Reading Comprehension

Chemical reactions in aqueous solutions usually proceed at rates that depend on the concentrations of the reactants. Consider a solution containing freely moving lead ions and iodide ions. As the ions combine to form lead iodide, a nearly insoluble solid that precipitates out of the solution, the solitary ions that remain in ever more dilute concentrations take longer to find each other, retarding the process. High temperatures generally accelerate chemical reactions (up to a point-a hot enough flame breaks all chemical bonds) by speeding up reactant particles, causing them to collide more frequently, and more importantly, by increasing the fraction of particles with sufficient velocity to overcome the activation energy barrier, the "hump" that reactants must first surmount in order to react. Nevertheless, lead iodide precipitation proceeds further with decreasing temperature. The reverse reaction happening simultaneously (the dissolution of lead iodide in water) slows down to a greater degree at low temperatures than the forward precipitation reaction, lowering the equilibrium concentrations of ionic species and decreasing the solubility of lead iodide.



The author mentions "a hot enough flame" (line 9) most probably in order to

explain the most efficient way to break chemical bonds

provide an example of a sufficiently accelerated chemical reaction

emphasize that the reaction rate is not an invariably increasing function of temperature

illustrate how the activation energy barrier of a typical reaction may be overcome

offer insight into the mechanisms by which forward and reverse reactions reach equilibrium

himu wrote:Chemical reactions in aqueous solutions usually proceed at rates that depend on the concentrations of the reactants. Consider a solution containing freely moving lead ions and iodide ions. As the ions combine to form lead iodide, a nearly insoluble solid that precipitates out of the solution, the solitary ions that remain in ever more dilute concentrations take longer to find each other, retarding the process. High temperatures generally accelerate chemical reactions (up to a point-a hot enough flame breaks all chemical bonds) by speeding up reactant particles, causing them to collide more frequently, and more importantly, by increasing the fraction of particles with sufficient velocity to overcome the activation energy barrier, the "hump" that reactants must first surmount in order to react. Nevertheless, lead iodide precipitation proceeds further with decreasing temperature. The reverse reaction happening simultaneously (the dissolution of lead iodide in water) slows down to a greater degree at low temperatures than the forward precipitation reaction, lowering the equilibrium concentrations of ionic species and decreasing the solubility of lead iodide.

The author mentions "a hot enough flame" (line 9) most probably in order to

A) explain the most efficient way to break chemical bonds
B) provide an example of a sufficiently accelerated chemical reaction
C) emphasize that the reaction rate is not an invariably increasing function of temperature
D) illustrate how the activation energy barrier of a typical reaction may be overcome
E) offer insight into the mechanisms by which forward and reverse reactions reach equilibrium

Is the answer B

Concentration is greater --> the reaction (precipitation) proceeds at a faster rate --> slows down as the concentration is lowered.
High temp --> Usually speeds up the reaction up to a point (of collision and breaking up of the bonds via a hot "enough" flame) --> Just sufficient to break the bonds.
Ions wont further react until the temp. is lowered

Hi Himu,

Can we have the OA and the source of the question ?