5. ThermoChemistry

First Law of Thermodynamics

The First Law of Thermodynamics is often called the Law of conservation of energy:

Energy can neither be created nor destroyed but only changed from one form to another.

ORThe energy of the universe is constant.

ORThe energy of a system which is isolated from its surroundings is constant.

We need a few more definitions so we can discus thermodynamics more easily.


The system is something we define ourselves and that definition then lets us write the equations we need and know what the symbols in the equations mean.  The system is generally defined to be whatever we are interested in.  That might be: the chemicals in a reaction, the reaction mixture, the chemicals and their container, etc.  Essentially, we want to define the system in a way that makes the calculations as simple as possible.


After we have defined the system, the surroundings are defined by default.  It is most simply defined to be the rest of the universe.  Although, in practical experiments, we often refer to the surroundings as the rest of the room we’re in, or perhaps the rest of the chemical apparatus. For example, a large water bath surrounding a reaction chamber might be considered the surroundings if it is insulated from the rest of the universe.

Closed System

Any system that is sealed but not insulated, for example, an aluminum water bottle will not lose water but it will get warm as the heat from the day is absorbed into the bottle.Thus, Heat may transfer into or out of the system but no material can transfer.

Isolated system

An isolated system is both sealed against material transfer and insulated against heat transfer.  For example, a thermos bottle is sealed so you don’t lose the contents and also insulated so the contents stay cold (or hot).Thus, No heat or matter can transfer between the system and its surroundings.

Open system

An open system can transfer both material and heat, for example, a paper coffee cup loses both water (as vapour) and heat to the surroundings.

Insulated system

An insulated system is designed so it does not lose heat to the surroundings or vice versa but it is not necessarily sealed against material transfer.  For example, a Styrofoam coffee cup might lose material but will retain heat.

Note that the experimental reality may not quite match up with the definitions we use here.  For example, An insulated system will still lose or gain heat because no amount of insulation is perfect.  However, we can often consider the amount to be negligible as long as the rate of heat loss is very slow compared to the time scale of the experiment we have carried out.  For example, it may take 30 minutes or so for the contents of a coffee cup to cool down to room temperature from the boiling point of water, so the cup is not a perfect insulator.  However, we might still use a coffee cup as an insulated container for a thermodynamics experiment that we can complete in a few seconds.  We will consider the coffee cup to be a perfectly insulated container in this experiment as long as the amount of heat lost over the short time of the experiment is quite small compared to the heat exchange involved in the experiment itself.

Energy transfer can be done in one of two ways:

  1. Work can be done on the system by the surroundings (or vice versa). It can take the form of mechanical work or of electrical energy transfer.
  2. Heat q  flows from the system to the surroundings (or vice versa)

There is a general sign convention that chemists use when describing q and w.  When energy flows into the system as a result of heat or work, the sign is positive (the system gains energy).  When energy flows out of a system, the sign is negative (the system loses energy.  Both and refer, not to an amount of energy, but to an amount of energy transferred as a result of the process.

(note the sign convention)

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