Chapter 10 Energy

10 | 2 Energy and Energy Changes Energy: ability to do work or produce heat –Chemical, mechanical, thermal, electrical, radiant, sound, nuclear –Potential and kinetic Energy may affect matter. –e.g. Raise its temperature, eventually causing a state change, or cause a chemical change such as decomposition All physical changes and chemical changes involve energy changes.

10 | 3 Energy and Energy Changes Potential Energy: energy due to composition or position Kinetic Energy: energy due to motion – - ½ mv 2

10 | 4 Energy and Energy Changes (cont.) Law of Conservation of Energy: energy can be converted from one form to another, but cannot be created or destroyed

10 | 5 Work and Energy Work: force acting over a distance –w = f d –Work done on a system will increase the energy of the system, whereas work done by the system will decrease the energy of the system State function: a property that changes independent of pathway

10 | 6 Temperature and Heat Heat: a flow of energy due to a temperature difference Temperature: a measure of the random motions of the components of a substance

10 | 7 Temperature and Heat (cont.)

10 | 8 Exothermic vs. Endothermic System: that part of the universe that we wish to study Surroundings: everything else in the universe Exothermic process: a process that results in the evolution of heat -Example: when a match is struck, it is an exothermic process because energy is produced as heat. Endothermic process: absorbs heat -Example: melting ice to form liquid water is an endothermic process because the ice absorbs heat in order to melt

10 | 9 Exothermic Process

10 | 10 Thermodynamics The Law of Conservation of Energy is also known as The First Law of Thermodynamics. It can be stated as “the energy of the universe is constant.” Internal Energy (E) = kinetic energy + potential energy ΔE = q + w = change in internal energy q = heat absorbed by the system w = work done on the system

10 | 11 Units of Energy One calorie = amount of energy needed to raise the temperature of one gram of water by 1°C –kcal = energy needed to raise the temperature of 1000 g of water 1°C joule –4.184 J = 1 cal In nutrition, calories are capitalized. –1 Cal = 1 kcal

10 | 12 Example - Converting Calories to Joules Convert 60.1 cal to joules.

10 | 13 Energy & Temperature of Matter The amount the temperature of an object increases depends on the amount of heat added (q). –If you double the added heat energy the temperature will increase twice as much. The amount the temperature of an object increases when heat is added depends on its mass –If you double the mass it will take twice as much heat energy to raise the temperature the same amount.

10 | 14 Specific Heat Capacity Specific heat (s): the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius Amount of Heat = Specific Heat x Mass x Temperature Change Q = s x m x  T

10 | 15 Specific Heat Capacity

10 | 16 Calculate the amount of heat energy (in joules) needed to raise the temperature of 7.40 g of water from 29.0°C to 46.0°C. Example #1:

10 | 17 Mass = 7.40 g Temperature change = 46.0°C – 29.0°C = 17.0°C Q = s m  T Example #1 (cont.) Specific heat of water = C g J 

10 | 18 A 1.6 g sample of metal that appears to be gold requires 5.8 J to raise the temperature from 23°C to 41°C. Is the metal pure gold? Example #2

10 | 19 Example #2 Table 10.1 lists the specific heat of gold as 0.13 Therefore the metal cannot be pure gold.

10 | 20 Enthalpy Change in enthalpy (ΔH p = q p ): the amount of heat exchanged when heat exchange occurs under conditions of constant pressure Enthalpy is a state function ΔH is independent of the path taken

10 | 21 Hess’s Law: in going from a set of reactants to a set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. Hess’s Law

10 | 22 Overall reaction: N 2 + 2O 2 2NO 2 ΔH = 68 kJ This reaction can be carried out in 2 steps: N 2 + O 2 2NO ΔH = 180 kJ 2NO + O 2 2NO 2 ΔH = -112 kJ N 2 + 2O 2 2NO 2 ΔH = 68 Kj Note: the sum of the two reactions gives the overall reaction and the same is true for the sum of the enthalpy change values. Hess’s Law (cont.)

10 | 23 Calorimetry The amount of heat flow transferred during a reaction is determined from temperature measurements made in a calorimeter.

10 | 24 Calorimetry (cont.)

10 | 25 Energy Quality & Quantity While the total amount or quantity of energy in the universe is constant (1st Law) the quality of energy is degraded as it is used. Burning of petroleum: High grade concentrated energy Low grade energy (heat)

10 | 26 Fuels Petroleum –A fossil fuel composed mainly of hydrocarbons Natural gas –Consists largely of methane –Also contains ethane, propane, and butane

10 | 27 Fuels (cont.)

10 | 28 Fuels (cont.)

10 | 29 Fuels (cont.) Coal –Matures geologically through stages

10 | 30 Global Warming

10 | 31 Global Warming (cont.)

10 | 32 Energy Use and Sources

10 | 33 Energy as a Driving Force Most processes that occur spontaneously involve an “energy spread.” –Heat flows from high to low temperature and “spreads” …or a “matter spread” –Salt dissolves or “spreads” in water

10 | 34 Entropy Entropy (S) is a measure of disorder or randomness. –As a system becomes more disordered, ΔS >0 Second Law of Thermodynamics: the entropy of the universe is always increasing.