Thermochemistry Energy

Glossary Terms thermochemistry: the study of energy changes that occur during chemical reactions and changes in state chemical potential energy: energy stored in chemical bonds heat (q): energy that transfers from one object to another because of a temperature difference between the objects system: a part of the universe on which you focus your attention surroundings: everything in the universe outside the system Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Glossary Terms law of conservation of energy: in any chemical or physical process, energy is neither created nor destroyed endothermic process: a process that absorbs heat from the surroundings exothermic process: a process that releases heat to its surroundings heat capacity: the amount of heat needed to increase the temperature of an object exactly 1°C specific heat: the amount of heat needed to increase the temperature of 1 g of a substance 1°C; also called specific heat capacity Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Energy Transformations Energy is the capacity for doing work or supplying heat. Unlike matter, energy has neither mass nor volume. Energy is detected only because of its effects. Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

1st Law of Thermodynamics Energy is always conserved. It can not be created or destroyed.

2nd Law of Thermodynamics Energy tends to disperse or spread out.

ENERGY (E) Described as the capacity for doing work.

WORK The application of force through distance. W = F x d W = work d = distance

FORMS OF ENERGY Potential energy – energy of position ( stored energy). Kinetic energy – energy of motion ( energy that is being used).

TYPES OF ENERGY Kinetic Potential Mechanic X X Electric X X Nuclear X X Radiant X Chemical X

Energy Transformations Every substance has a certain amount of energy stored inside it. The energy stored in the chemical bonds of a substance is called chemical potential energy. The kinds of atoms and the arrangement of the atoms in a substance determine the amount of energy stored in the substance. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Energy Transformations Every substance has a certain amount of energy stored inside it. When you buy gasoline, you are actually buying the stored potential energy it contains. The controlled explosions of the gasoline in a car’s engine transform the potential energy into useful work, which can be used to propel the car. Heat is also produced, making the car’s engine extremely hot. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Law of Conservation of Matter and Energy Matter and energy are interchangeable, the total matter and energy in the universe is constant. Matter can never be destroyed but converted from one form to another. Example: Burn a log. What kinds of energy is being converted when a log burns?

Heat The energy transferred between two objects as a consequence of the difference in temperature between the two objects. Energy in transit. Measure as a quantity of energy (calories) Depends on mass. Heat flows from a region of high heat intensity to a region of low heat intensity.

Thermal Equilibrium When heat flow between two objects is equal, therefore both objects are of equal temperature. No transfer of heat. No change in phases. Opposing tendencies are equal. Thermal: pertains to heat and also temperature.

Temperature The measure of an object ability to transfer heat to, or acquire heat from other object. If two objects are in direct contact, energy flow from one to other. Temperature is the property that determines the heat transfer. Measure as the heat intensity (degrees) of matter.

Example of Temperature and Heat Interaction If you put your hand in ice water and then in cool water How would your hand feel? It would feel warm. But if you put our hand in hot water first and then the cool water. How would your hand feel? It would feel cold. Explain Temperature depends on the transfer of heat energy to the hand or away. If a body of matter (object), has a greater temperature than its surrounding, energy flows from the object.

Example of Temperature A burning match and a camp fire may both be at the same temperature, but the quantities of heat given off are different.

Measuring Temperature Warm expand and cool contract. Mercury thermometer is based on this concept.

Temperature Scales Temperature Scales are Fahrenheit, Celsius, and Kelvin. have reference points for the boiling and freezing points of water. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Three Temperature Scales Fahrenheit – normally used in U.S., freezing point of water 32° F and boiling point 212 ° F. Celsius – most used by scientists, freezing point of water 0 ° C and 100° C. Kelvin – absolute temperature 273 K. The Celsius scale is determined by dividing the interval between 0° C and 100° C into 100 equal points.

Endothermic and Exothermic Processes What happens to the energy of the universe during a chemical or physical process? Chemical reactions and changes in physical state generally involve either the absorption or the release of heat. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Endothermic and Exothermic Processes You can define a system as the part of the universe on which you focus your attention. Everything else in the universe makes up the surroundings. Together, the system and its surroundings make up the universe. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Endothermic and Exothermic Processes Direction of Heat Flow The direction of heat flow is given from the point of view of the system. Heat is absorbed from the surroundings in an endothermic process. Heat flowing into a system from its surroundings is defined as positive; q has a positive value. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Endothermic and Exothermic Processes Direction of Heat Flow The direction of heat flow is given from the point of view of the system. An exothermic process is one that releases heat to its surroundings. Heat flowing out of a system into its surroundings is defined as negative; q has a negative value. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Endothermic and Exothermic Processes In an endothermic process, heat flows into the system from the surroundings. In an exothermic process, heat flows from the system to the surroundings. In both cases, energy is conserved. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Specific Heat Capacity Substances with low specific heat capacities can heat up and cool down easily. Which substance will warm up the fastest?

Specific Heats of Some Common Substances Interpret Data The specific heat capacity, or simply the specific heat, of a substance is the amount of heat it takes to raise the temperature of 1 g of the substance 1°C. Specific Heats of Some Common Substances Substance Specific heat J/(g·°C) cal/(g·°C) Liquid water 4.18 1.00 Ethanol 2.4 0.58 Ice 2.1 0.50 Steam 1.9 0.45 Chloroform 0.96 0.23 Aluminum 0.90 0.21 Iron 0.46 0.11 Silver 0.24 0.057 Water has a very high specific heat compared with the other substances. Metals generally have low specific heats. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Measuring Heat Calorimetry – the measurement of the thermal properties of matter. Thermal properties of matter are: Heat of solution Heat of combustion Heat of formation Heat of reaction

Units of Heat Calorie (cal) – the unit of heat energy commonly used in calorimetric. The quantity of heat required to raise the temperature of 1 gram of water through 1 Celsius degree. Very small unit. Kilocalorie (kcal) – most often used. The quantity of heat required to raise the temperature of 1 kilogram of water through 1 Celsius degree. 1 kcal = 103 cal

Calorie is defined as auxiliary heat unit in terms of a dimensional unit of energy, E. Kinetic energy (EK) – is determine by the mass of the moving body and its velocity (speed). EK = ½ mv2 m = mass (kg) v = speed (length/time – unit are meter/sec) Mass x (length/time) ml2/t2 EK = Kgm2/s2 is called a joule (J) 1 cal = 4.18 J 1kcal = 4.18 kJ

Heat Energy Conversion Identical units common to both numerator and denominator cancel. cal = J x 1cal/4.18J J = 4.18J/1cal Example: Given = 23.8 cal J = 23.8 cal x 4.18J/1cal J = 99.7J Given = 99.7J cal = 99.7J x 1cal/4.18J cal = 23.8cal

Problems 45.6 cal to J 54.8 J to cal 33.1 kcal to kJ 85.5 kJ to kcal

Energy Equations Energy (q) = m x ΔT x (Cp) M = mass (g) ΔT = final temperature – initial temperature (°C) Cp = specific heat capacity (J/(g·°C) or cal/(g·°C)). ΔT = q/ m x Cp Cp = q/ m x ΔT m = q/C x ΔT q = m x heat of fusion or vaporization (phase change)

Heat Capacity and Specific Heat Calculating Specific Heat C = q m  ΔT = mass (g)  change in temperature (°C) heat (J or cal) q is heat, expressed in terms of joules or calories. m is mass. ΔT is the change in temperature. ΔT = Tf – Ti The units of specific heat are either J/(g·°C) or cal/(g·°C). Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Calculating the Specific Heat of a Substance Sample Problem 17.2 Calculating the Specific Heat of a Substance The temperature of a 95.4-g piece of copper increases from 25.0°C to 48.0°C when the copper absorbs 849 J of heat. What is the specific heat of copper? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Analyze List the knowns and the unknown. Sample Problem 17.2 Analyze List the knowns and the unknown. 1 Use the known values and the definition of specific heat. KNOWNS mCu = 95.4 g ΔT = (48.0°C – 23.0°C) = 23.0°C q = 849 J UNKNOWN C = ? J/(g·°C) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Calculate Solve for the unknown. Sample Problem 17.2 Calculate Solve for the unknown. 2 Start with the equation for specific heat. CCu = q mCu  ΔT Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Calculate Solve for the unknown. Sample Problem 17.2 Calculate Solve for the unknown. 2 Substitute the known quantities into the equation to calculate the unknown value CCu. CCu = = 0.387 J/(g·°C) 849 J 95.4 g  23.0oC Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Evaluate Does the result make sense? Sample Problem 17.2 Evaluate Does the result make sense? 3 Remember that liquid water has a specific heat of 4.18 J/(g·°C). Metals have specific heats lower than water. Thus, the calculated value of 0.387 J/(g·°C) seems reasonable. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

The specific heat of ethanol is 2. 4 J/(g·°C) The specific heat of ethanol is 2.4 J/(g·°C). A sample of ethanol absorbs 676 J of heat, and the temperature rises from 22°C to 64°C. What is the mass of ethanol in the sample? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

The specific heat of ethanol is 2. 4 J/(g·°C) The specific heat of ethanol is 2.4 J/(g·°C). A sample of ethanol absorbs 676 J of heat, and the temperature rises from 22°C to 64°C. What is the mass of ethanol in the sample? C = q m  ΔT m = q C  ΔT m = = 6.7 g ethanol 676 J 2.4 J/(g·°C)  (64°C – 22°C) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.