The Nature of Energy

u Energy is the ability to do work or produce heat. u It exists in two basic forms, potential energy and kinetic energy.

The Nature of Energy u Kinetic energy is energy of motion. u Kinetic energy increases as the temperature of molecules goes up.

The Nature of Energy u Comparing any two samples, the one with the higher temperature has the higher kinetic energy.

The Nature of Energy u Temperature is a measure of an object’s average kinetic energy. u The unit for temperature commonly used by scientists is degrees Celsius or Kelvin.

The Nature of Energy u The potential energy of a substance depends upon its composition: the type of atoms in the substance, the number and type of chemical bonds joining the atoms, and the particular way the atoms are arranged.

The Nature of Energy u Chemical potential energy is stored in gasoline, wood, food, etc.

Law of Conservation of Energy u The law of conservation of energy states that in any chemical reaction or physical process, energy can be converted from one form to another, but it is neither created nor destroyed.

The Nature of Energy u Chemical systems contain both kinetic energy and potential energy. u Energy (kinetic and potential) of the particles of a substance changes when heated, cooled, or changing phase.

The Nature of Energy u As you consider the phases - solid, liquid, gas – this is in order of increasing potential energy. u Solids have the least potential energy. Liquids have a moderate amount of potential energy. Gases will have the most potential energy.

Heat u Heat, which is represented by the symbol q, is energy that is in the process of flowing from a warmer object to a cooler object. u The standard unit of heat and energy is the joule (J).

Heat u Heat involves a transfer of energy between 2 objects due to a temperature difference.

Heat flows from “hot to cold.”

Law of Conservation of Energy u When the warmer object loses heat, its temperature decreases and q is negative. u When the cooler object absorbs heat, its temperature rises and q is positive.

PHASE CHANGES

Phase Changes u A material will change from one state or phase to another at specific combinations of temperature and surrounding pressure. u Typically, the pressure is atmospheric pressure, so temperature is the determining factor to the change in state in those cases.

Liquid Vaporization Solid Gas ???? Condensation

Condensation and Vaporization

Vaporization u Vaporization is the process by which a liquid changes into a gas or vapor. u Vaporization is an endothermic process – the liquid absorbs heat.

Evaporization u When vaporization occurs only at the surface of an uncontained liquid (no lid on the container), the process is called evaporation.

Endothermic u Endothermic Absorbs heat Would feel cold if you were to touch it Pulls in heat from its surroundings – such as your hand, and converts it to chemical potential energy

u Condensation is the process by which a gas or vapor becomes a liquid. It is the reverse of vaporization. u Condensation is exothermic – heat is released. Condensation

Endothermic u Exothermic Releases heat Would feel hot if you were to touch it Release chemical potential energy and you would perceive it as heat

u Vaporization results in an increase in potential energy. u Condensation results in a decrease in potential energy. u Kinetic energy remains constant during vaporization and condensation. Energy During Vaporization & Condensation

u In a closed system, the rate of vaporization can equal the rate of condensation. u When the rates are equal the system is said to be in dynamic equilibrium. Dynamic Equilibrium

u Molecules are constantly changing phase - “Dynamic”  The total amount of liquid and vapor remains constant - “Equilibrium” Dynamic Equilibrium

Vapor Pressure u Vapor pressure is the pressure exerted by a vapor over a liquid.

Vapor Pressure u As temperature increases, water molecules gain kinetic energy and vapor pressure increases.

Vapor Pressure u Vapor pressure does not change with surface area.

Vapor Pressure u Vapor pressure for a liquid is lowered when sugar or salt is dissolved in the liquid.

u Matter/VaporPressure.htm Matter/VaporPressure.htm Dynamic Equilibrium

Boiling u When the vapor pressure of a liquid equals atmospheric pressure, the liquid has reached its boiling point, which is 100°C for water at sea level. u Standard atmospheric pressure equals 1 atm.

Boiling u At this point, molecules throughout the liquid have the energy to enter the gas or vapor phase. u The temperature of a liquid can never rise above its boiling point.

Liquid Melting Solid Gas ???? Freezing

Melting and Freezing

u The melting of a solid occurs when the forces holding the particles together are broken and the solid becomes a liquid. u Melting is an endothermic process – the solid absorbs heat. Melting

u Freezing occurs when a liquid becomes a crystalline solid. u Freezing is an exothermic process – the liquid releases heat. u The freezing point is the same as the melting point. Freezing

u Melting results in an increase in potential energy. u Freezing results in a decrease in potential energy. u Kinetic energy remains constant during melting and freezing. Energy During Melting & Freezing

Liquid Sublimation Solid Gas ???? Deposition

Sublimation and Deposition

u The process by which a solid changes directly into a gas without first becoming a liquid is called sublimation. u Solid air fresheners and dry ice are examples of solids that sublime. u Sublimation is endothermic. Sublimation

u When a substance changes from a gas or vapor directly into a solid without first becoming a liquid, the process is called deposition. u Deposition is the reverse of sublimation. Deposition

u Frost is an example of water deposition. u Deposition is exothermic. Deposition

u Sublimation results in an increase in potential energy. u Deposition results in a decrease in potential energy. u Kinetic energy remains constant during sublimation and deposition. Energy During Sublimation & Deposition

Phase Changes of Water and Energy

Solid Liquid Gas Melting Vaporization Condensation Freezing Sublimation Deposition

Question Classify the following phase change. 1. dry ice (solid carbon dioxide) to carbon dioxide gas sublimation

Question Classify the following phase change. 2. ice to liquid water melting

Question Classify the following phase change. 3. liquid water to ice freezing

Question Classify the following phase change. 4. water vapor to liquid water condensation