Chapter 8: Unit 1. Reaction Rates

Reaction Rates

The rate of reaction is defined as the amount of the reactants consumed per time or as the amount of products produced per time:

2 A +   3B  →   2 C +   5 D

Rate of Reaction = – 1/2 [∆ A] = – 1/3 [∆ B] = + 1/2 [∆ C] = + 1/5 [∆ D]

Where:

[∆ A] = final concentration of A – initial concentration of A

[∆ B] = final concentration of B – initial concentration of B

[∆ C] = final concentration of C – initial concentration of C

[∆ D] = final concentration of D – initial concentration of D

Example:

2 Mg(s)  + O₂(g)     …>  2 MgO(s)

If 5.00 grams of magnesium are used for this reaction and the reaction had lasted 30.0 seconds. Calculate the rate of the reaction.

First let us calculate first number of moles of magnesium used for the reaction: [Hint: Mg atomic mass is used]

Number of magnesium moles = [5.00 g Mg x (1 mol Mg / 24.3 g Mg)] = 0.206 mol Mg

Rate of reaction = [0.206 mol Mg / 30.0 seconds] = 0.00687 mol /seconds

In order for a chemical reaction to progress certain conditions are needed:

1. Molecules must collide with each other
2. A proper orientation and alignment of the atoms within the molecules

1. Activation energy is provided through the collision to help breaking the bonds among the atoms within the molecules. Activation energy is defined as amount of energy needed to break the atoms within the molecules.

The above conditions are the bases for the collision theory.

The figure below illustrates the progress of the reaction versus the energies involved in the reaction.

The activation energy of a chemical reaction is closely related to its rate. Specifically, the higher the activation energy, the slower the chemical reaction will be. This is because molecules can only complete the reaction once they have reached the top of the activation energy barrier.

The collision theory can be illustrated in the Phet Simulation below:

https://phet.colorado.edu/en/simulation/legacy/reactions-and-rates

The simulation discussed above leads to the fact that not all collisions result in a new substance and reactions are reversible. Reactions are the result of collisions and the products may collide and react to give reactant.

Questions:

Explain how the simulation model relates to test tube size experiments

What are the factors contributing to a successful reaction?

What would enable a reaction proceed or slow its progress with references to the reaction coordinate?

How does the reaction coordinate can show the potential energy changes with the separation of reactants and products?

Use the molecular model to explain why reactions have less than 100% yields.

Explain how can the effect of the heat of the chemical equilibrium?

The Chemistry Solution website illustrates the rate of the reaction and the collision among the reactants molecules to produce the products.

https://teachchemistry.org/periodical/issues/may-2018/reaction-rates

Explain the effects of the factors listed below on the rate of reaction using both the reaction simulation and the graph obtained from the corresponding simulation?

1. Concentration
2. Surface area
3. Temperature
4. Catalyst

A video of You Tube explains the rate of reaction concept in a very simple and systematic way: