5
QuizMultiple Choice

Reaction Pathways — Quiz

Question 5 • 26-27 - C - RUSD Chemistry in the Earth System

Iron(II) chloride is formed from the reaction between iron and copper(II) chloride. Fe plus CuCl sub 2 rightarrow FeCl sub 2 plus Cu If the reactants have triangle cap h sub f is equal to negative 256 kJ mol inverse and the products have triangle cap h sub f is equal to negative 321 kJ mol inverse , which reaction pathway graph would have a similar shape to this reaction?

Answer
A
Image with description A graph with reaction progression as the horizontal axis and Energy as the vertical axis. A line starts low on the vertical axis and negative 321 kilojoules and runs briefly parallel to the horizontal axis. The line rises sharply and then curves down to a point at negative 256 kilojoules and levels out.
B
Image with description A graph with reaction progression on the horizontal axis and energy on the vertical axis. A line starts high on the vertical axis at negative 256 kilojoules and runs level for a short distance. It rises a short distance and falls steeply to near the horizontal axis at negative 321 kilojoules; it runs parallel to the horizontal axis.
C
Image with description A graph with reaction progression on the horizontal axis and energy on the vertical axis. A line starts high on the vertical axis at negative 321 kilojoules and runs level for a short distance. It rises a short distance and falls steeply to near the horizontal axis at negative 256 kilojoules; it runs parallel to the horizontal axis.
D
Image with description A graph with reaction progression as the horizontal axis and Energy as the vertical axis. A line starts low on the vertical axis and negative 256 kilojoules and runs briefly parallel to the horizontal axis. The line rises sharply and then curves down to a point at negative 321 kilojoules and levels out.

Explanation

The reactants have an enthalpy of −256 kJ/mol, and the products have an enthalpy of −321 kJ/mol. Since the products are 65 kJ/mol lower in energy (−321 − (−256) = −65), the reaction is exothermic: the graph should start at −256, rise over an activation-energy hump, and finish lower at −321. Choice B shows that pattern, with the reactants higher than the products.

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