17.4 Calculating Heats of Reaction

17.4Calculating Heats of Reaction

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


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


Sometimes, you can use these old algebra tricks with chemical equations to solve for unknowns . . .


Which stone do you prefer? And why?

Diamond Graphite

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


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.


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.


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 . . .


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(g) OsO4(cr) DH = -56.4 kJ


HLEx2: Os(cr) + 2O2(g) OsO4(g) DH = -335 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.






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?






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

. . . use standard heats of formation to determine DHrxn

[Section under construction]

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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

Documents

17.4 Calculating Heats of Reaction