Which label corresponds to the Δ𝐻
of the reaction that forms CO2 gas from CO gas and O2 gas?
In this question, we’re being asked
to find the Δ𝐻, that is, the enthalpy change, of a reaction where carbon dioxide is
formed as a product from carbon monoxide and oxygen as reactants. The enthalpy change for a reaction
is defined with the symbol Δ𝐻. And it represents the change in
enthalpy between the products and reactants in a reaction at constant pressure.
Notice that the enthalpy is
represented as a vertical scale on the left side of this diagram. The diagram shows three separate
reactions occurring. These have been labeled number one,
two, and three.
In reaction one, we see one mole of
methane gas reacting with two moles of oxygen gas to produce one mole of carbon
dioxide gas and two moles of liquid water. This equation represents the
complete oxidation or complete combustion of one mole of methane. Because the products are at a lower
enthalpy relative to the reactants that we started with here, we would say this
reaction is exothermic. This reaction would release 890
kilojoules of heat to the surroundings.
Although carbon dioxide is formed
as a product in this reaction, the reactants are not carbon monoxide and oxygen as
specified in the question. Since we started with methane and
not carbon monoxide as specified in the question, this enthalpy change is not the
correct answer. We need to look for a reaction
where carbon monoxide is a reactant and carbon dioxide is in fact a product. The enthalpy change we’re looking
for, therefore, could be the enthalpy change for reaction two or three.
In reaction two, we start with one
mole of methane gas and two moles of oxygen gas as our reactants. According to the process in the
diagram, we form one mole of carbon monoxide gas, two moles of liquid water, and
half a mole of oxygen gas. Notice that the products of this
process are also at a lower enthalpy level than the reactants that we started
with. So this reaction is also
exothermic. In fact, it releases 607 kilojoules
of heat energy to the surroundings.
In this reaction, carbon monoxide
gas is one of the products. This is a lethally toxic gas. And it’s formed from the incomplete
combustion of methane in this reaction. Incomplete combustion occurs when
fuels are burned in a limited supply of oxygen. It’s clear to see from the equation
that although we started with two moles of oxygen gas, there’s half a mole of oxygen
gas left over after the process is completed, this is the unreacted oxygen. In this reaction, the reactants do
not match the reactants described in the question. We started with methane gas and not
carbon monoxide gas. Although oxygen gas is a reactant
in both situations, negative 607 kilojoules is not the enthalpy change for the
reaction that we are seeking.
In the process shown by reaction
three, we are taking one mole of carbon monoxide and reacting it with the unreacted
oxygen from process two, that is, half a mole of oxygen, to from a mole of carbon
dioxide gas. Carbon dioxide gas is the only new
product in this process. Although there are two moles of
liquid water on both sides of the equation, this remains unchanged. And we could ignore it as far as
the reaction is concerned.
Carbon monoxide gas does burn quite
nicely when you light it with a lighted splint. And it burns with a pale-blue
flame. Extra heat energy is released
during this process. So this reaction is also
exothermic, and its Δ𝐻 value is negative 283 kilojoules.
Notice at this stage that the
diagram represents two alternative reaction pathways to arrive at the same products
from the same reactants. Energetically, the two reaction
pathways must be just as exothermic as each other. In fact, we find that the enthalpy
change for pathway number one equates to the enthalpy change for pathway number two
plus the enthalpy change for pathway number three.
Since the reactants and products
are the same in both pathways, the enthalpy change is independent of the route
taken. If we look closely at reaction
three, we can see that the reactants and the only new product formed matches those
in the question. So negative 283 kilojoules is the
label that corresponds to the Δ𝐻 of the reaction that forms carbon dioxide from
carbon monoxide and oxygen gas. The other two labels in the diagram
are simply labels for the reactants and the products. They’re not labels for the enthalpy
change or a Δ𝐻 value. They’re not the correct
answers. So the correct Δ𝐻 for the reaction
in the question is negative 283 kilojoules.