Download Energetics II - Miller, Jonathan

Energetics
(a)
Explain that some chemical reactions are accompanied by energy
changes, principally in the form of heat energy; the energy changes
can be exothermic (DH, negative) or endothermic
(b)
Explain and use the terms:
(i)
Enthalpy change of reaction and standard conditions, with
particular reference to: formation, combustion, hydration, solution,
neutralisation, atomisation
Bond energy (DH positive, i.e. bond breaking)
(ii)
(c)
(d)
Calculate enthalpy changes from appropriate experimental results,
including the use of the relationship
Enthalpy change = mcDT
Apply Hess’ Law to construct simple energy cycles, and carry out
calculations involving such cycles and relevant energy terms, with
particular reference to:
(i) determining enthalpy changes that cannot be found by direct
experiment, e.g. an enthalpy change of formation from enthalpy
changes of combustion
(ii) average bond energies
(e)
Construct and interpret a reaction pathway diagram, in terms of the
enthalpy change of the reaction and of the activation energy
Direction of heat flow
(Energy accompanies all reactions)
Endothermic reactions
require energy to form
products (∆H is positive)
Exothermic reactions
release energy as a
product (∆H is negative)
Endothermic and exothermic
processes
Endothermic:
• Melting
• Vaporizing
• Chemical reactions
that have a positive
∆H (absorb energy)
Exothermic
• Freezing
• Condensing
• Combustion
• Chemical reactions
that have a negative
∆H (release energy)
Calculating the enthalpy (DH ) change of a reaction
DH = m x c x DT
The specific heat capacity, c is the amount of heat needed to raise
the temperature of 1 g of substance by 1 K. Its units are joules per
gram per Kelvin, or Jg-1K-1.
For example, the specific heat capacity of water is 4.2 g-1K-1, so it takes
4.2 joules to raise the temperature of 1 gram of water by 1 degree
kelvin.
Hess’s Law states that the total energy (or enthalpy) for
a chemical reaction is the same, whatever route is taken,
provided that the initial and final conditions are the same.
We can show this on a diagram called a thermochemical cycle
See book, 13.5.2 for calculations (p.132)
The standard molar enthalpy change of formation, DHf is
the enthalpy change when a mole of compound is formed
from its elements in their standard states under standard
conditions
The standard molar enthalpy of combustion DHc is the enthalpy change
at standard state when a mole of substance is completely burned in
oxygen