CALCULATING HEATS OF REACTION
OBJECTIVES•Apply Hess’s law of heat summation to find enthalpy changes for chemical and physical processes
•Calculate enthalpy changes using standard heats of formation
Requisite SkillsAlgebra I & II: System of
Equations
VOCABULARY:oHess’s law of heat of summationoStandard heat of formation
. . . manipulate and combine algebraic equations.
In algebra I, you learned that you can combine the like terms from two equations to make a third equation.
2x + 6y = 16
y = 2x + 12+
2x + 7y = 2x + 28
If you do this right, this allows you to solve for one of the variables by eliminating the other one.
7y = 28
y = 4
Once you know of the variables, you can plug it in and solve for the other variable.
4 = 2x + 12 4 - 12 = 2x + 12 - 12 -8 = 2x
x = -4
Sometimes, you need to flip one of the equations before adding them to get one of the terms to disappear.
9y = 3x - 57
2y + 22 = 3x +If we flip the second equation, and THEN add the two equations, the “x” terms will disappear.
9y = 3x - 57
3x = 2y + 22
3x + 9y = 3x + 2y - 35
9y = 2y - 35
7y = -35
y = -5
2(-5) + 22 = 3x
-10 + 22 = 3x
12 = 3x
x = 4
. . . manipulate and combine algebraic equations.
4x + 23y = 4x + 69
Other times, you need to multiply an equation by an integer to get what you want.
4y = x + 5
4x + 7y = 49 +
Even if we flip one of the equations, nobody disappears after adding the equations, but if we multiply the first equation by four first . . .
16y = 4x + 20
4x + 7y = 49
23y = 69
y = 34(3) = x + 5
12 = x + 5
x = 7
. . . manipulate and combine algebraic equations.
Sometimes, you can use these old algebra tricks with chemical equations to solve for unknowns . . .
. . . manipulate and combine algebraic equations.
Would you like Diamond to be converted into Graphite?
Such conversion or reaction will take millions of years to complete
And chemists are curious to know the enthalpy changes for the conversion of diamond to graphite.
Should they wait million of years when the reaction will be completed to get the data? Or
What do you recommend they should do? So that the enthalpy changes data would be available ASAP
Hess’s law
Mr. Hess came up with a quick solution of how to calculate heat of reaction, ΔHrxn for:
1. reactions that are too slow2. reactions that have
intermediate steps3. reactions that are dangerous4. reactions that are not very
useful
Hess’s law Heat of SummationStates that if you add to two or
more thermochemical equations to give final equation, then you can also add the heat of reaction to give the final heat of reaction
Hess’s law allows us to determine the heat of reaction indirectly.
Operations in Hess’s lawAny of these could be applied:Flipping a known chemical equationChanging the sign of the ΔH (-ve or
+ve) when you flipped Adding two or more chemical
equationsSubtracting two or more chemical
equationsMultiplying a chemical equation with
a whole number or fractionDividing a chemical equation with a
whole number
SWBAT . . .
C (s, diamond) = C (s, graphite) ΔH= ?kJ
a) C (s, graphite) + O2 (g) = CO2 (g) ΔH= -393.5kJ
b) C (s, diamond) + O2 (g) = CO2 (g) ΔH= -395.4kJ
CO2 and O2 need to be cancelled out to achieve the desired reaction we are looking for.
We need graphite to be on the product side, so we are going to flip equation (a)
C (s, diamond) + O2 (g) CO2 (g) ΔH= -395.4kJ
CO2 (g) C(s, graphite) + O2 (g) ΔH= 393.5kJ
C(s, diamond) C(s, graphite) ΔH= (-395.4 + 393.5)kJ
ΔH= (393.5 – 395.4)kJ
ΔH= - 1.9kJ
. . . use Hess’ law to determine DHrxn
Hess’s law allows us to figure out DHrxn for a reaction without
ever having to make the reaction happen in real life.
Sn(s) + Cl2(g) SnCl2(s) DH = -325.1 kJ
SnCl2(s) + Cl2(g) SnCl4(l) DH = -186.2 kJ
HLEx1: Imagine that experiments tell you the following:
If you add the two equations above, you get the following:
Sn(s) + SnCl2(s) + 2Cl2(g) SnCl2(s) + SnCl4(l)Just as with normal, algebraic equations, when the same term appears on the left and right, it can be crossed out . . . yielding . . .
Sn(s) + 2Cl2(g) SnCl4(l) DH = ? kJ
. . . use Hess’ law to determine DHrxn
Sn(s) + Cl2(g) SnCl2(s) DH = -325.1 kJ
SnCl2(s) + Cl2(g) SnCl4(l) DH = -186.2 kJ
Sn(s) + 2Cl2(g) SnCl4(l) DH = ? kJ
Now here’s the real magic. Hess’s law says that you can also add the DH’s for the reactions to get the DH for the final reaction.
(-325.11 kJ) + (-186.2 kJ) = -511.3 kJ
DH = -511.3 kJ
The sneaky miracle here is that we figured this out without ever having to make tin metal and chlorine gas react to form tin (IV) chloride in real life.
. . . use Hess’ law to determine DHrxn
That was a very simple usage of Hess’ law. We didn’t have to manipulate any equations before adding them. Let’s try a harder problem.
. . . use Hess’ law to determine DHrxn
HLEx2: Let’s say experiments have told us the following:
Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
OsO4(cr) OsO4(g) DH = 56.4 kJ
Use Hess’s Law to figure out DH for the following reaction:
Os(cr) + 2O2(g) OsO4(cr) DH = ? kJ
(The final equation looks a lot like the first equation, but notice that
OsO4 is a gas in the first equation and a crystal in the third.)What would you have to do to figure this one out? Think a moment . . .
. . . use Hess’ law to determine DHrxn
HLEx2:
OsO4(g) is in both of the initial equations, but doesn’t appear in
the final equation, so it needs to be eliminated somewhow.OsO4(g) is on the right on both equations, so they won’t cancel
each other out if you add the equations as they are.One filthy little trick you can do is to flip the second equation to put
OsO4(g) on the left, and THEN add the two equations together.
Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
OsO4(cr) OsO4(g) DH = 56.4 kJ
Os(cr) + 2O2(g) OsO4(cr) DH = ? kJ
OsO4(g) OsO4(cr) DH = -56.4 kJ
. . . use Hess’ law to determine DHrxn
HLEx2: Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
OsO4(cr) OsO4(g) DH = 56.4 kJ
Os(cr) + 2O2(g) OsO4(cr) DH = ? kJ
Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
Notice how the sign of DH changed on the equation that we
flipped. Remember that DHsolid = -DHfus and DHcond = -DHvap
Freezing is the opposite of melting and condensation is the opposite of vaporization, so their DH’s have opposite signs.
OsO4(cr) OsO4(g) DH = 56.4 kJ
OsO4(g) OsO4(cr) DH = -56.4 kJ
. . . use Hess’ law to determine DHrxn
HLEx2: Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
Os(cr) + 2O2(g) OsO4(cr) DH = ? kJ
Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
Now we can add the equations . . .Os(cr) + 2O2(g) + OsO4(g) OsO4(g) + OsO4(cr)
and simpify . . .
Now we have the equation we were looking for. Now what?
OsO4(cr) OsO4(g) DH = 56.4 kJ
OsO4(g) OsO4(cr) DH = -56.4 kJ
. . . use Hess’ law to determine DHrxn
HLEx2: Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
Os(cr) + 2O2(g) OsO4(cr) DH = ? kJ
Os(cr) + 2O2(g) OsO4(g) DH = -335 kJ
Add the DH’s to get the DH for the final equation.Os(cr) + 2O2(g) + OsO4(g) OsO4(g) + OsO4(cr)
DH = -335 kJ + (-56.4 kJ) = -391.4 kJ = DH
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17.1
17.1Q = m DT Cp
Problems
17.2
17.2 Calorimetry17.1 Temperature &
Heat
17.1 Endothermic & Exothermic
17.2Thermochemical
Equations
17.2Heat of
Combustion
17.3
17.3 Heating Curve for Water
17.3 State Change Math Problems
17.3 Heat of Solution Math
Problems
17.4
17.4 Alegbra Review
17.4 Hess’ Law
17.4 Standard Heat of Formation