Chemistry 2e © 2010 Pearson Australia

Law of Conservation of MassFigure 2.1

“We may lay it down as an incontestable axiom that, in all the operations of art and

nature, nothing is created; an equal amount of matter

exists both before and after the experiment. Upon this principle, the whole art of

performing chemical experiments depends.”

--Antoine Lavoisier, 1789


Chemical Equations

Concise representations of chemical

reactions.

They mainly give proportions phases

They do not imply mechanism


Anatomy of a Chemical Equation

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Figure 2.4 *



CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Reactants appear on the

left side of the equation.

Note that the equation contains molecules or elements and the phases they exist in during the reaction. e.g. Methane (gas)



CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Products appear on the right side of the equation.

The Arrow indicates which direction the reaction tends to go. If it points both ways it signifiies equilibrium.



The states of the reactants and products

are written in parentheses to the right of

each compound. In this example all are gases.

*

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)



CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Coefficients are inserted tobalance the equation.

Balanced Equations have the same elements

and numbers of atoms of each element on both sides

of the equation.

Numbers of Molecules are proportionate to

a complete reaction, not absolute.


Subscripts and Coefficients Give Different Information

Subscripts indicate the number of atoms of each element in a molecule

Coefficients indicate the number of molecules

Figure 2.2


Reaction Types: *Combination:

A + B C

Decomposition:

A B + C , often with a gas

Combustion:

Fuel + Oxidant Ash, Gases


Examples N2 (g) + 3 H2 (g) 2 NH3 (g)

C3H6 (g) + Br2 (l) C3H6Br2 (l)

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

Combination Reactions

Two or more substances react to form one product:

A+BC

Figure 2.5 *


Examples N2 (g) + 3 H2 (g) 2 NH3 (g)

C3H6 (g) + Br2 (l) C3H6Br2 (l)

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

Combination Reactions Nitrogen gas reacts with hydrogen gas to

form ammonia gas (Nitrogen fixation). Cyclopropane gas reacts with liquid bromine

to form 1,5 dibromopropane Magnesium metal reacts with oxygen gas to

form solid magnesium oxide.

Figure 2.5


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

Combination Reactions Figure 2.5 *

48.61 g Mg + 32.00 g O2 = 80.61 g MgO

What happens if you only have 16 g of O2 ?


Decomposition Reactions

One substance breaks down into two or more substances

Examples CaCO3 (s) CaO (s) + CO2 (g) 2 KClO3 (s) 2 KCl (s) + 3 O2 (g)

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

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

Figure 2.6 *




Sodium azide solid reacts to form sodium metal and nitrogen gas. (So what does it mean if somebody

text’s you: “ U NaN3 “ ?)

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

Figure 2.6


You Airbag!




Calcium carbonate (limestone) decomposes to form solid unslaked lime and carbon dioxide gas.

This is the 1st step to make portland cement.

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


Decomposition ReactionsOne substance breaks down into two or more substances

2 KClO3 (s) 2 KCl (s) + 3 O2 (g)

POP QUIZ!

2 moles of solid _______decomposes to form

2 moles of solid _ _ and 3 moles of gas.


Decomposition ReactionsOne substance breaks down into two or more substances

2 KClO3 (s) 2 KCl (s) + 3 O2 (g)

POP QUIZ!

2 moles of solid Potassium Chlorate decomposes to form

2 moles of solid Potassium Chloride and3 moles of Oxygen gas.


Combustion Reactions Rapid reactions that

produce a flame

Most often involve hydrocarbons reacting with oxygen in the air

Examples CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

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

Figure 2.7


Combustion ReactionsMost often involve hydrocarbons reacting with oxygen in the air

Which hydrocarbon fuel generates the least amount of greenhouse gas?

Examples CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

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

Figure 2.7


Formula Mass


Formula Mass ( or Weight)

Sum of the atomic weights of the atoms in a chemical formula

The formula mass of calcium chloride, CaCl2, would be

Ca: 1 x 40.1 g = 40.1 g

+ Cl: 2 x 35.5 g = 71.0 g

111.1 g

According to Dalton’s Law what would the decomposition of 1 mole of Calcium Chloride generate?


Molecular Mass

Sum of the atomic weights of the atoms in a molecule

For the molecule ethane, C2H6, the molecular weight would be

C: 2 x 12.0 g = 24.1 g + H: 6 x 1.0 g = 6.0 g

30.1 g

For the complete combustion of 1 mole of ethane, what weight of water would be produced?


Mass of Water from Ethane

For the complete combustion of 1 mole of ethane, what weight of water would be produced?

C2 H 6 (g) 2 CO2 (g) + 3 H2O (g) .

6 g H makes 3 moles of H2O,

each mole of water is 1 mole of Oxygen for 16 g and 2 moles of Hydrogen for 2 g, so 18 g.

But there are 3 moles of water so:

3 moles water X 18 g/mole = 54g water


Percent Composition

One can find the percentage of the mass of a compound that comes from each of the elements in the compound by using this equation:

% element =(number of atoms of that element)x(atomic mass of that element)

(formula mass of the compound)x 100


Percent Composition

For example, the percentage of carbon in ethane, C2H6, is:

% C =2 x 12.0 g

30.0 g

24.0 g

30.0 g= x 100

= 80.0 %

If there are 3X as many Hydrogens, why is the percentage of carbon so high?

powerpoint to accompany chapter 2 part 1 stoichiometry: calculations with chemical formulae and...

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