Rate of Reaction | Basic Concept and Formulas
Hi, guys. Now, we’re gonna discuss about ‘Rate of Reaction’. I’m sure you are familiar with the term of ‘rate’. Yeah, we usually use that term in physics. However, do you know that in chemistry, we also use that term? Yep, we use the ‘rate of reaction’ to define the loss of reactants or the increase of products in chemical reaction over time.
Anyway, how can reactants change into product?
Now, we introduce you another term, collision theory. The theory says that, to undergo a reaction, particles need to come into contact with one another or in the other words collide with sufficient energy and correct orientation. Effective collision will change reactants into products. If particles do not have enough energy, they won’t change into reactant even though there is a collision. That is called ineffective collision.
So, how much energy do the particles need in order to collide effectively? There is another term, known as activation energy, symbolised by Ea which is the minimum energy required for the particles to collide effectively so the reactant will turn into product.
Figure below shows activation energy for exothermic and endothermic reaction,
Based on collision theory, there are several factors that effect rate of reaction. What are they? Let’s check one by one!
The Effect of Concentration on Rate of Reaction
A higher concentration implies a greater quantity of solute particles dissolved within a specific volume of solvent. In reactions that involve solutions, more concentrated reactants exhibit a swifter reaction rate due to the increased frequency of collisions between reacting particles. If we see the figure above, the left picture has more particles means the solution is more concentrated than the right one. Thus, the collision happened more frequent for the solution on the left. In conclusion, the increase of concentration will increase rate of reaction because the frequency of collision increases.
The Effect of Temperature on Rate of Reaction
When the temperature of a reaction mixture is elevated, it leads to an increase in the average kinetic energy of the particles. This results in faster movement of particles in both solutions and gases, leading to a higher frequency of collisions. The likelihood of successful collisions, meaning those resulting in a reaction, rises due to the increase in particles surpassing the activation energy threshold. This aspect holds greater significance in influencing the outcome.
Catalysts enhance the reaction rate by guiding the reaction along an Catalysts enhance the reaction rate by facilitating an alternate reaction pathway or mechanism with a lower activation energy compared to the uncatalyzed reaction.
From the picture above, we see that there is the coloured area under the curve, that was the area which the particles or molecules have greater energy than activation energy. Without catalyst, that area is the darker one. With the present of catalyst, there are more particles or molecules that have energy greater than activation energy (the total of dark and light purple area). Therefore, more effective collusions happen and consequently, rate of reaction increases.
After we talked about the concept of rate of reaction and some factors that affect it, now let’s discuss about the formulas we use to calculate rate of reaction. Before we go there, let’s discuss about some useful terms. Look at the table and the graph below!
The table shows concentration of cyclopropane and rate of rection. From the graph, we can see the trend, when the concentration of cyclopropane doubled, the rate of reaction was also doubled. Likewise, when the concentration of cyclopropane decreased one third, the rate of the reaction also reduced one third. Let’s, look at the table, in the third column, there are constant numbers which we get by dividing rate with the concentration. That numbers indicates, the rate of reaction is constantly change proportional to the concentration of reactants. We call that numbers as rate constant, symbolised by k. Unit of k varies, depending on order of the reaction.
So, what is order?
The reaction order indicates how the rate of a reaction is influenced by the concentration of a specific reactant. It represents the exponent to which the concentration of that reactant is raised in the rate equation.
Look at the rate equations below,
A + B -> C + D
Rate = k [A]m [B]n
m and n are the order of reaction, m is the order for reactant A and n is the order for reactant B. m + n is the total order of the reaction
To make uou more understand, see the example below,
Rate = k [H2] [NO]2
From that equation, we know that the order with respect to H2 is 1 and the order with respect to NO is 2. The total order of the reaction is 1 + 2 = 3.
Units of rate constant and the other formulas summarised in the following table.
Source: Chemistry by Cambridge International As and A Level 3rd Edition.