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ThermochemistryChapter 17
The temperature of lava from a volcano ranges from 550°C to 1400°C. As lava flows, it loses heat and begins to cool. You will learn about heat flow and why some substances cool down or heat up more quickly than others.
The Flow of Energy—Heat and Work
17.1
Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state.
Every substance has a certain amount of energy stored in chemical bonds, and this is called chemical potential energy.
Energy Transformations17.1
◦ During chemical reactions bonds break and form, so there is a change in chemical potential energy. This results in energy being absorbed or released as heat.
◦ During changes of state heat is absorbed or released causing molecules to move closer together or farther apart. This results in a change in potential energy.
Energy Transformations
Heat, represented by q, is the energy that transfers from one object to another because of a temperature difference between them.
Heat always flows from a warmer object to a cooler object.
Energy Transformations
◦ The law of conservation of energy states that in any chemical or physical process, energy is neither created nor destroyed. Therefore heat is not created or destroyed, it merely
transfers from one substance to another.
In an endothermic process, heat is absorbed.
In an exothermic process heat is released.
Exothermic and Endothermic Processes
17.1
An endothermic process is one that absorbs heat from the surroundings.
Exothermic and Endothermic Processes
17.1
An exothermic process is one that releases heat to its surroundings.
Exothermic and Endothermic Processes
17.1
Heat flow is measured in two common units, the calorie and the joule.◦ The energy in food is usually expressed in
Calories.◦ In this course we will use the Joule (J)
Ref. table D
Units for Measuring Heat Flow17.1
The amount of heat needed to increase the temperature of an object exactly 1°C is the heat capacity of that object.
◦ The heat capacity of an object depends on both its mass and its chemical composition.
Heat Capacity and Specific Heat
17.1
The specific heat capacity (C), 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 heat of water = 4.18 J/g-°C see ref. table B
Heat Capacity and Specific Heat
17.1
Water releases a lot of heat as it cools. During freezing weather, farmers protect citrus crops by spraying them with water.
Heat Capacity and Specific Heat
17.1
Heat Capacity and Specific Heat
17.1
Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.
Heat Capacity and Specific Heat
17.1
A burning match releases heat to its surroundings in all directions. How much heat does this exothermic reaction release? You will learn to measure heat flow in chemical and physical processes by applying the concept of specific heat.
Measuring and Expressing Enthalpy Changes
17.2
Calorimetry◦ Calorimetry is the
precise measurement of the heat flow into or out of a system for chemical and physical processes.
◦ The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter.
17.2
◦ In calorimetry, the assumption is made that there is a total heat transfer between the system and the surroundings. No heat is lost.
Ref. table T – heat transfer equation:
q = heat absorbed or released (J) M = mass of water C= specific heat ΔT = temperature change
Calorimetry17.2
q = m x C x ΔT
◦ The heat content of a system at constant pressure is the same as a property called the enthalpy (H) of the system. Most ordinary reactions take place at constant
pressure, so the terms enthalpy and heat are interchangeable.
Calorimetry17.2
Nutritionists use a bomb calorimeter to measure the heat (energy) content of foods.
Calorimetry17.2
A chemical equation that includes the enthalpy change is called a thermochemical equation.
Thermochemical Equations17.2
In exothermic reactions, heat is given off to the surroundings, and shown as a product.
In endothermic reactions, heat is taken in from the surroundings, and is shown as a reactant.
Thermochemical Equations17.2
The heat of reaction is the enthalpy change (∆H) for the chemical equation.
Exothermic reactions have a negative heat of reaction, and endothermic reactions have a positive heat of reaction.
See reference table I
Thermochemical Equations17.2
Thermochemical Equations17.2
Exothermic Reaction
Thermochemical Equations17.2
Endothermic Reaction
The heat of combustion is the heat of reaction for the complete burning of one mole of a substance.
Thermochemical Equations17.2
During a race, an athlete can burn a lot of calories that either do work or are released as heat. This section will help you to understand how the evaporation of sweat from your skin helps to rid your body of excess heat.
Heat in Changes of State17.3
The heat of fusion (Hf) is the heat absorbed by one gram of a solid substance as it melts to a liquid at a constant temperature.
The heat of solidification (Hs) is the heat lost when one mole of a liquid solidifies at a constant temperature.
For water:Hf = 334 J/g
Heats of Fusion and Solidification
◦ The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies; that is, Hf = -Hs
◦ To calculate heat gained during melting:q = mHf
Reference table T
Heats of Fusion and Solidification
17.3
The amount of heat necessary to vaporize one gram of a given liquid is called its heat of vaporization (Hv).
The amount of heat released when 1 gram of vapor condenses at the normal boiling point is called its heat of condensation (Hc).
For water: Hv= 2260 J/g
Heats of Vaporization and Condensation
17.3
◦ The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses.
◦ Hv = –Hc
◦ To calculate heat gained during boiling: q = mHv
Reference table T
Heats of Vaporization and Condensation
17.3
Enthalpy changes accompany changes in state.
Heats of Vaporization and Condensation
17.3
Heats of Vaporization and Condensation
17.3
◦ During the formation of a solution, heat is either released or absorbed. The enthalpy change caused by dissolution of one
mole of substance is the molar heat of solution (∆Hsoln).
Heat of Solution17.3
When ammonium nitrate crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve.
Heat of Solution17.3
