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Chemical change
Learning Objectives
A checklist of concepts to learn and skills to master in this section.
Before You Start
Prerequisite skills and concepts to review.
Lecture Slides
Lecture Notes
Internet sites and paper references for further exploration.
Frequently Asked Questions
Find an answer, or ask a question.
Terms and definitions from the glossary are marked with an asterisk (*).
Quiz: Ionic equations
Write net ionic equations for chemical reactions, and recognize spectator ions.
/chem/senese/101/reactions/ionic-equations-quiz.shtml (10/07/98)

Learning objectives

Before you start...

  • 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.

Lecture outline

Notes Notes
Ten symptoms of chemical change

What is a chemical change?

  • 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

  1. 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."
  1. Separate reactants & products with a process symbol:
    rightarrow reaction goes to completion; sometimes used for one-step reactions
    doublearrow 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 gasrightarrowoxygen gas
  1. Replace names with formulas; put plus signs between formulas.
O3 gasright arrowO2 gas
  1. Include symbols for physical states
    (g) gas
    (l) liquid
    (s) solid
    (aq) aqueous (dissolved in water)
O3 (g)right arrowO2(g)
Notes Notes
Ten tips on balancing chemical equations
  1. "Balance" the equation by adding coefficients so atoms and charge are conserved. Each side of a balanced equation must have:
    • the same number of atoms of each type
    • the same total charge
2 O3 (g)right arrow 3 O2(g)
Notes Notes
Types of chemical reactions

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.

synthesis a compound forms from simpler reactants C2H4(g) + H2O(g)right arrow C2H5OH(g)
H2(g) + ½ O2(g) right arrow H2O(l)
decomposition a compound breaks down into simpler products CaCO3(s)right arrowCaO(s) + CO2(g)
2NH4NO3(s)right arrow2N2(g) + 4H2O(g) + O2(g)
2 H2O(l) right arrow 2 H2(g) + O2(g)
displacement AB + Cright arrowA + BC Zn(s) + 2 HCl(aq)right arrowZnCl2(aq) + H2(g)
Zn(s) + CuCl2(aq)right arrowZnCl2(aq) + Cu(s)
neutralization H+ transfer from acid to base NaOH(aq) + HCl(aq) right arrowH2O(l) + NaCl(aq)
KOH + HC2H3O2(aq)right arrow 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)right arrow 4 CO2(g) + 6 H2O(g)

Introduction to solutions

  • 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

Why things dissolve in water

If a molecule is lopsided, it's usually polar. If it is symmetrical, or if all the atoms share electrons equally, it's usually nonpolar. We'll learn how to predict polarity later in the course.
  • 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:
Ice Glucose dissolving in water
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 waterNaCl dissolving 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) rightarrow Na+(aq) + Cl-(aq)
      K2SO4(s) rightarrow 2 K+(aq) + SO42-(aq)


  • experimental evidence for ionic dissociation*
    • solutions of ionic compounds conduct electricity
    • conductivity varies from compound to compound
strong electrolyte weak electrolyte weak electrolyte
    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
  • no dissociation
  • solutions don't conduct electricity
  • typical compounds:
    molecular compounds

Predicting solubility of ionic compounds

compounds containing these ions are soluble... ... unless combined with:
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General Chemistry Online! Chemical change

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