Chapter 17 Thermochemistry

In what direction does heat flow?
heat always flows from a warmer object to a cooler object.
What happens in endothermic and exothermic processes?
in an endothermic process, the system gains heat as the surroundings cool down. in an exothermic process, the system loses heat as the surroundings heat up.
In what units is heat flow measured?
heat flow is measured in two common units, the calorie and the joule.
On what two factors does the heat capacity of an object depend?
the heat capacity of an object depends on both its mass and its chemical composition.
Thermochemistry
the study of energy changes that occur during chemical reactions and changes in state.
Chemical potential energy
the energy stored in the chemical bonds of a substance.
Heat (q)
energy that transfers from one object to another because of a temperature difference between them.
System
the part of the universe on which you focus your attention.
Surroundings
include everything else in the universe.
Law of conservation of energy
states that in any chemical or physical process, energy is neither created nor destroyed.
Endothermic process
one that absorbs heat from the surroundings.
Exothermic process
one that releases heat to its surroundings.
Heat capacity
the amount of heat needed to increase the temperature of an object exactly 1 degree C.
Specific heat
the amount of heat it takes to raise the temperature of 1g of the substance 1 degree C.
What basic concepts apply to calorimetry?
in calorimetry, the heat released by the system is equal to the heat absorbed by its surroundings. Conversely, the heat absorbed by a system is equal to the heat released by its surroundings.
How can you express the enthalpy change for a reaction in a chemical equation?
in a chemical equation, the enthalpy change for the reaction can be written as either a reactant or a product.
Calorimetry
the precise measurement of the heat flow into or our of a system for chemical and physical processes.
Calorimeter
the insulated device used to measure the absorption or release of heat in chemical or physical processes.
Enthalpy
the heat content of a system at constant pressure.
Thermochemical equation
a chemical equation that includes the enthalpy change.
Heat of reaction
the enthalpy change for the chemical equation exactly as it is written.
Heat of combustion
the heat of reaction for the complete burning of one mole of a substance.
How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies?
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, delta Hfus = -delta Hsolid.
How does the quantity of heat absorbed by a vaporizing liquid compare to the quantity of heat released when the vapor condenses?
the quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, delta Hvap = -delta Hcond.
What thermochemical changes can occur when a solution forms?
during the formation of a solution, heat is either released or absorbed.
Molar heat of fusion (delta Hfus)
the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature.
Molar heat of solidification (delta Hsolid)
the heat lost whem one mole of a liquid solidifies at a constant temperature.
Molar heat of vaporization (delta Hvap)
the amount of heat necessary to vaporize one mole of a given liquid.
Molar heat of condensation (delta Hcond)
the amount of heat released when one mole of vapor condenses at the normal boiling point.
Molar heat of solution (delta Hsoln)
the enthalpy change caused by dissolution of one mole of substance.
What are two ways that you can determine the heat of reaction when it cannot be directly measured?
Hess’s law allows you to determine the heat of a reaction indirectly.
Hess’s law of heat summation
states that if you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction.
Standard heat of formation (delta Hf^0)
the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states at 25 degrees C. (delta Hf^0=0)
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