Scope, Sequence, and Coordination

A Framework for High School Science Education

Based on the National Science Education Standards

Identifying and Explaining Reactions

Chemical Reaction Rates
Chemical reactions can take place in time periods ranging from the few femtoseconds (10-15 seconds) required for an atom to move a fraction of a chemical bond distance to geologic time scales of billions of years. Reaction rates depend on how often the reacting atoms and molecules encounter one another, on the temperature, and on the properties--including shape--of the reacting species.

Further Description:

Reactions are observed to take place at different rates. These rates differ according to the species present, concentration, pressure, temperature, and the presence of other substances such as catalysts. Predictions of reaction rates can be made qualitatively and quantitatively. The latter are based on the frequency and angles at which the species collide and on the energy of the collisions. Each reaction has a specific energy of activation that can be measured. Many reactions involve several steps, each of which plays an important role in determining the reaction rate.

Many reactions do not go to completion but establish an equilibrium, that is, a state in which the forward reaction and the reverse reaction have equal rates and the concentrations of the reactants and products remain constant. When changes are made to an equilibrium system (pressure, volume, temperature, concentration), predictions can be made both qualitatively and quantitatively about concentrations of the reacting species.

Concepts Needed:

Grade 9

Concentration, temperature, pressure, kinetic energy

Grade 10

Reaction rate, collision frequency

Grade 11

Potential energy, kinetics, activation energy, reaction mechanism, reversible reaction, equilibrium, static, dynamic, steady state, rate-determining step, activated complex

Grade 12

Reaction coordinate, equilibrium constant, mass action

Empirical Laws or Observed Relationships:

Law of mass action, Le Chatelier=s principle

Theories or Models:

Chemical bond, kinetic-molecular theory, collision theory

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Micro-Unit Description:

Identifying and Explaining Reactions
Using the particulate nature of matter, students should explain why temperature, concentration, and surface area are important factors in determining reaction rate.

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