CHAPTER 10 Energy

10.1 The Nature of Energy

THE NATURE OF ENERGY Energy is the ability to do work or produce heat. 2 Types Potential energy: energy due to position or composition. Example: Water behind a dam Kinetic energy: energy due to the motion of the object. Depends on the mass (m) and velocity (v) of the object KE = ½mv 2 Important Characteristic of energy: It is conserved!

THE NATURE OF ENERGY The Law of Conservation of Energy states that energy cannot be created or destroyed, only converted from one form to another. Similar to the Law of Conservation of Matter The energy of the universe is constant!! Important definitions for later… Work is the application of a force over a distance. A state function is a property of the system that changes independently of its pathway.

10.2 Temperature and Heat

TEMPERATURE AND HEAT Temperature is a measure of the random motions of the components of a substance. Example: Cold H 2 O- molecules moving slowly Hot H 2 O- molecules moving rapidly Symbol for temperature change: ΔT Heat can be defined as a flow of energy because of a temperature difference.

TEMPERATURE AND HEAT Heat example: Figure 10.2, 10.3 and 10.4 in book Energy from the hot water is transferred through the thin metal to the cold water and as the hot water cools down the cold water warms up. Energy is conserved so the energy lost by the hot water is the same as the energy gained by the cold water The final temperature is the average of the original temperatures: / 2 = 50 degrees C Temperature changes: ∆T hot = 90.°C – 50.°C = 40.°C ∆T cold = 50.°C – 10.°C = 40.°C

10.3 Exothermic and Endothermic Processes

EXOTHERMIC AND ENDOTHERMIC PROCESSES When considering energy, we must divide the universe into two parts: The system is part of the universe on which we wish to focus attention. The surroundings include everything else in the universe. Example: Striking a match and allowing it to burn System? Surroundings?

EXOTHERMIC AND ENDOTHERMIC PROCESSES Energy flow between system and surroundings Exothermic means that energy is being released (flows out of the system). Feels warm. Endothermic means that energy is being absorbed (flows into the system). Feels cold. Remember! The energy gained by the surroundings must be equal to the energy lost by the system. The energy lost by the surroundings must be equal to the energy gained by the system. In any exothermic reaction, some of the potential energy stored in the chemical bonds is converted to thermal energy (random kinetic energy) via heat.

Define the following as either exothermic or endothermic A hot pack being used by a hunter in the woods A cold pack being used by a lifeguard on a swimmers ankle Water boiling Methane reacting with oxygen in a lit Bunsen burner EXOTHERMIC AND ENDOTHERMIC PROCESSES

10.4 Thermodynamics

THERMODYNAMICS The study of energy is called thermodynamics. There are four laws of thermodynamics. The First Law of Thermodynamics is often called the law of conservation of energy and is stated as: The energy of the universe is constant. The internal energy, E, of a system can be defined most precisely as the sum of the kinetic and potential energies of all “particles” in the system. E = KE + PE

THERMODYNAMICS The energy of a system can be changed by a flow of work, heat, or both. Where Δ means a change in the function that follows q represents heat w respresents work

10.5 Measuring Energy Changes

MEASURING ENERGY CHANGES In the metric system, the calorie is defined as the amount of energy (heat) that is required to raise the temperature of one gram of water by one Celsius degree. The “calorie” with which you are probably familiar is used to measure the energy content of food and is actually a kilocalorie (1000 calories), written with a capital C (Calorie) to distinguish it from the calorie used in chemistry. The joule (an SI unit) can be most conveniently defined in terms of the calorie. 1 calorie = joules 1 cal = J

MEASURING ENERGY CHANGES The amount of energy required to change the temperature of one gram of a substance by one Celsius degree is called its specific heat capacity, or more commonly, its specific heat. Where q is the energy (heat) required m is the mass of the sample in grams c is the specific heat in J/g 0 C ΔT is the change in temperature in Celsius degrees (Tf-Ti) ‏

MEASURING ENERGY CHANGES Remember: Thermodynamic quantities always consist of two parts: a number a sign- indicates the direction of the energy flow Heat Example- when a quantity of energy flows into the system, q is (+) because the systems energy is increasing Example- when a quantity of energy flows out of the system, q is (-) because the systems energy is decreasing Work Example- if a system does work on its surroundings, w is (-) because energy is flowing out of the system Example- if surroundings do work on the system, w is (+) because energy is flowing into the system

10.6 Thermochemistry (Enthalpy) ‏

THERMOCHEMISTRY Enthalpy Enthalpy is a special energy function, designated by H. For a reaction occurring under conditions of constant pressure, the change in enthalpy ( ∆ H) is equal to the energy that flows as heat. Tells chemists how much energy is produced (released) or absorbed in a chemical reaction. The enthalpy change for a reaction (that occurs at constant pressure) is the same as the heat for that reaction.

THERMOCHEMISTRY Calorimetry is the science of measuring heat and is based on observing the temperature change when a body absorbs or discharges energy as heat. A calorimeter is a device used to determine the heat associated with a chemical reaction. How does it work? A reaction is run in the calorimeter and the temperature change of the calorimeter is observed Knowing the temperature change and the heat capacity in the calorimeter enables us to calculate the heat energy released or absorbed by the reaction Helps determine ∆H

10.7 Hess’s Law

HESS’S LAW One of the most important characteristics of enthalpy is that it is a state function. That is, the change in enthalpy for a given process is independent of the pathway for the process. In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. This principle is known as Hess’s Law.

CHARACTERISTICS OF ENERGY CHANGES To use Hess’s Law to compute enthalpy changes for reactions, it is important to understand two characteristics of Δ H for a reaction: 1. If a reaction is reversed, the sign of Δ H is also reversed. 2. The magnitude of Δ H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of Δ H is multiplied by the same integer.

10.8 Quality vs. Quantity of Energy

QUALITY vs. QUANTITY OF ENERGY Most important characteristic of energy it is conserved Total energy of universe has always been and will always be the same So why should we worry about conserving energy such as petroleum? The energy crisis is not about quantity of energy… but about quality When we utilize energy to do work, we degrade its usefulness

Example: Automobile trip from Chicago to Denver Need gas along the way… what happens to this energy? Gasoline (l) + O 2 (g)  CO 2 (g) + H 2 O (l) + energy The gasoline combines with oxygen and is changed to thermal energy The total quantity of energy remains the same as before the trip but the energy concentrated in the gasoline becomes widely distributed in the environment QUALITY vs. QUANTITY OF ENERGY

Concentrated energy  Spread energy Work Where do we get most of our fuel for energy? PETROLEUM! Petroleum is formed over very long periods of time through simple plants and animals absorbing energy from the sun and using this energy to construct molecules. As these organisms died and became buried, natural processes changed them into petroleum deposits. This is our oil READ: Energy and Our World (10.9)  you are responsible for understanding the information in the section. QUALITY vs. QUANTITY OF ENERGY

10.10 Energy as a Driving Force

ENTROPY Entropy, a state function, is a measure of the disorder or randomness. Entropy is designated by S. As randomness increases, S also increases. The Second Law of Thermodynamics states that the entropy of the universe is always increasing. A spontaneous process is one that occurs in nature without outside intervention – it happens “on its own.”