Speed of Chemical Reaction

Introduction: * A chemical reaction changes a substance into another substance. * The speed of a chemical reaction measures how quickly reactants are changed into products. * Some reactions occur in a short time such as fireworks. * The reaction between oil & caustic soda takes long time. * Iron rusting also takes long time to occur. * The reactions which form petroleum oil in the earth take millions of years.

Example:

Conclusion: The concentration of nitrogen pentaoxide is 0.16 mole/liter; i.e. 100% while the concentration of both nitrogen oxide and oxygen is zero%.

As time passes, the concentration of nitrogen pentaoxide starts to decrease whereas the concentration of both nitrogen oxide and oxygen starts to increase. By the end of the reaction, the concentration of nitrogen pentaoxide becomes zero mole/litre; i.e. zero % whereas the concentration of both nitrogen oxide and oxygen increases; i.e. 100%.

Definitions: Concentration is: The strength of a solution; it’s the number of molecules of a substance in a given volume. The speed of chemical reaction is the change in the concentration of reactants & products in a unit of time.

Example

Note: The speed of this reaction is measured by the rate of disappearance of the reactants or the rate of formation of the product.

Factors affect the speed of chemical reaction: * The nature of reactants (the bonds in the molecules, area of reactants exposed to reaction). * The concentration of reactants * The temperature of reactants * Catalysts

1) The nature of reactants: a. Type of bond: Covalent compounds such as organic compounds react slowly because they don’t break into ions. Ionic compounds react fast because they break into ions.

Example:

b. The area of reactants exposed to reaction (surface area): The surface area of the reactants exposed to the reaction affects the speed of the chemical reaction. The bigger the area exposed to the reaction, the faster the chemical reaction is.

2) The effect of the concentration of reactants on the speed of reaction: One factor of the increase in the speed of chemical reaction is the increase in the concentration of reactants. This increases the number of collisions between molecules and consequently the speed of the reaction increases.

Example: The reaction of magnesium with concentrated acid is much faster

3) The effect of temperature on the speed of reaction: * Another factor that increases the chemical reaction is the increase in temperature. This increases the number of collisions between molecules and consequently the speed of the reaction increases. Most of the chemical reactions speed up when temperature increases.

4) Catalyst: A catalyst is a substance which speeds up chemical reaction without sharing in it. Positive catalysts speed up reactions while negative catalysts slow down reactions.

The properties of catalysts: 1- They change the rate of the reaction without changing the reactants & products. 2- The catalyst isn’t changed chemically. 3- The catalyst combines with the reactants & separates from the products at the end of the reaction. 4- The catalyst is used in small amount. 5- The catalyst decreases the energy needed for the reaction.

The effect of the catalyst on the rate of reaction:

The effect of enzymes on the rate of reaction. An enzyme is a biological molecule which acts as a catalyst. An example of an enzyme is the oxidize enzyme found in sweet potato. Oxidize enzyme in the sweet potato speeds up the breaking of hydrogen peroxide.