Mr. Bouyer
Day 1 | Day 2 | Lab | Skills Test

Describe rate of reaction and its units. Describe the factors affecting reaction rates. Write the rate expression for a single-step chemical reaction. Use experimental data to determine the mechanism for a reaction.

Chemical Reaction Rates

Different chemical reactions take place at different rates. The study of reaction rates is the study of the forming and breaking of chemical bonds, a very complex subject. 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.
• 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.

Rate expression: 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.

  Here is an example:

  • For the reaction: H₂O₂ + 2HI 2H₂O
  • The rate expression should be: 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 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 indicates that the reaction occurs in more than one step.

Use this webtest to practice Rate Expression calculations.

Portfolio Assignment 221:
Scoring criteria

1. Iron combines with oxygen to produce iron (III) oxide in a single-step reaction.
   1. Write a balanced equation for the reaction.
   2. Write the rate expression for the reaction.
   3. What is meant by the expression [Fe₂O₃]?
   4. What units are indicated by [  ]?

2. For the single-step reaction, 2NO + Br₂ 2NOBr₂ , the k is 0.8 dm⁶ / mole^{2 .} sec. If the initial [NO] was 1.4M, what initial [Br₂] would produce NOBr₂ at an initial rate of 0.75 moles/dm³/sec?

   

Day 2

Reaction mechanism - the series of steps that must occur for a reaction to go to completion.

Most chemical reactions occur in a series of steps. Each step normally involves the collision of only two particles. There is very little chance of three or more particles colliding with the proper energy and geometry to cause a reaction. Some reactions have several steps and others have very few. The only way to determine the number of steps in a reaction mechanism is by interpreting experimental data.

In any reaction mechanism, one of the steps will be slower than all the others. This is known as the rate determining step. It is the only step that will affect the reaction rate. At a given temperature, the reaction rate varies directly as the product of the concentration of the reactants in the slowest step.

Test Your Concept Understanding: Nitrogen (IV) oxide gas was combined with oxygen gas to produce nitrogen (V) oxide gas in the laboratory producing the following data: Trial [NO2] [O2] Rate of N2O5 Formation 1 0.025 0.011 3.1 X 10-4 mole/dm3/sec 2 0.025 0.022 6.2 X 10-4 mole/dm3/sec 3 0.050 0.011 6.2 X 10-4 mole/dm3/sec Write the rate expression for the reaction. Calculate the rate constant. Use the rate expression and calculated value for k to compute the initial rate of formation of N2O5 if the concentrations of NO2 and O2 were each 0.0030M.