Transcript Enthalpy and Hess's Law - X-Colloid Chemistry Home Page

Chemistry 100
Enthalpy and Hess’s Law
Energy Changes in Chemical
Reactions

Let’s take a typical reaction
CH4 (g) + O2 (g)  CO2 (g) + 2 H2O (l)


This is an example of ‘burning a fossil
fuel.’ This reaction releases energy!
What about this reaction?
KCl (s)  KCl (aq)

This reaction requires energy!
Energy Changes

Almost all chemical and physical
processes are accompanied by the
loss or gain of energy from the
system.
energy
System
surroundings
Enthalpy

Enthalpy H  the heat content
of a substance.
Enthalpy change - H
H  qp
thermal energy (heat)
evolved or absorbed
under constant
pressure conditions.
Exothermic vs. Endothermic
Processes

Exothermic process  heat content of
system decreases, the system
enthalpy decreases (i.e., H < 0)


e.g., the combustion of methane
CH4 (g) + O2 (g)  CO2 (g) + 2 H2O (l)
Endothermic process  energy of the
system is increased (i.e. H > 0).

e.g., dissolving potassium chloride in water.
KCl (s)  KCl (aq)
The Enthalpy Change in a
Chemical Reaction

We want to know how much energy is
released when we burn ethane!
C2H6 (g) + 7/2 O2 (g)  2 CO2 (g) + 3 H2O (l)

We wish to know the difference in the heat
content of the reactants vs. the products.


H   npH (products) -  nr H (reactants)
np and nr represent the number of moles of
products and reactants, respectively.
The Enthalpy Change (cont’d)

For the ethane combustion reaction
1 C2H6 (g) + 7/2 O2 (g)  2 CO2 (g) + 3
H2O (l)
H   npH (products) -  nr H (reactants)
= 3 H[H2O (l)] + 2 H[CO2 (g)] - (7/2 H[O2(g)]
+
1 H[C2H6 (g)] )
Conventions for Writing
Thermochemical Equations


For exothermic reactions, H <0; for
endothermic reactions, H > 0.
H values are given for processes
occurring at 25C and 1 atm pressure

STTP - standard thermodynamic
temperature and pressure.
Conventions (cont’d)

The physical state of each substance
participating in the process must be
stated




s  solid
l  liquid
g  gas
aq  aqueous solutions
Conventions (cont’d)

The stoichiometric coefficients = the
number of moles of each substance
involved in the transformation
Fractional stoichiometric coefficients
are permitted in thermochemical
equations.
Conventions (cont’d)

Multiply or divide a chemical equation
by a factor, the H value must also be
multiplied or divided by that factor


Enthalpy is an extensive property.
When an equation is reversed, the
sign of the H value is changed but its
magnitude stays the same.
H Values and Hess’s Law

Calculating enthalpy changes for
physical and chemical transformations.


tabulated values of reaction enthalpies
(largely measured with a calorimeter).
indirect Method – Hess’s Law.
Hess’s Law

Hess’s Law - enthalpy changes for
sequences of reactions.

Enthalpy change accompanying a
reaction is the same whether the reaction
occurs in a single step or in many steps.