Ask A Scientist Chemistry Archive

name         Greg
status       educator
grade        other
location     VA
	
Question -   IF I could replace letters with numbers, then this that I
found would mean something.
I found:
"Note that H2O vapor and H2O liquid have
different heats of formation, so the state of matter is important.
delta H = Hf(CO) + Hf(H2) - Hf(H2O) - Hf(C) = Hf(CO) - Hf(H2O)"
(because H2 and C are @ ground state and are 0)

but where does one look up heats of formation?

Searching "heats of formation" finds much information, but no actual
numbers for 'heats of formation' are found.

A chemical reaction that produces heat (has a negative DH)(delta H)
(just an attempt, so please correct me if wrong)
If H2O has greater Hf than C0, then:
delta H is less than zero:
energy is released as heat, and zero joules are required for formation.

And do DH numbers tell us how much energy?

Do these numbers convert to:
temperature * time:
that is:
does +DH tell me what temperature and for how long?

Joules = watts x seconds, therefore watts = joules/seconds. A 1000 watt
fire will dissipate 1000 joules per second.
Joules. A joule is a unit of energy. A calorie is also a measure of
energy. 1 calorie = 4.186 joules.
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You have several questions intertwined here so let us step back and start
from scratch:
1. Joules and calories are measures of ENERGY. The ENERGY changes in
chemical reactions do not tell you anything about HOW FAST the reaction
occurs. The TIME enters is because POWER is the rate of production of the
ENERGY, in the sense that a 100 watt bulb uses 100 joules of electrical
ENERGY per SECOND. So the thermochemistry which calculates the amount of
ENERGY released (or absorbed) in a chemical reaction is the same whether the
reaction occurs rapidly or slowly -- it depends only on the initial state
(that is: temperature and pressure) of the reaction products and reactants.
It DOES NOT depend on HOW FAST the reaction is carried out.
2. In tabulating the ENERGY changes of chemical several conventions are
applied. First, unless specified otherwise, each product and reactant is
assumed to be in its pure state (i.e. not in solution), the applied pressure
is usually 1 atm., and the temperature of each is the same (usually 298.15
kelvins). Second, the physical form of each is taken to be the most stable
form at 298.15 kelvins and 1 atm pressure. So, for example, graphite is the
most stable form of carbon under these conditions, not diamonds, nor
buckeyballs.
So for water, unless otherwise specified, liquid water is form assumed at 1
atm and 298.15 kelvins. These set of conditions are collectively referred to
as the "standard state" of the compound.
3. If energy is given off the ENERGY change is assigned a negative value.
If energy is absorbed the ENERGY change is assigned a positive value.
4. The ENERGY of formation of EVERY ELEMENT at 1 atm and 298.15 kelvins is
assigned a value of ZERO.
5. Most chemical reactions are run at constant pressure (usually 1 atm) not
constant volume. As a result if a net amount of gaseous products is PRODUCED
by the reaction, the chemical reaction has to expend some of the energy
produced pushing back the atmosphere. Conversely, if a net amount of gaseous
products is consumed by the reaction, the atmosphere squeezes the gaseous
products so that 1 atm. is remains constant. When that work (against of by)
the atmosphere is accounted for the amount of heat varies a corresponding
amount (P* delta V), and the thermal ENERGY given off or absorbed is called
the ENTHALPY of reaction. It is these numbers one usually finds tabulated in
the literature or in handbooks.
6. So applying all the above to a compound, say CO2: C(graphite) + O2(gas) =
CO2(gas)
one finds the "heat of formation" (strictly speaking the ENTHALPY of
formation) of CO2 tabulated in tables. By the convention the value zero is
assigned to the reactants.
I hope this gives you a start on how to use the tables that you can find in
the chemistry handbooks.

Vince Calder
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