Chemical Kinetics II

Collision Theory

Collision Theory attempts to explain reaction rates.  It is based on the basic ideas:

• Molecules must collide to react.
• Concentration affects the rate.  A higher concentration allows for more particles to have a chance of colliding, therefore increasing the rate.  Fewer particles would lessen the chance of collision, therefore slowing the rate.
• Molecules must collide hard enough (but not too hard) to cause a reaction.
• Temperature and rate are related.
• Only a small number of collisions actually result in a reaction.

Activation Energy is the energy needed to form an activated complex or transition state so a reaction can occur.  In other words, how much energy is needed to get the reaction started.  Think of it as the amount of energy it takes to cut up all the vegetables you need to prepare a meal.

Take the synthesis reaction that forms hydrogen iodide gas.

Before the hydrogen and iodine can combine, the diatomic molecules must be broken apart in a redox reaction.  After the positive hydrogen ions and the iodine ions have been formed, the electrostatic force (attraction of a + for a -) will pull the ions together.

This reaction needs energy to break up the diatomic molecules and form ions.  This is the activation energy.  The H+ and I- are the activated complex (the transition state or intermediate step) in the mechanism of the reaction.

Chemical Kinetics

Chemical Kinetics

The Rate of Reaction is measured by the change in molarity over time.  By convention, we use the decreasing molarity of the reactants instead of the increasing molarity of the products.  The rate of a reaction depends upon many factors: the number of molecules, how fast they are moving, how they are colliding, …  Because of this, reaction rate is not linear, it is an exponential decay.

Instantaneous rate is the rate at a specific moment in time.  It is determined by the slope of the tangent line to the curve.  The only way to determine this without calculus is to graph the data and manually approximate a tangent line.

Average rate is the rate between 2 specific times.  It is determined by the slope of the line between the 2 times on the curve.  You do not need an actually graph the data to determine average rate, just use the (time, molarity) as (x,y) and determine the slope by ∆x/∆y.

The formula for the rate law for an equation is always    rate=k[reactant]^order

The power or order of the reaction for each reactant must be determined from experimental data.  The total order for a reaction is equal to the sum of the orders for each reactant. 

• This method requires that a reaction be run several times.
• The initial concentrations of the reactants are varied.
• The reaction rate is measured just after the reactants are mixed.
• Eliminates the effect of the reverse reaction.