By the end of this article, you should be able to describe what is Rate of reaction-Definition, units, equation, formula. what are the factors affecting, how to calculate, and how does concentration/ Temperature affect the rate of reaction. Let’s start discussing one by one.
We know that some reactions are slow while others are fast. For example, iron combines with oxygen in presence of moist air, when it rusts (a slow reaction); while a piece of phosphorus when exposed to air bursts into flame (a spontaneous or fast reaction). Thus chemical reactions are said to proceed at different rates. The rate of fast reactions and hence kinetics cannot be studied, that is the study of kinetics is limited to slow reactions. The study of Chemical kinetics deals with the reaction rate, the effect of temperature and pressure and the mechanism by which the reaction takes place.
What is Rate of Reaction- Definition
|The rate of reaction is the change in concentration (number of moles per litre) of reactant or product per unit time. The rate of a reaction is always positive.|
We can write the formula for the rate of a chemical reaction in terms of the change in concentration of a reactant or of a product.
(i) In terms of the change in concentration of the reactant, the formula for the rate of a reaction is:
It should be noted that the minus sign in the above formula for the rate of a reaction. Since the concentration of a reactant decreases during a reaction, the change in concentration is negative. We include a minus sign in the reaction rate formula to ensure that the rate of reaction comes as a positive number.
(ii) In terms of the change in concentration of a product, the formula for the rate of a reaction is :
Since the concentration of a product increases during a reaction, the change in concentration of a product is positive. So, the rate of reaction will be positive.
Rate of Reaction Units
The concentration is normally expressed in moles/litre and time in seconds or in minutes. Therefore, the units of reaction rate are:
- mol L -1s-1.
- mol L -1min-1.
For the Gaseous reactants, the pressure is normally used and is expressed in – atmosphere. Therefore, units are
- atm s-1.
- atm min-1.
Factors Affecting Rate of Reaction
The rates of chemical reactions are influenced by a number of factors. These are :
|Concentration of Reactants||Temperature|
|Presence of Catalyst||Nature of Reactants|
|Surface Area||Exposure to Radiation|
- Increase in concentration the of reactants always gives more yield of the products irrespective of the nature of the reaction and the conditions which govern the reaction.
- Most of the chemical reactions which are slow at lower temperature become fast upon heating. In fact, temperature provides heat energy to the reactant species. They move and collide at the faster rate and give more products.
- The effect of the catalyst on reaction rate is very significant. A number of catalysts are used in chemical reactions of industrial importance.
- Chemical reactions involving different reactants do not proceed at the same speed. For example, combustion of nitric oxide (NO) is faster as compared to that of carbon monoxide (CO).
- In the laboratory, we quite often prefer granulated zinc in preference of lump of the metal while preparing hydrogen gas on reacting with the dilute hydrochloric acid or dilute sulphuric acid. Actually, granulated zinc has the Greater surface area available for the attack by the acid than the lump of zinc. Therefore, it reacts at a faster rate.
- Many chemical reactions known as photochemical reactions are carried in the presence of sunlight. For example,
H2(g) + Cl2(g) → 2HCl(g)
In these types of reactions, the photons of light are the source of energy which helps in breaking the bonds in the reacting molecules so that may react and form products.
How Does Concentration Affect The Rate of Reaction?
To understand it better, let’s take two conical flasks A and B.
- Flask A – When we have put in 3 grams of zinc granules and 5 ml of 1 M hydrochloric acid in flask A, then hydrogen gas is produced gradually. We note down the volume of hydrogen gas formed after every 10 seconds by using a stopwatch.
- Flask B – We now take another similar flat-bottomed flask B and put the same mass of 3 grams of zinc granules in it. But we now add 5 ml of 2 M hydrochloric acid into the flask (which has double the concentration than in the previous case). The volume of hydrogen gas produced is noted after every 10 seconds by using a stopwatch.
We plot the graphs between time readings and the corresponding volume readings of hydrogen gas evolved for both the experiments on the same graph paper. We get two curves showing the reaction rate in the two cases.
The curve OA is for the reaction between 3 grams zinc and 1M hydrochloric acid whereas curve OB is for the reaction between 3 grams zinc and 2 M hydrochloric acid.
We can see from Figure curve OB is steeper than curve OA. This means that the reaction in flask B is faster than in flask A. Since of concentration of the reactant on the rate of a flask B has the higher concentration of hydrochloric acid. Form this we conclude that when the concentration of hydrochloric acid is increased (from 1 M to 2 M), then the rate of formation of hydrogen gas increases (or the reaction rate increases).
The volume of hydrogen gas evolved in these experiments is a measure of the rate of the two reactions (taking place in two separate flasks). So, in general, we can say that when the concentration of a reactant is increased, the reaction rate increases (the reaction goes faster). It has been found by experiments that in the reaction :
Zn (s) + 2HCl (aq) → ZnCl2 (aq) + H2(g)
If the concentration of the reactant hydrochloric acid is doubled, then the rate of evolution of hydrogen gas also gets doubled, In other words, if the concentration of hydrochloric acid is doubled, the reaction rate also gets doubled. If, however, we decrease the concentration of a reactant, then the reaction rate decreases. From this discussion, we conclude that a reaction can be made to go faster or slower by changing the concentration of reactants.
How Does Temperature Affect The Rate of Reaction?
On increasing the temperature of reactants, the energy of the molecules increases. Due to increased energy the frequency of collision increases and finally the rate of reaction increases. This we conclude that :
- Increase in temperature increases the rate of a reaction
- Decrease in temperature decreases the rate of a reaction
Two examples of the effect of temperature on the rate of a reaction from everyday life.
- In order to cook food faster, we use a pressure cooker. In a pressure cooker, water boils at a temperature higher than its normal boiling point of 100°C. So, due to the higher temperature inside the pressure cooker, the chemical reactions involved in the cooking of food go at a faster rate and cook the food in lesser time.
- We keep the perishable food material at a low temperature inside a refrigerator. Due to the low temperature inside the refrigerator, the chemical reactions responsible for the spoilage of food become very slow and hence the food remains good for a much longer period.
Rate of reaction Formula
The rate of a chemical reaction means the speed with which a chemical reaction takes place. As a result, the reactants are consumed and the products are formed. Thus, the rate of a reaction implies the decrease in molar concentration of the reactants per unit time or increase in the molar concentration of the product per unit time.
Let us consider a general reaction,
R (reactant) → P (product)
When we start a chemical reaction, we have reactants only and there are no products. As the reaction proceeds, the amount of reactants becomes less and less whereas the amount of products becomes more and more. In other words, as a chemical reaction proceeds, the concentration of reactants goes on decreasing but the concentration of products goes on increasing.
An intermediate time comes when Reactant is equal to Product.
Rate of Reaction Equation (Calculate)
When a reaction commences, the concentration of the reactant(s) starts decreasing while the concentration of product(s) starts appearing or increasing. If dc is the change in concentration in a very small time interval dt, then the reaction rate is dc/dt.
Conventionally(rule wise), the rate of change in concentration is taken to be negative for reactant (i.e. reaction rate for reactant is written as -dc/dt) and positive for a product (i.e. reaction rate for the product is written as dc/dt).
A + B → C
According to the law of mass action, the reaction rate is proportional to the active mass of the reactants. Thus for the general reaction,
nA + mB → Product
Rate =k[A]n [B]m
where K is a constant and known as rate constant or velocity constant. This relation is known as rate equation or rate law.
This is all about the basics of Rate of reaction-Definition, units, equation, formula, factors affecting, how to calculate, and how does concentration/ Temperature affect the rate of reaction.