Activity :: Demonstration : Spontaneous and non-spontaneous reactions
Apparatus and materials:
A length of magnesium ribbon, thick copper wire and a bunsen burner
Scrape the length of magnesium ribbon and copper wire clean.
Heat each piece of metal over the bunsen burner, in a non-luminous flame. Observe whether any chemical reaction takes place.
Remove the metals from the flame and observe whether the reaction stops. If the reaction stops, return the metal to the bunsen flame and continue to heat it.
• Did any reaction take place before the metals were heated?
• Did either of the reactions continue after they were removed from the flame?
• Write a balanced equation for each of the chemical reactions that takes place.
In the demonstration above, the reaction between magnesium and oxygen, and the reaction between copper and oxygen are both non-spontaneous. Before the metals were held over the bunsen burner, no reaction was observed. They need energy to initiate the reaction. After the reaction has started, it may then carry on spontaneously. This is what happened when the magnesium reacted with oxygen. Even after the magnesium was removed from the flame, the reaction continued. Other reactions will not carry on unless there is a constant addition of energy. This was the case when copper reacted with oxygen. As soon as the copper was removed from the flame, the reaction stopped.
Now try adding a solution of dilute sulfuric acid with a solution of sodium hydroxide. What do you observe? This is an example of a spontaneous reaction because the reaction takes place without any energy being added.
Activation energy and the activated complex
From the demonstrations of spontaneous and non-spontaneous reactions, it should be clear that most reactions will not take place until the system has some minimum amount of energy added to it. This energy is called the activation energy. Activation energy is the ’threshold energy’ or the energy that must be overcome in order for a chemical reaction to occur.
It is possible to draw an energy diagram to show the energy changes that take place during a particular reaction. Let’s consider an example:
The reaction between H2(g) and F2(g) (figure 14.1) needs energy in order to proceed, and this is the activation energy. Once the reaction has started, an in-between, temporary state is reached where the two reactants combine to give H2F2. This state is sometimes called a transition state and the energy that is needed to reach this state is equal to the activation energy for the reaction. The compound that is formed in this transition state is called the activated complex. The transition state lasts for only a very short time, after which either the original bonds reform, or the bonds are broken and a new product forms. In this example, the final product is HF and it has a lower energy than the reactants. The reaction is exothermic and ∆H is negative.
In endothermic reactions, the final products have a higher energy than the reactants. An energy diagram is shown below (figure 14.2) for the endothermic reaction XY + Z → X + Y Z. In this example, the activated complex has the formula XYZ. Notice that the activation energy for the endothermic reaction is much greater than for the exothermic reaction.
Important: Enzymes and activation energy
An enzyme is a catalyst that helps to speed up the rate of a reaction by lowering the activation energy of a reaction. There are many enzymes in the human body, without which lots of important reactions would never take place. Cellular respiration is one example of a reaction that is catalysed by enzymes.
Exercise: Energy and reactions
1. Carbon reacts with water according to the following equation: C+H2O⇔CO+H2 ∆H>0
(a) Is this reaction endothermic or exothermic? (b) Give a reason for your answer.
2. Refer to the graph below and then answer the questions that follow:
(a) What is the energy of the reactants?
(b) What is the energy of the products?
(c) Calculate ∆H.
(d) What is the activation energy for this reaction?