Energy changes occur between a system and its surroundings.
SYSTEM—the part of the universe under consideration. Ex. Atom, or a lab set up
Most common form of energy transfer involves heat
Ex. A = 25ºC , B =20ºC
What happens when we put them together?
Energy transfers from A to B. (High to low).
HEAT (q)—energy transferred as a result of a temperature difference. Represented by the letter q.
If left undisturbed, energy will transfer until A and B are the same temperature.
JOULE (J)—SI unit for energy = 1 kg m2 / s2
ENERGY AND CHEMICAL CHANGE
Chemical changes are always accompanied by changes in energy.
ENDOTHERMIC—if energy is absorbed. Since energy is taken in, the products of the reaction have higher energy levels than the substances that reacted.
EXOTHERMIC—if energy is given off (usually in the form of heat). Products have less energy than the reactants. Rule of thumb, exothermic reactions usually occur spontaneously (without outside help). Lighting a match (friction is the activation energy).
Both reactions take a certain amount of energy to get started (rolling a rock). This is called
ACTIVATION ENERGY—THE MINIMUM AMOUNT OF ENERGY TO GET A REACTION STARTED.
MEASURING ENERGY CHANGES
Calorimeter—a device used to measure energy given off or absorbed during chemical changes.
SPECIFIC HEAT (Cp)—the heat required to raise the temperature of one gram of substance by one Celcius degree. Every substance has its own specific heat.
Heat required to raise the temp. of one gram of water, one Celcius degree is 4.184 Joules. The specific heat of water is 4.184 J/gCº