Thursday, June 12, 2014
Chapter 17: Reaction Rates
Section 17.1: A Model for Reaction Rates
Expressing Reaction Rates -
The average rate of an action or process is the change in a given quantity during a specific period of time, and is indicated by the Greek letter delta (Δ).
Average rate or speed is written as:
Average Rate = Δquantity/Δt
This equation defines the average rate at which reactants produce products, which is the amount of change of a reactant in a given period of time.
The reaction rate of a chemical reaction is stated as the change in concentration of a reactant or product per unit time, expressed as mol/(L*s).
Brackets around the formula for a substance denote the molar concentration.
Example: [NO2] represents the molar concentration of NO2.
Reaction rates are determined by measuring the concentrations of reactants and/or products in an actual chemical reaction. They cannot be calculated from balanced equations as stoichiometric amounts can.
Reaction rates must always be positive. When the rate is measured by the consumption of a reactant, a negative sign is applied to the calculation to get a positive reaction rate.
The rate of consumption is calculated by this form of the average rate equation:
Average Reaction Rate = -(Δquantity/Δt)
The Collision Theory -
The reactants in a chemical reaction must collide in order to form products.
The collision theory states that atoms, ions, and molecules must collide in order to react.
Reacting substances must collide with sufficient energy to form the activated complex.
Example: 2H2 + O2 → 2H2O
According to the collision theory, H2 and O2 molecules must collide in order to react and produce H2O.
Orientation and the Activated Complex -
Activated Complex: a temporary, unstable arrangement of atoms that may form products or may break apart to re-form the reactants.
Because the activated complex is as likely to form reactants as it is to form products, it is sometimes referred to as the transition state.
An activated complex is the first step leading to the resulting chemical reaction.
Activation Energy and Reaction -
The minimum amount of energy that reacting particles must have to form the activated complex and lead to a reaction is called the activation energy, Ea.
Activation energy has a direct influence on the rate of a reaction.
A high Ea means that relatively few collisions will have the required energy to produce the activated complex and the reaction rate will be low.
A low Ea means that more collisions will have sufficient energy to react, and the reaction rate will be higher. Section 17.2: Factors Affecting Reaction Rates The Nature of Reactants -
An important factor that affects the rate of a chemical reaction is the reactive nature of the reactants.
The tendency of a substance to react influence the rate of a reaction involving the substance.
The more reactive a substance is, the faster the reaction rate.
Reactions speed up when the concentrations of reacting particles are increased.
One of the fundamental principles of the collision theory is that particles must collide to react.
The number of particles in a reaction makes a difference in the rate at which the reaction takes place.
Example: The concentration of oxygen in the air surrounding steel wool is much less than that of the pure oxygen in a flask. The higher oxygen concentration accounts for the faster reaction.
Surface Area -
Increasing the surface area of reactants provides more opportunity for collisions with other reactants, thereby increasing the reaction rate.
Pulverizing (or grinding) a substance is one way to increase its rate of reaction.
This is because, for the same mass, many small particles possess more total surface area than one large particle.
Increasing the surface area of a reactant does not change its concentration, but it does increase the rate of reaction by increasing the collision rate between reacting particles.…