• Write balanced chemical equations from descriptions of chemical changes.
• Classify chemical reactions as synthesis, formation, decomposition, thermolysis, electrolysis, displacement, single displacement, metathesis, precipitation, neutralization, redox, and combustion reactions.
• Write formation and combustion reactions for given compounds.
• Understand the concept of equilibrium solubility and use it to recognize saturated and supersaturated solutions.
• Explain why water can dissolve polar and ionic substances.
• Visualize the link between a solution's ability to conduct electricity and the degree of ionic dissociation.
• Classify solutes as strong acids, weak acids, strong bases, and salts.
• Classify solutes as strong electrolytes, weak electrolytes, and nonelectrolytes.

• Review inorganic nomenclature. You'll need to translate compound names into formulas when writing chemical equations.
• Review compound classification; in particular, be able to distinguish ionic and covalent compounds. This will be essential in writing molecular and ionic chemical equations, and in classifying solutes as electrolytes.
• Review conversion of moles to grams using the molar mass. You'll need this skill to predict the amount of reactant consumed or product formed in a chemical reaction, and to prepare solutions of known concentration.

• Definition: A chemical change occurs whenever compounds are formed or decomposed.
  • reactants disappear as chemical change occurs.
  • products appear as chemical change occurs.
  • catalysts speed up the reaction, but aren't produced or consumed.
• Microscopic view: A chemical change occurs whenever there is a rearrangement of atoms that makes or breaks chemical bonds.

Writing chemical equations

Write a sentence describing the reaction that includes the names and states of all reactants and products, and any special conditions the reaction requires.	"Ozone gas decomposes into oxygen gas when exposed to ultraviolet light."
Separate reactants & products with a process symbol: reaction goes to completion; sometimes used for one-step reactions reaction is reversible; at equilibrium, reactants and products are mixed = net reaction; reaction occurs in several steps. Catalysts are usually written on top of the process symbol.	Ozone gas oxygen gas
Replace names with formulas; put plus signs between formulas.	O3 gas O2 gas
Include symbols for physical states (g) gas (l) liquid (s) solid (aq) aqueous (dissolved in water)	O3 (g) O2(g)

Types of chemical reactions

Several common reaction types. Somes reactions can be classified in more than one way, and some reactions fit into none of these categories.

Type	Definition	Examples
synthesis	a compound forms from simpler reactants	C2H4(g) + H2O(g) C2H5OH(g) H2(g) + ½ O2(g) H2O(l)
decomposition	a compound breaks down into simpler products	CaCO3(s)CaO(s) + CO2(g) 2NH4NO3(s)2N2(g) + 4H2O(g) + O2(g) 2 H2O(l) 2 H2(g) + O2(g)
displacement	AB + CA + BC	Zn(s) + 2 HCl(aq)ZnCl2(aq) + H2(g) Zn(s) + CuCl2(aq)ZnCl2(aq) + Cu(s)
neutralization	H+ transfer from acid to base	NaOH(aq) + HCl(aq) H2O(l) + NaCl(aq) KOH + HC2H3O2(aq) H2O(l) + KC2H3O2(aq)
precipitation	a solid product from aqueous reactants	BaCl2(aq) + Na2SO4(aq) = BaSO4(s) + 2 NaCl(aq) AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq)
redox	electron transfer between reactants	2 C2H6(g) + 7 O2(g) 4 CO2(g) + 6 H2O(g)

• A solution is a homogeneous mixture
• The most abundant component in a solution is called the solvent
• Other components are called solutes
• When pure solute and a solution are placed in contact and no more solute dissolves (or comes out of solution) the solution is said to be saturated
  • The concentration of a saturated solution is called the equilibrium solubility
  • Solubility is different for different materials
  • A solution with concentration less than the equilibrium solubility is called unsaturated
  • A solution with concentration above the equilibrium solubility is called supersaturated
  • Solutes drop out of supersaturated solutions that have been "seeded", e. g. with crystals or dust

• water is a polar molecule
  • electrons are shared unevenly between the oxygen & hydrogens in water
  • result: water molecules have a partial negative charge on O, and a partial positive charge on H
  • water molecules can form strong electrostatic attractions to:
    • other water molecules
    • other polar molecules
    • ions

The shape & polarity of the water molecule explains the structure of ice	Polar water molecules surround polar sugar molecules, lifting them off the surface of a solid sugar crystal. Click the thumbnail for a full size image.

• How ionic compounds dissolve in water
  • polar waters stick to ions on the surface of an ionic crystal, partially neutralizing the ion's charge
  • attraction of surface ions to the rest of the crystal is weakened
  • water molecules surround the surface ions and carry them away from the crystal
  • result: soluble ionic compounds dissociate into ions in water
    NaCl(s) Na⁺(aq) + Cl^-(aq)
    K₂SO₄(s) 2 K⁺(aq) + SO₄^2-(aq)
    

• experimental evidence for ionic dissociation
  • solutions of ionic compounds conduct electricity
  • conductivity varies from compound to compound

strong electrolytes completely dissociate into ions solutions strongly conduct electricity typical compounds: soluble ionic compounds strong acids	weak electrolytes incompletely dissociate into ions solutions weakly conduct electricity typical compounds: weak acids	nonelectrolytes no dissociation solutions don't conduct electricity typical compounds: molecular compounds

Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 06/16/05.URL: http://antoine.frostburg.edu/chem/senese/101/reactions/index.shtml