Exothermic & Endothermic

Thermochemistry is the study of energy changes in a chemical reaction.  First, you must understand a few basic definitions about energy from a physics standpoint.  Energy is defined as the ability to perform work. Work is a force applied through a distance, and force is defined as a push or a pull.  Therefore energy is the ability to make something move through a distance.  In physics, we are generally thinking about making an object move- like a ball rolling down a hill or dropping a rock.  For chemistry, we are concerned about the constant movement of atoms, ions and molecules.

Energy is measure in 2 different units.  A Joule (J) is defined as a Newton*meter.  A Newton is the metric unit of force.  Remember Sir Isaac Newton's 2nd law of motion states that a Force=mass*acceleration (F=ma).  In SI units, a Newton (N) = mass (kg) * acceleration (m/s^2).  So if work is defined as a force applied through a distance, a Joule is a Newton*meter.

Another unit of energy is the calorie (cal).  A calorie is defined as the amount of energy needed to raise the temperature of 1.0 g of water 1 C.  Temperature is the average amount of Kinetic Energy (E of motion) is a substance.  So a calorie is basically the amount of energy we need to add to 1g of water to make it's molecules move faster, so that it raises the temperature of the water 1 C.

Since we have 2 units, we need a conversion to change one unit to another.

 1 calorie = 4.184 Joules

You've probably heard about calories before in relation to food.  These are actually Calories, with a large C.  Sometimes manufacturers and advertisers mix them up, but a food Calorie is actually a kilocalorie.

If a system LOSES energy, it is called EXOTHERMIC.  Since heat is leaving the system, it will feel hot when you touch it.  The energy is leaving the system and going into your finger.

If a system GAINS energy, it is called ENDOTHERMIC.  Since heat is being added to the system, it will feel cool when you touch it.  The energy is leaving your finger and entering the system.