Chemical Reaction Rates

The units of reaction rate are (moles / dm³) / second. Different chemical reactions take place at different rates. The study of reaction rates is the study of the forming and breaking of chemical bonds.

The nature of the reactants involved in a chemical reaction will determine the kind of bonding that occurs.

Reactions with bond rearrangement or electron transfer generally take longer than ionic reactions.
Ionic reactions are almost instantaneous because of the strong attraction between the charged particles.

Because of electron cloud repulsion, most neutral molecules that come in contact with one another bounce off without reacting. For these molecules to react, they must collide with enough kinetic energy to cause changes in the electron clouds of both molecules. When such a change occurs, an activated complex is formed, which allows the reaction to proceed. The energy required to give molecules enough kinetic energy to form an activated complex is known as the activation energy

for the reaction.

Other factors affecting reaction rate:

Concentration - For a chemical reaction to occur, the particles must collide. An increase in the number of particles per unit volume (concentration) increases the chance of their colliding. Increasing the concentration of the reacting particles generally increases the reaction rate.
Temperature - An increase in the speed of molecules increases the number of molecules that have the required activation energy. Increasing the number of molecules with activation energy generally increases the reaction rate. Increasing the temperature of the reacting particles generally increases the reaction rate.
Catalysts - A catalyst is a substance that increases a reaction rate without being permanently changed. The catalyst appears to be chemically unaffected by the reaction. A catalyst changes the reaction mechanism in such a way that the activation energy required is less than that in the uncatalyzed reaction.
Inhibitors - An inhibitor ties up a reactant in a complex so that it will not react.

Stable compounds do not spontaneously decompose in air.

Thermodynamically stable - At room temperature, the compound does not spontaneously decompose in air. The overall energy change in the decomposition reaction is positive.
Kinetically stable - At room temperature, the compound spontaneously decomposes in air. The overall energy change in the decomposition reaction is negative.
However, the reaction takes place so slowly, at room temperature, that no observable change takes place for years. Since the rate of change is imperceptible, the substance is still considered stable.

Unstable compounds spontaneously decompose in air. The reaction takes place at such a rate that the change is observable.

A rate expression is a mathematical equation used to calculate the rate of a chemical reaction.

The rate expression for a reaction is expressed as k times the product of the concentrations of the reactants.

r a t e = k [ a ] [ b ]

The brackets [ ] in the rate expression indicate concentration in moles/dm3.
k in the rate expression is the specific rate constant.
Any reaction has only one value for the constant, k, at a given temperature.

If a chemical reaction occurs in one step, the concentrations of the reactants in the rate expression have exponents equal to the coefficients of the reactants in the balanced equation for the reaction.

For the reaction: H₂O₂ + 2 HI → 2 H₂O + I₂
The rate expression is: rate = k [H₂O₂] [HI]²

If a chemical reaction occurs in more than one step, the concentrations of the reactants in the rate determining step (the step that has the slowest reaction rate) are the only ones that are important.

The only way to be sure of the rate expression is to use experimental data.
Experimental data proves the actual rate expression for the reaction above is:

rate = k [H₂O₂] [HI]
This shows that the reaction occurs in more than one step.

Reaction Rate Problems

Computer Lab