Chemical Kinetics - Rates of Chemical Reactions

CHEMICAL KINETICS – RATES OF CHEMICAL REACTIONS

Chemistry Kinetics is all about the description of the rates at which a reaction proceeds, which is also influenced on how long (time duration) it takes the reaction to occur and get completed, i.e how fast or slow it goes. Also, it is quite interesting to know that these rates can be measured, and the pathways through which the reactants are converted into products are known (Mechanism). Knowledge of mechanisms makes it possible to choose reaction conditions that favour one path over others. Thus, with the information given, scientists are able to:

- Predict on how to speed up the rate of a reaction that is slow to take place and vice versa.

- Control and direct the desired outcome of a reaction.

- Apply measures to slow down the rate of a reaction from taking place, like in preserving perishable foods from spoilage by refrigerating them, etc.

Chemical reactions go at different rates based on their chemical properties or makeup, and there are factors which alter/change their rates.

1. It takes less time to cook rice, than to cook the same quantity of beans on equal amount of heat. Raw beans and raw rice have their own different rates of getting cooked.

2. Some reactions will not occur unless heat is applied to them, or the reactants are ignited, like in;

- Calcium trioxocarbonate (iv) requires heating for it to decompose.

CaCO3(s) ➡️ CaO(s) + CO2(g)

- Raw egg requires heating (boiling) for it to form cooked egg.

Raw egg(l) ➡️ Cooked egg(s)

3. Some reactions are so fast that their rates cannot be measured, and these reactions involve ions, eg. Precipitation reactions.

Determining Rates of Chemical Reactions

The rate of a chemical reaction is the change in quantity (in concentration/mass/volume) of a reactant or product per unit time. It is usually expressed as the concentration of reactant consumed per unit or the concentration of product formed per unit time;

Rate = Change in quantity /

Time taken = gs-1

From the above illustration, reaction rate is directly proportional to the change in quantity of substance (reactant consumed or product formed), and inversely proportional to the time taken. Thus, a very fast reaction takes a short time for completion, and conversely, a very slow reaction will take much time.

In rate reaction, the quantity used in calculating for the rate is either from the product formed or reactant consumed.

1. From Product: As a reaction proceeds, the quantity of product increases rapidly initially, then, slowly, towards the end of the reaction, until it is consumed. In the decomposition of CaCO3 to produce CaO and CO2, the rate of reaction can be determined by measuring the volume(quantity) of CO2 evolved/given off over a given time at a constant temperature.

CaCO3(s) ➡️ CaO(s) + CO2(g)

2. From Reactant: As a reaction proceeds, the quantity of reactant decreases rapidly initially, then, slowly, towards the end of the reaction, until it is completely used up. When a known mass of Zinc reacts with an acid, HCl, the rate of the reaction can be determined by measuring the loss in mass of Zinc over a given time.

Zn(s) + HCl(aq) ➡️ ZnCl2 + H2(g).

Solved Example

1. Marble chips weighing 4.0g were dropped into a large volume of hydrochloric acid, 2.5g of the marble remained for 5 minutes. Calculate the rate of the reaction.

Solution:

Given parameters: Initial mass of marble before reaction = 4.0g

Final mass of marble used up after reaction = 2.5g

Time taken = 5 mins converted to secs: 5 x 60secs = 300secs

If, Rate = Change in quantity / Time

Substituting… Rate = 4.0 – 2.5 / 300 = 0.005gs-1