About 75 percent of hydrogen for industrial use is produced by the steam-reforming process. This process is carried out in two stages called primary and secondary reforming. In the primary stage, a mixture of steam and methane at about 30 atm is heated over a nickel catalyst at
The secondary stage is carried out at about
(a) What conditions of temperature and pressure would favor the formation of products in both the primary and secondany stages? (b) The equilibrium constant Kc for the primary stage is 18 at
Interpretation:
The favorable conditions for the formation of products, the partial pressure constant of thereaction, and the pressures of all the gases at equilibrium are to be determined.
Concept introduction:
When the enthalpy of a reaction is less than zero, it is called an exothermic reaction (enthalpy is negative), and when it is more than zero, it is an endothermic reaction (enthalpy is positive).
Equilibrium constant is the ratio of the concentration of reactants and products present in the chemical reaction.
For a general reaction:
The general formula for writing equilibrium expression for the reaction is given as:
Here,
A and B are reactants, C is products, and
For the general reaction:
The general formula for writing equilibrium expression for the reaction is given as:
Here,
A and B are reactants, C and D are products, and
Equilibrium constants of gas phase reaction are written in terms of partial pressures because concentration of gases is directly proportional to partial pressures.
The relationship between
Answer to Problem 96AP
Solution:
(a)
A high temperature favors product formation.
(b)
i)
ii)
Explanation of Solution
Given information:
The temperature at the first stage is
The temperature at thesecond stage is
a)The conditions of temperature and pressure that would favor that formation of products both in primary and secondary stage.
The optimum condition for pressure is
Standard conditions for pressure in air is,
At a temperature of
Based on the given reactions, the enthalpy is positive. So, the forward reaction is an endothermic reaction. According to LeChatelier’s principle, at high temperatures, more products are formed. In the steam-reforming process, alow temperature is favored, according to Le Chatelier’s principle.
An interesting fact to note is that the plants use natural gases, such as methane, for heating and generating hydrogen. To maintain the high temperatures, one-third of the methane gas is burned.
The first stage of the reaction is as follows:
The second stage of the reaction is as follows:
In both the primary and the secondary reaction, a high temperature is favorable for product formation because both are endothermic reactions.
The sum of the number of moles of the product is more than that of the reactants in both the reactions. So, a decrease in pressure favors the forward reaction to form more products.
b)The equilibrium constant is
i)
The value of
The relation between
Here,
The number of moles of the reaction is calculated as follows:
Substitute the values of
The value of
ii) The pressure of all gases at equilibrium
The partial pressures of methane and steam start at
The initial change equilibrium table for thereaction is as follows:
The equilibrium constant is:
Here,
Substitute the value of
Take the square root on both sides,
From the quadratic expression,
The partial pressure of
The partial pressure of
The partial pressure of
The partial pressure of
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