· Thermochemistry is the study of the quantity of heat energy released or absorbed in a chemical reaction. Example: the burning of fuel: is a heatevolving reaction · Heat : · Energy: is a form of energy the potential to do work (to move matter) exists in many different forms: Electrical energy Kinetic Energy (energy of motion) Light energy Heat energy Chemical energy (energy of substances)
· Different forms of energy can be interconverted Examples: Light bulb: electrical energy is converted to light energy and heat energy Car engine: chemical energy (from gasoline) heat energy kinetic energy
KINETIC ENERGY (Ek) · Definition: is the energy associated with a moving object · Formula: 2 Ek = ½ mv , where
m = mass of the moving object v = speed of the moving object
(in kg) (in m/s)
Two objects moving at different speeds may have the same kinetic energy Example: slow moving truck and fast moving sports’ car
2 2 SI unit of energy = from formula of energy = kg . m /s = joule (J) The joule (J) is a very a very small unit
NonSI unit of heat energy : The calorie (cal) amount of energy required to raise the temperature of one gram of water by one degree Celsius 1 cal = 4.184 J
POTENTIAL ENERGY (Ep) · Definition: the energy of an object has because of its position in a field of force is stored energy · Example: Any object above ground level possesses Potential Energy since it is attracted by the gravitational force. As such, it has the potential to fall, and therefore to move. (Potential Energy changes into Kinetic Energy) Ep = mgh, where m = mass of the object g = constant acceleration of gravity h = height (level) of the object aboveground level
NOTE: differences of level are usually more important As an object falls toward ground level: Ø its Ep decreases (h decreases) Ø its Ek increases ( object speeds up; v increases) INTERNAL ENERGY (U) · Definition: the sum of the kinetic and potential energies of the particles making up a substance. In general: Total Energy of a Substance = U + Ek + Ep NOTE: A substance at rest, in a flask in the laboratory Ø possesses no kinetic energy Ø potential energy is ignored Ø possesses Internal Energy Total Energy of the Substance = Internal Energy of the Substance = U Conversions of different forms of energy are governed by : The Law of Conservation of Energy: · Energy may be converted from one form to another, but the total quantity of energy remains constant.
HEAT OF REACTION
· Thermodynamics: is the science of the relationships between heat energy and other forms of energy :are thermodynamic properties of matter the substance or mixture of substance undergoing a physical or a chemical change everything around the thermodynamic system
Ek , Ep , U · Thermodynamic System:
· The Surroundings:
Definition of Heat: · The energy that flows into, or out of a system because of a difference in temperature between the thermodynamic system and its surroundings. This assumes that the system and its surroundings are in thermal contact, (that is, they are not thermally insulated). Example: hot coffee in a thermos is thermally insulated from its surroundings
Direction of Heat Flow : · Heat always flows from a region of higher temperature to a region of lower temperature until the temperatures of the two regions become equal (thermal equilibrium). 1. If heat energy flows into a system from its surroundings, its internal energy increases: Example: A glass of ice water slowly reaches room temperature. The internal energy (U) of the water increases, as it warms up.
2. If heat energy flows out from a system into the surroundings, its internal energy decreases Example: ...