Common Mistakes

Plotting Kinetics Experiments

In only first-order reactions, the half-life is independent of the initial concentration.

Arrhenius Equation

The units of R are in joules, while activation energy is often reported in kilojoules. Be sure to do this conversion.

Dynamic Equilibrium

Dynamic equilibrium may take a significant amount of time to reach in an experiment. Be patient.

Le Chatelier's Principle

  1. Make sure that you use gas pressures when working with Kp and molar concentrations when working with Kc.
  2. Sometimes it is possible to simplify the equilibrium expression. If the value of x is much smaller than the concentrations in the initial concentrations, then it is possible to remove x from the equilibrium expression. Like anyone else, chemists like to avoid work whenever possible. See below for an example.

Thermodynamics

An exothermic reaction has a positive ΔH value while an endothermic reaction has a negative ΔH value, which sometimes seems counterintuitive.

Entropy for an element or compound is always greater than zero. However, the change in entropy (ΔS) can be negative. A common example: water freezing.