Law of Conservation of Energy

Law of Conservation of Energy • “Energy can neither be created nor destroyed.” • The total amount of energy in the universe is constant. There is no process that can increase or decrease that amount. • However, we can transfer energy from one place in the universe to another, and we can change its form. Tro's "Introductory Chemistry", Chapter 3

Matter Possesses Energy • When a piece of matter possesses energy, it can give some or all of it to another object. • It can do work on the other object. • All chemical and physical changes result in the matter changing energy. Tro's "Introductory Chemistry", Chapter 3

Kinds of EnergyKinetic and Potential • Potential energy is energy that is stored. • Water flows because gravity pulls it downstream. • However, the dam won’t allow it to move, so it has to store that energy. • Kinetic energy is energy of motion, or energy that is being transferred from one object to another. • When the water flows over the dam, some of its potential energy is converted to kinetic energy of motion. Tro's "Introductory Chemistry", Chapter 3

Some Forms of Energy • Electrical • Kinetic energy associated with the flow of electrical charge. • Heat or Thermal Energy • Kinetic energy associated with molecular motion. • Light or Radiant Energy • Kinetic energy associated with energy transitions in an atom. • Nuclear • Potential energy in the nucleus of atoms. • Chemical • Potential energy in the attachment of atoms or because of their position. Tro's "Introductory Chemistry", Chapter 3

Units of Energy • Calorie (cal) is the amount of energy needed to raise one gram of water by 1 °C. • kcal = energy needed to raise 1000 g of water 1 °C. • food calories = kcals. Tro's "Introductory Chemistry", Chapter 3

Energy Use Tro's "Introductory Chemistry", Chapter 3

Exothermic Processes • When a change results in the release of energy it is called an exothermic process. • An exothermic chemical reaction occurs when the reactants have more chemical potential energy than the products. • The excess energy is released into the surrounding materials, adding energy to them. • Often the surrounding materials get hotter from the energy released by the reaction. Tro's "Introductory Chemistry", Chapter 3

Surroundings reaction Reactants Amount of energy released Potential energy Products An Exothermic Reaction Tro's "Introductory Chemistry", Chapter 3

Endothermic Processes • When a change requires the absorption of energy it is called an endothermic process. • An endothermic chemical reaction occurs when the products have more chemical potential energy than the reactants. • The required energy is absorbed from the surrounding materials, taking energy from them. • Often the surrounding materials get colder due to the energy being removed by the reaction. Tro's "Introductory Chemistry", Chapter 3

Surroundings reaction Products Amount of energy absorbed Potential energy Reactants An Endothermic Reaction Tro's "Introductory Chemistry", Chapter 3

Temperature Scales • Fahrenheit scale, °F. • Used in the U.S. • Celsius scale, °C. • Used in all other countries. • A Celsius degree is 1.8 times larger than a Fahrenheit degree. • Kelvin scale, K. • Absolute scale. Tro's "Introductory Chemistry", Chapter 3

Temperature Scales 100°C 373 K 212°F 671 R Boiling point water 298 K 75°F 534 R Room temp 25°C 0°C 273 K 32°F 459 R Melting point ice -38.9°C 234.1 K -38°F 421 R Boiling point mercury -183°C 90 K -297°F 162 R Boiling point oxygen BP helium -269°C 4 K -452°F 7 R -273°C 0 K -459 °F 0 R Absolute zero Celsius Kelvin Fahrenheit Rankine

Temperature Scales • The Fahrenheit temperature scale used as its two reference points the freezing point of concentrated saltwater (0 °F) and average body temperature (96 °F). • More accurate measure now sets average body temperature at 98.6 °F. • Room temperature is about 72 °F. Tro's "Introductory Chemistry", Chapter 3

Temperature Scales, Continued • The Celsius temperature scale used as its two reference points the freezing point of distilled water (0 °C) and boiling point of distilled water (100 °C). • More reproducible standards. • Most commonly used in science. • Room temperature is about 22 °C. Tro's "Introductory Chemistry", Chapter 3

Fahrenheit vs. Celsius • A Celsius degree is 1.8 times larger than a Fahrenheit degree. • The standard used for 0° on the Fahrenheit scale is a lower temperature than the standard used for 0° on the Celsius scale. Tro's "Introductory Chemistry", Chapter 3

The Kelvin Temperature Scale • Both the Celsius and Fahrenheit scales have negative numbers. • Yet, real physical things are always positive amounts! • The Kelvin scale is an absolute scale, meaning it measures the actual temperature of an object. • 0 K is called absolute zero. It is too cold for matter to exist because all molecular motion would stop. • 0 K = -273 °C = -459 °F. Tro's "Introductory Chemistry", Chapter 3

Kelvin vs. Celsius • The size of a “degree” on the Kelvin scale is the same as on the Celsius scale. • The 0 standard on the Kelvin scale is a much lower temperature than on the Celsius scale. • When converting between kelvins and °C, remember that the kelvin temperature is always the larger number and always positive! Tro's "Introductory Chemistry", Chapter 3

Energy and the Temperature of Matter • The amount the temperature of an object increases depends on the amount of heat energy added (q). • If you double the added heat energy the temperature will increase twice as much. • The amount the temperature of an object increases depending on its mass. • If you double the mass, it will take twice as much heat energy to raise the temperature the same amount. Tro's "Introductory Chemistry", Chapter 3

Law of Conservation of Energy