Transcript Thermochemistry

Thermochemistry
THERMOCHEMISTRY
The study of heat released or required by
chemical reactions
Fuel is burnt to produce energy - combustion (e.g. when
fossil fuels are burnt)
CH4(g) + 2O2(g)
CO2(g) + 2H2O(l) + energy
Energy is the capacity to do work – there are
many types of energy!
•
Thermal energy is the energy associated
with the movement of molecules in a
substance – it is the same as kinetic
energy!
•
Chemical energy is the energy stored
within the bonds of chemical substances
•
Nuclear energy is the energy stored within
the collection of neutrons and protons in
the atom
•
Electrical energy is the energy associated
with the flow of electrons
•
Potential energy is the energy available
by virtue of an object’s position
6.1
Two main general forms of
energy
Kinetic energy
(EK) = ½ mv2
• Energy is measured in the
standard unit of Joules
• 1 J = 1 kg ∙ m2/s2
• mass must be in kg
• velocity must be in m/s
Energy due
to motion
• height must be in meters
• g = acceleration due to
gravity must be in m/s2
Potential
energy
(EP) = mgh
Energy due to
position (stored
energy)
Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between two bodies that
are at different temperatures.
Temperature is an indirect measurement of the thermal
or heat energy.
Temperature is NOT heat Energy
400C
greater temperature
But less heat energy
900C
There is a greater amount of
Heat energy in a bathub at 40 degrees
Than in a coffee cup at 90 degrees!
UNITS OF ENERGY
S.I. unit of energy is the joule (J)
Heat and work ( energy in transit) also
measured in joules
The calorie (cal) is another metric unit
for energy –
1 cal = 4.184 J
A Food calorie, with a capital C – is equal
to a 1000 chemistry calories:
1 Food Calorie (1 Calorie) = 1000 calories
A Candy bar with 480 Calories actually
contains 480,000 chemistry calories!
The specific heat (C) of a substance is the amount of heat (q)
required to raise the temperature of one gram of the
substance by one degree Celsius.
To measure the Heat (q) absorbed or
released by a substance:
q = m C Dt
Q = heat absorbed or released
m = mass of substance
C = specific heat of substance
Dt = tfinal - tinitial
How much heat is given off when an 869 g iron bar cools
from 940C to 50C?
C of Fe = 0.444 J/g • 0C
Dt = tfinal – tinitial = 50C – 940C = -890C
q = mCDt
= 869 g x 0.444 J/g • 0C x –890C = -34,000 J
Exothermic process is any process that gives off heat –
transfers thermal energy from the system to the surroundings.
2H2 (g) + O2 (g)
H2O (g)
2H2O (l) + energy
H2O (l) + energy
Endothermic process is any process in which heat has to be
supplied to the system from the surroundings.
energy + 2HgO (s)
energy + H2O (s)
2Hg (l) + O2 (g)
H2O (l)
Burning fossil
fuels is an
exothermic
reaction
Fireworks exploding is
an exothermic reaction
Photosynthesis is an
endothermic reaction
(requires energy input
from sun)
Ice melting is
an endothermic
process!
Enthalpy (H) is used to quantify the heat flow into or out of a system in a process
that occurs at constant pressure. (comes from Greek for “heat inside”)
DH = H (products) – H (reactants)
DH = heat given off or absorbed during a reaction at constant pressure
Hproducts < Hreactants
DH = -
Hproducts > Hreactants
DH = +
Thermochemical Equations
Is DH negative or positive?
System absorbs heat
Endothermic
DH is positive!
6.01 kJ are absorbed for every 1 mole of ice that
melts at 00C and 1 atm.
H2O (s)
H2O (l)
DH = 6.01 kJ
Thermochemical Equations
Is DH negative or positive?
System gives off heat
Exothermic
DH is negative!
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (l) DH = -890.4 kJ
Heat, or DH, can be written as part of a chemical
reaction!
If the reaction has a positive DH, then the reaction
needs heat, and heat is written on the left side of the
arrow as a reactant
If the reaction has a negative DH, then the reaction
releases heat, and heat is written on the left side of
the arrow as a product
C3H8 (g) + 5 O2 (g)
3 CO2 (g) + 4H2O (g) DH = -2043 kJ
or
C3H8 (g) + 5 O2 (g)
3 CO2 (g) + 4H2O (g) + 2043 kJ
We can treat heat, then, like a reactant or product….
And perform stoichiometry problems…
How many kJ of heat are released when 355 grams
of propane are burned with excess oxygen?
C3H8 (g) + 5 O2 (g)
3 CO2 (g) + 4H2O (g) + 2043 kJ
355 grams C3H8 x 1 mole C3H8 = 8.07 moles C3H8
44 grams
8.07 moles C3H8 x 2043 kJ heat
1 mole C3H8
= 16,483.3
kJ heat released
How do we measure the heat of a
reaction in an experiment…?
There are three ways:
1.Using a calorimeter
2.Using Hess’ Law
3.Using a table of heats of formation
Let’s look at each method….
1. Using a calorimeter
• A calorimeter is an insulated
device used to capture all of
the heat either absorbed or
released by a reaction!
• The reaction is usually
surrounded by water….why?
• Water is stable, and has a
high specific heat
• It changes temperature
slowly!
• q reaction = - q surroundings
No heat enters or leaves!
• By measuring the heat that
the water absorbs or releases,
we can calculate the heat of
the reaction!
A 0.1964-g sample of solid quinone (C6H4O2) is burned in a bomb
calorimeter that contains 373 grams of water. The temperature of the
water inside the calorimeter increases by 3.2°C. Calculate the
energy of combustion of quinone per mole.
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•
•
•
•
First – write a balanced chemical equation!
1 C6H4O2 (s) + 6 O2 (g) → 6 CO2 (g) + 2 H2O (g)
The heat released by the reaction is absorbed by the calorimeter:
q reaction = - q calorimeter
q = mcDT
q = (373 grams H2O)(4.184 J/g0C)(3.2°C) = 4,994.02 J gained by
calorimeter
q reaction = -4,994.02 J (released by reaction)
This is not the DH, though!
• 1 C6H4O2 (s) + 6 O2 (g) → 6 CO2 (g) + 2 H2O (g)
• The change in heat, or DH, is the energy
released for the reaction the way it was written!
• We only used .1964 grams of the chemical!
• The reaction calls for one mole of the chemical!
• 1 mole C6H4O2 (s) = 108 grams
• So I set up a ratio:
• -4,994.02 J/.1964 g = X/108 g
• X = -2,746,202.4 J = -2746.2024 kJ
• So, DH = -2700 kJ (2 significant figures)
2. Using Hess’ Law
What if a reaction is too costly or dangerous
to conduct, but we still want to calculate its DH?
4NH3(g) + 5O2(g)

4NO(g) + 6H2O(g)
We can use algebra to manipulate other reactions to look
like the desired reaction – making sure we change the
energies as well!
This is called Hess’ Law:
N2(g) + O2(g)  2NO(g)
DH = 180.6 kJ
N2(g) + 3H2(g)  2NH3(g)
DH = -91.8 kJ
2H2(g) + O2(g)  2H2O(g)
DH = -483.7 kJ
Goal:
4NH3(g) + 5O2(g)

4NO(g) + 6H2O(g)
Using the following sets of reactions:
N2(g) + O2(g)  2NO(g)
N2(g) + 3H2(g)  2NH3(g)
2H2(g) +
O2(g)  2H2O(g)
NH3: Reverse and x 2
DH = 180.6 kJ
DH = -91.8 kJ
DH = -483.7 kJ
4NH3  2N2 + 6H2 DH =
+183.6 kJ
Any chemical in more than one reaction - skip
O2 :
NO:
x2
H2O:
x3
2N2 + 2O2  4NO
6H2 +
3O2  6H2O
DH = 361.2 kJ
DH = -1451.1 kJ
Goal:
4NH3(g) + 5O2(g)
NH3: Reverse and x2

4NO(g) + 6H2O(g)
4NH3  2N2 + 6H2 DH =
+183.6 kJ
O2 :
NO:
x2
2N2 + 2O2  4NO
H2O:
x3
6H2 +
3O2  6H2O
DH = 361.2 kJ
DH = -1451.1 kJ
Cancel terms and take sum.
4NH3
+ 5O2

4NO
+ 6H2O
DH = -906.3 kJ
Is the reaction endothermic or exothermic?
Determine the heat of reaction for the reaction:
C2H4(g) + H2(g)  C2H6(g)
Use the following reactions:
C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(l)
DH = -1401 kJ
C2H6(g) + 7/2O2(g)  2CO2(g) + 3H2O(l)
DH = -1550 kJ
H2(g) +
1/2O2(g)  H2O(l)
DH = -286 kJ
23
Determine the heat of reaction for the reaction:
Goal:
C2H4(g) + H2(g)  C2H6(g)
DH = ?
Use the following reactions:
C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(l)
DH = -1401 kJ
C2H6(g) + 7/2O2(g)  2CO2(g) + 3H2O(l) DH = -1550 kJ
H2(g) +
1/2O2(g)  H2O(l)
DH = -286 kJ
C2H4(g) :use 1 as is C2H4(g) + 3O2(g)  2CO2(g) + 2H2O(l) DH = -1401 kJ
H2(g) :# 3 as is
H2(g) + 1/2O2(g)  H2O(l)
DH = -286 kJ
C2H6(g) : rev #2
2CO2(g) + 3H2O(l)  C2H6(g) + 7/2O2(g)
DH = +1550 kJ
C2H4(g) + H2(g)  C2H6(g)
DH = -137 kJ
3. Using Heats of Formation Tables
A. Heat of Formation (DHfº) – the heat released or absorbed
when one mole of a substance is formed from its elements.
EX: H2(g) + ½ O2(g)  H2O(l) DHfº = -289 kJ
The reactant elements are in their “standard state” –
their most stable form at 25 ºC and 1 atm. This is
usually indicated with a 0 by the DHf.
The DHfº of an element in its standard state is zero. You
Cannot make an element from elements!
The heat of formation for a substance is like having its
Potential energy – it is a measurement of how stable or
Unstable it is!
How do we use the table to figure out the DH
For a reaction?
The DH of a rxn is equal to the sum of the DHfº’s of
the products minus the sum of the DHfº’s of the reactants.
(Each product’s or reactant’s DHfº must be multiplied
by its coefficient.)
DHrxn = S DHfº(products) – S DHfº(reactants)
YOU MUST PRINT OFF THE TABLE FROM MY
WEBSITE – IT WAS NOT INCLUDED IN YOUR
PACKET!
1.
Calculate the DH for:
2 Na(s) + 2 H2O(l)  2 NaOH(aq) + H2(g) DH = ?
DHrxn = S DHfº(products) – S DHfº(reactants)
DHrxn = [(2)(NaOH(aq)) + (1)(H2(g))] – [(2)(Na(s)) +
(2)(H2O(l))]
1.
2 Na(s) + 2 H2O(l)  2 NaOH(aq) + H2(g) DH = ?
DHrxn = S DHfº(products) – S DHfº(reactants)
DHrxn = [(2)(NaOH(aq)) + (1)(H2(g))] – [(2)(Na(s)) + (2)(H2O(l))]
DHrxn = [(2)(-469.6 kJ) + (1)(0 kJ)] – [(2)(0 kJ) + (2)(-285.84 kJ)]
DHrxn = -367.52 kJ
Notice that the table from my website has some values listed twice –
and they are slightly different – you might get slightly differing
answers based on the values that you use – that is fine!
2.
Calculate the DH for:
C2H5OH (l) + 3 O2(g)  2 CO2(g) + 3 H2O(l) DH = ?
DH = -1366.89 kJ