chemical reactions and equations reactants →products reactants are starting materials and are...
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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)
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.
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)
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
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
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?