Chemical Reactions and Equations

Reactants → Products

Reactants are starting materials and are shown at left of arrow

Products are final materials and are shown on right of arrow

Phase Labels

Phase labels

• show state of materials

• written as subscript in parentheses after formula

(s) = solid

(l) = liquid

(g) = gas

(aq) = aqueous (dissolved in water)

Greek Letter Delta Δ indicates that heat must be added to make reaction go

Δ shown over arrow

Evidence for a Chemical Reaction

• Formation of a gas (Fizzes)

• Formation of a solid (precipitate) when liquids are mixed

• Change in color (not just dilution of color)

• Change in heat (released or absorbed)

Equations must be BALANCED

• According to Law of Conservation of Matter (Mass), matter cannot be created or destroyed in a chemical reaction

• Thus equations must have same number of each kind of atom on left and right sides

Rules for Balancing Equations

1. Write correct formula for each reactant and product.

2. Do NOT change formula----ever!

3. Change the number of each kind of atom in equation by writing coefficients in front of the formula.

4. Keep changing coefficients until the equation is balanced and there are the same number of atoms of each kind on both sides.

5. Double check your equation.

Balance the following equation

H2 + N2 NH3

3 H2 + N2 2 NH3

Avogadro’s Number and the Mole

Atomic mass unit (amu) - unit of measure for the mass of atoms.– carbon-12 assigned the mass of exactly 12

amu– 1 amu = 1.66 x 10-24 g – periodic table gives atomic weights in amu.

What is atomic weight ofone F atom?

Atomic weight of one F atom (from periodic table) is 19.00 amu

What is mass of one F atom in grams?

atom F

F g10156.3

Famu 1

g101.661

atom F

Famu 19.00 23-24

More convenient to work with grams than amu, but mass of one atom is VERY SMALL

Therefore, we use a new unit:

mole (mol)

Mole

1 mole = 6.022 x 1023 (of anything)

Unit similar to other common units

dozen (12)

gross (12 dozen or 144)

Avogadro’s number = 6.022 x 1023

Number of atoms of C found in 12.01 grams of Carbon

Molar Mass of element

Mass in grams of one mole of atoms

Can convert between atoms, moles and mass

Mass ↔ Moles

Use molar mass 1 mol = “x” g

How many grams are there in 0.20 mol of H?

0.20 mol H x 1.008 g H = 0.20 g H 1 mol H

How many mol of C are there in 4.0 g of C?

Molar Mass

Mass of one mole of compound

Molar mass has same numerical value as formula weight, but different units (g/mol)

1 formula weight of NaCl = 58.5 amu

1 mole of NaCl = 58.5 g

Compounds

Formula mass (formula weight):

Mass of one molecule or formula unit

Given in amu

Add up atomic mass of all atoms in compound

Calculation of Formula Weight

NaCl

1 Na @ 23.0 amu = 23.0 amu

1 Cl @ 35.45 amu = 35.45 “

1 formula weight of NaCl = 58.5 amu

What do the coefficients in a balanced equation mean?

3 H2 + 1 N2 2 NH3

Moles 3 1 2Molecules 3 1 2

Coefficients do not give mass relationship

Conversions between moles and atoms

• 1 mol = 6.02 x 1023

Need to convert moles to mass in order to get mass relationships (not coefficients)

3 H2 + 1 N2 2 NH3

3 mol H2 + 1 mol N2 → 2 mol NH3

3 mol H2 x 1 mol N2 x 2 mol NH3 x

2 g H2/mol H2 28 g N2/mol N2 17 g NH3/mol NH3

6 g H2 + 28 g N2 = 34 g NH3

2 Mg + O2 2 MgO

• How much MgO can be made if you start with 6.07 g of Mg?

• Convert g Mg to mol Mg• Convert mol Mg to mol MgO (Use equation)• Convert mol MgO to g Mg

1 mol Mg = 24.3 g1 mol MgO = 40.3 g

1 mol O2 = 32 g

2 Mg + O2 2 MgO

• How much MgO can be made if you start with 6.08 g of Mg?

2 Mg + O2 2 MgO

• How much MgO can be made if you start with 6.08 g of Mg?

Limiting Reagent

• May run out of one compound before you use up all of the other compound.

If you start with 6.08 g Mg and 32 g O2 which will be used up first?

Limiting Reagent

• May run out of one compound before you use up all of the other compound.

If you start with 6.08 g Mg and 32 g O2 which will be used up first?