Speed of Reaction (Introduction)

What will you learn in module 6.1?

  • What is the speed of reaction.
  • How does activation energy of a reaction affect the speed of reaction (6.1c).

What is the Speed of Reaction?

The speed of reaction tells us how fast or slow a reaction is. There are reactions that are very slow, such as the rusting of iron, which is the oxidation of iron to form Fe3O4.

rusting chempenguin

There are also reactions that occur very fast, such as the reaction of dilute hydrochloric acid with magnesium. When magnesium is added to dilute hydrochloric acid, you can see some reaction happening almost immediately.

Effervescence of colourless hydrogen gas is seen almost immediately, this is because magnesium is a reactive metal and readily reacts with acids to form hydrogen gas.

magnesium in hydrochloric acid chempenguin

But How do we Determine the Speed of a Reaction?

We can do this by monitoring either the amount of reactants used up or products formed per unit time. Or we can say the rate of reactants used up or the rate of products formed.

In a chemical reaction, reactants are substances that have not gone through the reaction. They are original substances that has not changed. Reactants are written at the left hand side of a chemical equation.

Products, however, are substances that have gone through a chemical reaction. Products are written at the right hand side of the chemical equation.

Let’s look at an example that we have seen just now: the reaction of magnesium metal and dilute hydrochloric acid.

HCl(aq) +Mg(s) → MgCl2(aq) + H2(g) —(1)

In reaction 1, hydrochloric acid (HCl) and magnesium metal (Mg) are the reactants. Reactants always appear at the left hand side of the chemical equation.

The products of the reaction are magnesium chloride (MgCl2) and hydrogen gas (H2). They are the substances formed after the reaction has occurred.

The higher the rate of reactants that are used up in the reaction, the faster the speed of reaction. Similarly, the higher the rate the products are formed in the reaction, the faster the speed of reaction.

We can use equations to calculate the rate of reaction.

Rate of reaction = amount of reactant used up / time taken

Rate of reaction = amount of product formed / time taken

When Does a Reaction Occur?

There are two main factors that determine whether a reaction will occur or not. They are: (1) thermodynamic feasibility and (2) kinetics (speed of reaction).

In other words, these two factors tell us whether a reaction is possible or not in certain conditions, such as, temperature.

(1): Thermodynamic feasibility

Thermodynamic is one of the two factors that determine whether a reaction is feasible or not. Enthalpy change of a reaction is one of the factor that affect the thermodynamic feasibility.

Enthalpy change of a reaction is the energy change when a reaction goes to completion,

Actually, there is another factor, which is entropy, that affect the thermodynamic feasibility. But, in O Level syllabus, you do not need to know the details about entropy, you only need to know about enthalpy.

But you have to know that exothermic reaction are generally more feasible to occur than endothermic reaction. This is because in an exothermic reaction, the products are in lower energy state than the reactants.

Exothermic reaction is where energy (not necessarily heat energy) is given out from the reaction to the surroundings. This is illustrated by a negative enthalpy change.

Always remember, a reaction tends to achieve a lower energy state.

(2): Kinetics

Another factor, that determines whether a reaction can “carry out” or not, is the kinetics, which is concerned with the speed of reaction. Both factors determine whether a reaction can occur or not.

As mentioned above, the speed of reaction tells us how fast the reaction occurs. Even if a reaction is thermodynamically feasible, but it may not happen because the rate of reaction is too low.

An example would be the reaction of graphite to diamond (2). Both graphite and diamond are made up of the same element which is carbon.

C(graphite) → C(diamond) H≤0

Graphite is more thermodynamically unstable as compared to diamond, and you may think the conversion of graphite to diamond is feasible.

But this is not true. Although graphite is thermodynamically unstable as compare to diamond, the reaction does not occur.

This is because the speed of reaction of the conversion of graphite to diamond is too slow, due to the high activation energy of graphite to diamond.

Direct factors affecting the speed of reaction

For a reaction to happen in a reasonable speed, molecules must:

  • Collide in the correct orientation.
  • Collide with energy greater than or equal to a minimum energy, known as the activation energy.

Any factors that affect the frequency of collision or activation energy will affect the rate of reaction.

What happens during a chemical reaction?

Collision theory tells us what happen during a chemical reaction, and this can be placed into a series of steps.

  1. Particles are in a constant state of random motion.
  2. Particles collide together.
  3. If energy of collision is equal to or greater than activation energy, chemical reaction will occur. If the energy of collision is lower than activation energy, no reaction occur.
  4. All bonds of the reactants break, this is an endothermic process. New bonds formed in products, this is an exothermic process. (*Remember the
  5. Reaction completes.

Activation energy is the minimum energy that must be supplied to a chemical system before a reaction can occur. If the reaction has a high activation energy, it has a lower speed of reaction.

This is because the probability of molecules having energy equal to or greater than the higher activation energy is very low. If the molecules collide with energy equal to or greater than the activation energy, the collision is successful and this an effective collision.

If any factor can lower the activation energy, the frequency of collisions and therefore effective collisions increase, thus the speed of reaction increases.

When you are answering exam questions, use phrases like “frequency of effective collision” to describe the number of effective collisions per unit time.

The activation energy is a direct factor that affects the speed of reaction. In the later section of this chapter, we will learn some indirect factors that affect speed of reaction, which is by affecting the activation energy.



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