Download Note Guide 7-4

Note Guide 7-4
•Branch of chemistry that studies energy changes which take
place during reactions and changes in state of matter =
thermochemistry
•Potential energy = energy of position/stored energy. But in
chemistry we have chemical potential energy = energy stored
in the chemical bonds of a substance
--how much energy stored is determined by kinds of atoms
and how they are arranged.
•Heat(q) = energy that transfers from one object to another due
to a difference in temperature between them
--how does heat transfer? From the WARMER to the
COLDER OBJECT
•Law of conservation of energy =matter will neither be
created nor destroyed, including chemical processes.
--this means if a system gives off heat (loses), it must go
somewhere, therefore it goes into the surrounding area of the
system.
--2 Process
1. Endothermic – system gains heat as the surroundings cool
off.
--heat is absorbed
EX: the person is
absorbing the heat
given off
2. Exothermic – system loses heat as the surroundings heat up
--giving off heat.
EX: the body (system)
loses heat to the
surroundings (skin/air)—
sweat/perspire
•Heat movement is measured 2 ways
1. Calories – quantity of heat needed to raise the temp.
of 1 g of pure water 1 deg. C
2. Joule – SI unit of energy.
•Heat capacity = how much heat is needed to raise the temp
of an object 1 deg C.
--depends on
a. Mass – more mass of the object, more heat it can
.
hold
b. Chemical make-up – different substance can have
diff. Heat capacity even though mass may be equal.
--of course, we cant call it just heat capacity, we have to
change the name = specific heat capacity
--amount of heat needed to raise the temp of 1 g of the stuff
1 deg C.
--COPY FORMULA FROM PAGE 509
7-4 Note Guide, part deux
•Enthalpy = heat content of a system at constant pressure
•In a chemical reaction, the enthalpy change for the reaction
can be written as either a reactant or a product.
--calcium oxide and water react as an exothermic reaction
CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ
•Heat of reaction = enthalpy change for the chemical
equation exactly as it is written
--heats of reaction usually shown as ΔH
CaO(s) + H2O(l)  Ca(OH)2(s) ΔH =-65.2kJ
--other reactions are endothermic.
Baking soda decomposes when heated
2NaHCO3(s) + 129 kJ  Na2CO3(s) +H2O(g) + CO2(g)
ΔH=129kJ
--The amount of heat released or absorbed during a reaction
depends on the number of moles of the reactants invovled
Pg. 516 Sample problem 17.3
•Heat of combustion = heat of reaction for the complete
burning of one mole of a substance.
Normally, the actual yield in a chemical reaction will
be equal to or less than the theoretical yield.
1. Always True
2. Sometimes True
3. Never True
So
0%
Tr
ue
er
ev
N
m
et
im
es
Tr
ue
s
ay
lw
A
0%
Tr
ue
0%
The amount of product can be determined from the
amount of excess reagent.
1. Always True
2. Sometimes True
3. Never True
So
0%
Tr
ue
er
ev
N
m
et
im
es
Tr
ue
s
ay
lw
A
0%
Tr
ue
0%
What would likely happen if you were to touch the flask in
which an endothermic reaction were occuring?
0%
1.
0%
2.
0%
3.
0%
4.
The flask would probably feel cooler than before
the reaction started.
The flask would probably feel warmer than before
the reaction started.
The flask would feel the same as before the
reaction started.
None of the above
What does the symbol ΔH stand for?
0%
0%
0%
0%
1. The specific heat of a substance
2. The heat capacity of a substance
3. The heat of reaction for a chem. reaction
4. One Calorie given off by a reaction
The amount of heat released by the complete burning of 1
mol of a substance is the
0%
0%
0%
0%
1.
2.
3.
4.
Specific heat
Heat of combustion
Heat capacity
Heat of fusion.