1.7 Energy changes in chemical reactions  (Page 4/5)

Endothermic and exothermic reactions

1. In each of the following reactions, say whether the reaction is endothermic or exothermic, and give a reason for your answer.
   1. $H_2+I_2\to 2HI+21kJ.mo{l}^{-1}$
   2. $C{H}_4+2O_2\to C{O}_2+2H_{2}O$ $\Delta$ H = -802 kJ.mol ${}^{-1}$
   3. The following reaction takes place in a flask: $Ba{\left(OH\right)}_2.8H_{2}O+2N{H}_{4}N{O}_3\to Ba{\left(N{O}_3\right)}_2+2N{H}_3+10H_{2}O$ Within a few minutes, the temperature of the flask drops by approximately 20C.
   4. $2Na+C{l}_2\to 2NaCl$ $\Delta$ H = -411 kJ
   5. $C+O_2\to C{O}_2$
2. For each of the following descriptions, say whether the process is endothermic or exothermic and give a reason for your answer.
   1. evaporation
   2. the combustion reaction in a car engine
   3. bomb explosions
   4. melting ice
   5. digestion of food
   6. condensation

Spontaneous and non-spontaneous reactions

Demonstration : spontaneous and non-spontaneous reactions

Apparatus and materials:

A length of magnesium ribbon, thick copper wire and a bunsen burner.

Method:

1. Scrape the length of magnesium ribbon and copper wire clean.
2. Heat each piece of metal over the bunsen burner, in a non-luminous flame. Do Not look directly at the flame. Observe whether any chemical reaction takes place.
3. 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.

Results:

• 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 carefuly adding a solution of dilute sulfuric acid to 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.

Spontaneous reaction

A spontaneous reaction is a physical or chemical change that occurs without the addition of energy.

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.