# What is Rate of reaction:[units, equation, formula]

At the end of this article, you should be able to describe what is Rate of reaction-Definition, units, equation, formula. 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.

## 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.

1. 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.
2. 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).

For example,

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.