CHEMISTRY
Thermochemistry
Thermochemistry is interested in the heat
changes that occur during chemical
reactions.
Let’s define some words that may be helpful.
When is WORK done?
______________________________________________________________________________
What is ENERGY?
______________________________________________________________________________
Chemical Potential Energy is ______________________________________________
_____________________________________________________________________________.
What is HEAT? ____________________________________________________________
______________________________________________________________________________
BOND BREAKING ________________ energy.
BOND FORMING _________________ energy.
Exothermic Reaction
(RELEASE HEAT)
Endothermic Reaction
(ABSORB HEAT)
Hproducts < Hreactants
∆H = Hp - Hr
= Negative ( - )
Hproducts > Hreactants
∆H = Hp - Hr
= Positive ( + )
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Since most chemical reactions and physical changes carried out in the laboratory are open to the
atmosphere, these changes occur at constant pressure. FOR SYSTEMS AT CONSTANT PRESSURE,
THE HEAT CONTENT IS THE SAME AS A PROPERTY CALLED THE ENTHALPY(H) OF
THE SYSTEM.
Heat changes for reactions carried out at constant pressure are the same as
changes in enthalpy, symbolized as ΔH (read “delta H”). q = ΔH.
q = ΔH = m x C x
ΔT
m = mass of the water
C = specific heat capacity of water
ΔT = Tf - Ti
EXAMPLES
1. 15.3 g of NaNO3 were dissolved in 100. g of water in a calorimeter. The temperature of the water
dropped from 25.00oC to 21.56oC. Calculate ΔH for the solution process.
2. A 1.0 g sample of octane (C8H18) is burned in a calorimeter containing 1200 g of water. The
temperature of the water rises from 25.00oC to 33.20oC. Calculate ΔH for this process.
3. A student mixed 50.0 ml of water containing 0.50 mol of HCl at 22.5oC with 50.0 ml of water
containing 0.50 mol NaOH at 22.5oC in a foam cup calorimeter. The temperature of the resulting
solution increased to 26.0oC. How much heat in kilojoules (kJ) was released by this reaction?
The enthalpy of the products is (lower or higher…circle the best) than that of the
reactants in an exothermic reaction.
The enthalpy of the products is (lesser or greater…circle the best) than that of the
reactants in an endothermic reaction.
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The standard state is the MOST STABLE FORM OF AN ELEMENT UNDER
STANDARD CONDITIONS. (25oC and 1 atm of pressure)
When discussing heat changes in a chemical equation…we need to define a few terms:
(a) System = ___________________________________________________________________
(b) Surroundings = ____________________________________________________________
(c) Universe = ________________________________________________________________
In thermochemical calculations, the direction of the heat flow is given from the point of view of the
system.
Refer back to the diagrams above:
ENDOTHERMIC PROCESSES: ___________________________________________________
________________________________________________________________________________
EXOTHERMIC PROCESSES: _____________________________________________________
________________________________________________________________________________
1 calorie = quantity of heat needed to raise the
temperature of 1 g of pure water 1oC.
Calorie defines energy in food.
1 Calorie = 1 kilocalorie = 1000 calories
1 joule = 0.2390 calories
4.184 J = 1 calorie
Heat capacity = amount of heat needed to
increase the temperature of an object exactly
1oC (depends on mass and chemical
composition).
Specific Heat Capacity (specific heat) of a
substance is the amount of heat it takes to raise
the temperature of 1 g of the substance 1oC.
Metals have LOW specific heats (Al = 0.90 J/ g∙oC) and water has a very high specific heat (4.184
J/g∙oC or 1 calorie/ g∙oC). Heat affects the temperature of objects with a high specific heat much less
than the temperature of those with a low specific heat.
IT TAKES A LOT OF HEAT TO RAISE THE TEMPERATURE OF A BODY OF WATER.
WATER RELEASES A LOT OF ENERGY AS IT COOLS.
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EXAMPLES
1. When 435 J of heat is added to 3.4 g of olive oil at 21oC, the temperature increases to 85oC. What is
the specific heat of olive oil?
2. How much heat is required to raise the temperature of 20.0 g of iron from 26oC to 72.30oC? The
specific heat of iron is 0.447 J/g∙oC.
Chemistry problems involving enthalpy changes are similar to stoichiometry problems. The amount of heat
released or absorbed during a reaction depends on the number of moles of the reactants involved.
1. Calculate the amount of heat released by the combustion of 1.75 mol of benzene.
2C6H6 + 15O2  12CO2 + 6H2O
∆Ho = - 98.0 kJ
2. How much heat is transferred when 100.0 g of calcium oxide (CaO) reacts with carbon according to
the equation below? Is this reaction endothermic or exothermic?
CaO + 3C  CaC2 + CO
ΔHo = + 464.8 kJ
3. Ammonium dichromate decomposes in a vigorous reaction when it is heated. Calculate the heat
transferred for the decomposition of 53.0 g of ammonium dichromate according to the following
equation.
(NH4)2Cr2O7  N2 + 4H2O + Cr2O3
ΔHo = - 315 kJ
The heat of combustion is _____________________________________________________________.
ALL SOLIDS ABSORB HEAT AS THEY MELT TO BECOME LIQUIDS.
Molar heat of fusion (∆Hfusion) = ____________________________________________________________
Molar heat of solidification (ΔHsolidification) = ___________________________________________________
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H2O(s)  H2O(l)
ΔH fus = 6.01 kJ/mol
H2O(l)  H2O(s)
ΔH solid = - 6.01 kJ/mol
Molar heat of vaporization = amount of heat necessary to vaporize one
mole of a given liquid.
H2O(l)  H2O(g)
ΔH vap = 40.7 kJ/mol
H2O(g)  H2O(l)
ΔH cond = - 40.7 kJ/mol
Molar heat of condensation = amount of heat released when 1 mol of
vapor condenses.
1. How much heat (in kJ) is absorbed when 63.7 g H2O(l) at 100oC is converted to steam at 100oC?
2. Why is a burn from steam potentially far more serious than a burn from very hot water?
DID YOU KNOW THAT HEAT CHANGES CAN OCCUR WHEN A SOLUTE DISSOLVES IN A
SOLVENT? The heat change caused by dissolution of one mole of substance is the molar heat of solution
(ΔH soln).
A practical application of an exothermic reaction is how a hot pack works (CaCl2 + water).
A practical application of an endothermic reaction is how a cold pack works (NH4NO3 + water).
HESS’S LAW
Since most reactions take place in a series of steps. Since it is impossible to obtain the heat of reaction of an
intermediate step…we rely on Hess’s law to help us. It makes it possible to measure a heat of reaction
indirectly.
If a series of reactions are added together, the enthalpy change for the NET equation will be the sum
of the enthalpy changes for the individual steps.
ΔH net = ΔH1 + ΔH2 … etc.
N2(g) + O2(g)  2NO(g)
ΔH = +181 kJ
+ 2NO(g) + O2(g)  2NO2(g) ΔH = - 113 kJ
ΔHnet =
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RULES:
1. If the coefficients of an equation are multiplied by a factor, the enthalpy change for the reaction is
multiplied by the same factor.
2. If the equation is reversed, the sign of ΔH changes also.
EXAMPLE #1
C(s) + ½ O2(g)  CO(g)
ΔHo = - 110.5 kJ
CO(g) + ½ O2(g)  CO2(g) ΔHo = - 283.0 kJ
Calculate the value of ΔHo for the reaction C(s) + O2(g)  CO2(g).
EXAMPLE #2
Cu(s) + Cl2(g)  CuCl2(s)
ΔHo = - 206 kJ
2Cu(s) + Cl2(g)  2CuCl(s) ΔHo = - 136 kJ
Calculate the value of ΔHo for the reaction CuCl2(s) + Cu(s)  2CuCl(s).
EXAMPLE #3
C(s) + O2(g)  CO2(g)
ΔHo = - 393.5 kJ
H2(g) + ½ O2(g)  H2O(l)
ΔHo = - 285.8 kJ
2C2H2(g) + 5O2(g)  4CO2(g) + 2H2O(l)
ΔHo = - 2598.8 kJ
o
Calculate the value of ΔH for the reaction 2C(s) + H2(g)  C2H2(g).
Standard Heats of Formation (ΔHo f)
This is measured for a compound. It is the change in enthalpy that accompanies the formation of one mole of
a compound from its elements with all substances in their standard states at 25oC.
The ΔHof of a free element in its standard state is arbitrarily set at zero [H2(g), N2(g), O2(g), F2(g), Cl2(g),
Br2(l), I2(s); this is also true of the graphite form of carbon, C(s).
The standard heat of reaction (ΔHo) is the difference between the standard heats of formation of all the
reactants and products.
ΔHo = ΔHof (products) - ΔHof (reactants)
EXAMPLE #4
What is the standard heat of reaction (ΔHo) for the reaction of gaseous carbon monoxide with oxygen to form
gaseous carbon dioxide?
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