Video Transcript
What is the correct expression for
𝐾 𝑐 for the following equilibrium system: CaCO3 solid reacts reversibly to produce
CaO solid plus CO2 gas, which of the following statements is correct? (A) 𝐾 𝑐 equals CaO concentration
times CO2 concentration divided by CaCO3 concentration. (B) 𝐾 𝑐 equals CaCO3
concentration divided by CaO concentration times CO2 concentration. (C) 𝐾 𝑐 equals one divided by CO2
concentration. Or (D) 𝐾 𝑐 equals CO2
concentration.
In this question, we need to
determine the correct 𝐾 𝑐 equation for the reversible decomposition of the calcium
carbonate compound. We first need to note that not all
the species in the reaction are in the same state or phase. This is a heterogeneous equilibrium
system. A heterogeneous equilibrium system
is a system with chemicals that are not all in the same state or phase. Other than the equation in the
question, another example of a heterogeneous equilibrium system is the reversible
reaction of carbon and water.
A homogeneous equilibrium system,
on the other hand, is a system with chemicals that are all in the same state or
phase. An example would be the formation
of ammonia molecules. All of the reactant and product
substances are in the gas state.
Let’s clear a little space and look
at the expressions used to determine the equilibrium constant.
It is relatively straightforward to
determine the 𝐾 𝑐 equation for a homogeneous equilibrium system. The algebraic equations explain how
we can always figure out the 𝐾 𝑐 equation for the reaction of a homogeneous
equilibrium system. The numerator of the 𝐾 𝑐 equation
has the concentration of the products, and the denominator has reactant
concentrations. The stoichiometric coefficients of
the reactants and products are the exponents of these concentration terms.
If we use the production of ammonia
as an example, we find that 𝐾 𝑐 equals NH3 concentration to the power of two
divided by N2 concentration times H2 concentration to the power of three. It is less straightforward to
determine the 𝐾 𝑐 equation for a heterogeneous equilibrium system. We tend to discount concentration
terms for solids and liquids in chemical systems with other gaseous chemical
compounds or elements. We do not include the solid and
liquid concentrations, as these values are essentially constant.
We can now look at the reversible
reaction in the question. The equation has two chemicals in
the solid state, and one is a gas. We do not need to consider the
concentration of the solid calcium carbonate and calcium oxide compounds. The answer must be the surprisingly
simple equation 𝐾 𝑐 equals CO2 concentration.
Let’s bring back the answer
choices. The correct expression for 𝐾 𝑐
for the given equilibrium system is (D), 𝐾 𝑐 equals CO2 concentration.
