Transcript SCI 111 - Onondaga Community College

Chemical Reactions
Potassium iodide (aq) reacts with lead nitrate (aq)
producing a yellow precipitate of lead iodide
Chemical Formulas
1. Empirical formula
2. Molecular formula
3. Structural formula
Formulas?
Empirical
• Metal and non-metal
• Ionic - lacking discrete unit,
or molecule
• Simplest whole number ratio
Molecular
• Covalent compounds
• Molecular and empirical
formulas can be different
– Glucose:
molecular C6H12O6 versus
empirical CH2O.
Formula Weight
• General term ; Molecular Weight used more often
• “Sum” of the atomic weights of all the atoms in a chemical formula
Hydrocarbons and Carbohydrates
(Organic Chemistry)
• Hydrocarbons
– Composed of H and C
– Some simple ; some complex
– Examples: C3H8 (propane)
C4H10 (butane)
– Complete “combustion” yields:
• CO2, H2O + energy
• Carbohydrates
–
–
–
–
Composed of H, C, and O
Sugars, starches, cellulose
Examples: C12H22O11 (sugar)
Complete “oxidation” yields:
• CO2, H2O + energy.
Chemical Reactions
• Occur through formation and breaking of
chemical bonds between atoms
• Involve changes in matter, creation of new
materials, and energy exchange
• Chemical equations
– Concise representation of chemical reactions.
Chemical Equations
• Reactants - substances existing before reaction
• Products - substances existing after reaction
• Chemical symbols and formulas needed for
quantitative purposes.
Balancing Equations
• Law of conservation of mass: atoms are neither created
nor destroyed in chemical reactions
– Mass of reactants = mass of products (i.e. balanced)
• To balance a chemical equation
– Change coefficients in front of chemical formulas
– Do not change the subscripts (numbers within formulas).
Subscripts vs Coefficients
Fig 10.5
Example: Fig. 10.6
Stepwise balancing procedure
page 279
Like an Inventory or “Bean Counting”
1.
2.
3.
Law of conservation of mass (atoms are conserved)
Don’t change subscripts of formulas (compounds)
Multiply everything within a compound by the Coefficient
•
Look for the most complex reactants and products
–
•
•
Try to balance atoms within them first
Treat “Polyatomic” ions that appear on both sides as
independent units with a charge
Cross-over technique and use of fractional coefficients top
find least common multiple to balance the equation
–
See the next few examples: 10.5, 10.6, and 10.7
Chemical Reactions
Potassium iodide (aq) reacts with lead nitrate (aq)
producing a yellow precipitate of lead iodide
Bal Eq
Classifications of
Chemical Reactions
1. Combination reactions
2. Decomposition reactions
3. Replacement reactions

(1-3 = redox reaction subclasses)
4. Ion exchange reactions
Combination Reactions
Rust: Fig 10.10
• Two or more substances
combine to form a single
compound
4 Fe(s) + 3 O2(g)
2 Fe2O3(s)
Decomposition Reactions
2 HgO (s)
• Breakdown into simpler
compounds or elements
• Usually require some form
of energy for Rx to occur
Δ
2 Hg (s) + O2
Example: Replacement Reaction
2 Al (s) + 3 CuCl2 (aq)
Fig 10.13
2 AlCl3 (aq) + 3 Cu (s)
Replacement Reaction
Fig 10.12
• Occur because some
elements have a stronger
electron-holding ability
• More active metals (Li, K,
Ca, Na) give up electrons to
elements lower on the list
2 Al (s) + 3 CuCl2 (aq)
2 AlCl3 (aq) + 3 Cu (s)
Ag (s) + CuCl2 (aq)
No Rx
Ion Exchange Reaction
AX + BY
3 Ca(OH)2 (aq) + Al2(SO4)3 (aq)
AY + BX
3 CaSO4 (aq) + 2 Al(OH)3
•
Ion Exchange:
 ions of one compound interact with ions of
another compound
•
Possible results:
1.
2.
3.
•
Solid precipitates: ↓
Gas forms: ↑
Water formed: H2O (l)
No ion exchange reaction occurred if both
products are soluble (See appendix B)
 “ S ” versus “ i ”
Information from
Chemical Equations
• Atoms are conserved
• Mass is conserved
• Law of combining
volumes (gases)
– Gases at the same
temperature and pressure
contain equal numbers of
molecules
Units of Measurement used
with Equations
• Atomic mass unit (u) = 1/12th mass of carbon-12
• One mole of a substance contains Avogadro’s number (6.02x1023) of
the basic chemical unit of that substance (atoms, molecules, ions, …)
• Example: A mole of carbon-12 atoms is defined as having 6.02 x 1023
atoms totaling a mass of 12.00g
Molar Weights
• Gram-atomic weight:
mass in grams equal to
atomic weight
• Gram-formula weight:
mass in grams equal to
formula weight
• Gram-molecular weight:
mass in grams equal to
molecular weight
Quantitative use of Equations
Next Time:
Water and Solutions