Factors Affecting the Rate of Reactions

[cvc121]

Homework Statement

Classify the following actions based on how they could affect a reaction rate (rate increases, rate decreases, rate is unaffected) and provide a brief explanation as to why:

• Add a catalyst
• Decrease the concentration of a reactant
• Decrease the surface area of a solid reactant
• Increase the temperature

Homework Equations

N/A

The Attempt at a Solution

Add a catalyst
- Rate increases; change reaction mechanism to include steps with lower activation energies.

Decrease the concentration of a reactant
- Rate decreases; decreases the chance for collisions since there are less reactant molecules in a given area.

Decrease the surface area of a solid reactant
- Rate decreases; less collisions because less particles exposed to the other reactant.

Increase the temperature
- Rate increases; increase in the kinetic energy of the molecules and causing them to move faster and collide more often. Thus, more molecules can achieve the required activation energy.

Can anyone confirm if I am on the right track? I am especially unsure about the decrease in the surface area of a solid reactant. Is the phase significant in this case?

Thanks. All help Is very much appreciated.

 

[Borek]

Looks OK to me.

Changing surface doesn't change the equilibrium position, but changes the reaction speed. It does so both for the forward reaction and the backward reaction, as their ratio remains constant equilibrium is unaffected.

 

[epenguin]

Certainly on the right track. The answers are all what you mostly expect. 1 is true almost by definition.

For your second answer, this is not absolute. For example in heterogeneous catalysis and enzyme catalysis you can often easily reach a concentration where nearly 100% of the catalytic sites are occupied. This is called saturation. When you reduce the concentration nearly 100% are still occupied and working at their maximum rate, so then the result is no change in reaction rate. Reducing it still further though there will come a point where the reaction rate does decrease with decreasing concentration.

You can also have the situation where surface- or enzyme- adsorbed molecules get in each other's way - with the result that an increase of concentration causes decrease of reaction rate (so in these ranges decrease of concentration results in increased reaction rate). Called "excess substrate inhibition" or just "substrate inhibition".

These situations are not uncommon. Maybe a bit less common is that when reaction mechanisms are complicated, with several steps, you can actually have situations where increased temperature causes slower reaction due to the favouring of higher concentration of an intermediate. Or there can be an equilibrium between a more and a less reactive intermediate, with higher temperature favoring the latter. More common, though trivial in a way, is inactivation of catalysts, particularly enzymes, by temperature. Most enzymes will be destroyed if you keep them at 100°, even 60°, 50°, or less, at first but this tails off and the reaction stops.

However if you are only being asked at the level of simple yes or no quizzes, you'd probably be misunderstood and lose points by worrying about these qualifications.