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Unformatted text preview: el J. Sailor, UC San Diego Internal Energy vs Enthalpy
Chemical reactions are usually performed in open
containers, so it is difficult to measure w, the work
performed on the atmosphere.
We define the term enthalpy
ΔH (the change in
(H) to eliminate the need to
enthalpy) is separately
consider PΔV workthe
when calculating internal
heat gained or lost
energy changes in reactions: when a reaction is
ΔH = ΔE +
for reactions performed at
ΔH = q
Chem 6A Michael J. Sailor, UC San Diego Example: endothermic process
Water evaporation is an endothermic process. Your body
(the surroundings) gets colder when water on it (the system)
your body heat (q)
ΔH www.ehow.com q > 0 (+)
ΔH > 0 (+) Heat flows from the surroundings to the
system: an endothermic process Chem 6A Michael J. Sailor, UC San Diego Example: exothermic process
Water condensation is an exothermic process. Your body
(the surroundings) gets a burn when hot water vapor (the
system) condenses on it.
your body heat (q) q < 0 (-) ΔH ΔH < 0 (-) Heat flows from the system to the
surroundings: an exothermic process Chem 6A Michael J. Sailor, UC San Diego www.roadfood.com Chem 6A Michael J. Sailor, UC San Diego Heat Capacity
Heat Capacity is the relationship between heat absorbed
and temperature change in a substance.
Specific heat capacity:
quantity of heat required to
change the temperature of 1
gram of a substance by 1 K.
How much heat does it take to
warm up your jacuzzi?
www.clearleadinc.com Heat capacity of water: 4.184 J g -1 K-1
1000 of 1000 g 4.184
Volume L jacuzzi: 1000 L J ( 39 $ 20)K
= 79,500 kJ
Initial temperature: 20 °CK
Final temperature: 39 °C
Chem 6A Michael J. Sailor, UC San Diego Measuring the enthalpy of a reaction:
The chemical transformation involved in
medical hot packs is the dissolution of
calcium chloride in water. The enthalpy
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This note was uploaded on 11/16/2011 for the course CHEM 6A taught by Professor Pomeroy during the Spring '08 term at UCSD.
- Spring '08