When dealing with chemical reactions we often talk about the system and the
surroundings. The system is
the reacting chemicals, the reactants and the products
while the surroundings are everything else... the beaker,
the room, the building, the universe!
The systems can be open or closed. An open system is one where both matter (reacting chemicals) and energy can be exchanged freely with the surroundings. Carrying out a reaction in an open beaker where for example heat and gases can escape into the atmosphere is an example of a reaction taking place in an open system. A closed system is one which can exchange energy (usually heat) with the surrounding but not matter. The diagram below shows both open and closed systems. In the closed system below two gases are reacting in a sealed container which has a movable piston on top. The piston will allow energy to enter and leave the system as it moves up and down but no gases (matter) can enter or leave.
Energy is a word which has probably been used many times in your science lessons, but what exactly is energy? The word energy is derived from the Greek word ergon; meaning work. A simple working picture of energy might be the capacity to do work or supply heat
One of the main forms of energy we are concerned with in chemistry is
potential energy. There is
stored potential energy or chemical
energy within the bonds of a molecule. When a susbstance
undergoes a chemical reaction some of this stored chemical
energy maybe released as heat and light energy.
When a strip of magnesium metal is held in a hot Bunsen flame or even the
methane gas used in the
Bunsen burner reacts with the oxygen in the air the reacting chemicals lose some of the
stored potential or chemical energy in their bonds as
heat and light as the products of the reaction form.
Consider for a second a chemical system (reactants and products), it could be a flask filled with say hydrogen
and oxygen or a beaker of acid reacting with
magnesium ribbon, now consider the fact that all the atoms in this system are moving and so have
kinetic energy. However
the individual atoms consist of electrons
which are also moving and will also have kinetic energy. There will also be
potential energy present as a result of the separation of the protons
and electrons and we also need to think about potential energy
due to all the interactions of the molecules, atoms
and sub-atomic particles that make up the system. The
internal energy of a system (symbol E) is the
sum of all the kinetic and potential
all the atoms and sub-atomic particles
that make up the system. Not surprisingly it is not possible to calculate
the internal energy (E) of a system, even
with the most powerful and advanced computers available today.
From your gcse science course you may remember that in physics we used a definition of work which was: work is done when a force moves an object a distance, d.
So far we have seen that a system can exchange energy with its surrounding as heat and work:
However like internal energy (E) it is not possible to calculate the enthalpy of a system. To be honest as chemists even if it were possible to calculate the enthalpy of a system, the calculated value would be of little practical use. What is much more useful is the change in enthalpy that takes place during a chemical reaction. The enthalpy change (ΔH) is simply the heat energy lost or gained by the system at constant pressure plus any work done or gained by the system. For most practical purposes the value of ΔH and ΔE vary little, especially when there is a small change in the volume in going from the reactants to the products in a chemical reaction, since little or no work will be done. The enthalpy change for a reaction is simply:
For us chemists this basically means that any energy lost by a reacting system will be gianed by the surroundings and in the case of an endothermic reaction any heat energy lost by the surrounding will be gained by the system.
Well all these reactions release heat energy to the surroudings. They are all exothermic reactions.
An exothermic reaction release heat energy to the surroundings. A thermometer (part of the surroundings would measure an increase in temperature during an exothermic reaction.
As we have seen above the reacting chemicals (the system) in a reaction
act as a store of chemical energy.
During an exothermic reaction
the system (chemicals which are reacting) loses energy to the surrounding,
mainly as heat. The temperature of the
surrounding will increase. Remember the law of conservation of energy,
this states that energy cannot be created or
destroyed, only changed from one form to another. So if the reacting chemical lose energy then the surrounding must
gain the energy lost by the reacting chemicals (the system).
Have you ever sucked on a sherbet sweet and felt your mouth getting slightly cooler? Well this is because there is an endothermic reaction taking place in your mouth. Sour sherbet sweets contain citric acid and a base called sodium bicarbonate. In your mouth these chemical react to release carbon dioxide, which gives the fizzing sensation when you eat sherbet, but the reaction between citric acid and sodium bicarbonate is an endothermic one. It removes heat energy from the surrounding, and in this case the surroundings are your mouth! So you should feel a cooling sensation in your mouth as you enjoy your fizzy sherbet sweets.
Endothermic reactions are much less common than exothermic reactions. One of the most spectacular endothermic reactions to see is one where solid ammonium chloride is added to a beaker containing solid barium hydroxide. The beaker containing the solid barium hydroxide is sitting on top of a block of wood on which a small pool of water has been added. When the two solids are added together and stirred vigorously so much heat energy is removed from the surroundings that the pool of water under the beaker freezes and forms a solid block of ice. This ice is so thick that it will stick the beaker to the wooden block. The temperature changes are very dramatic. If the experiment is done in a lab at room temperature, 250C, then after the experiment is over the temperature may have dropped to -100C. A temperature drop of 350C. In this experiment the chemicals, the system, has gained energy from the surrounding. As a consequence the temperature of the surroundings has dropped.
An endothermic reaction removes heat energy from the surroundings. A thermometer (part of the surroundings) would measure a decrease in temperature during an endothermic reaction.