Question

C. Why use an Excess of a reactant?

1. Some reactions stop while portions of the reactants are still present.

a. This is inefficient and wasteful.

b. Use an excess of the least expensive reactant can be driven to continue
until all of the limiting reactant is used up.

c. Excess of one can speed up a reaction.

Answer 1

Here’s a clearer, corrected, and slightly expanded version of your notes plus a few examples and caveats.

Why use an excess of a reactant?

1. Some reactions stop even though some reagents remain.
a. That is inefficient and wasteful if you don’t convert all of the limiting reagent.
b. Using an excess of the least expensive reactant drives the reaction to completion (consumes the limiting reagent).
c. An excess of one reactant increases its concentration, which often increases the reaction rate (see rate laws).

Additional reasons and principles
- Equilibrium control: For reversible reactions, Le Châtelier’s principle says increasing the concentration of a reactant shifts the equilibrium toward products, increasing yield.
- Kinetic advantage: Many rate laws include reactant concentrations; raising one reactant’s concentration raises the reaction rate.
- Compensate for losses/impurities: Excess makes up for partial impurity, side reactions, adsorption, or volatilization.
- Practicality: It can be easier/cheaper to use excess of a cheap reagent than to recover unreacted expensive reagent.

Common examples
- Esterification: use excess alcohol or remove water to shift equilibrium toward ester.
- SN2 reactions: using excess nucleophile can increase conversion and rate.
- Hydrogenations: using excess H2 (cheap gas) helps drive complete hydrogenation.

Drawbacks to consider
- Increased cost and waste if the “excess” reagent is not cheap or recyclable.
- Extra separation and purification steps to remove leftover reagent.
- Possible side reactions or safety hazards when a reagent is present in large excess.

Practical guideline
- Use excess when it’s inexpensive, easy to separate or recycle, and when higher conversion or faster rate is desirable; avoid when separation costs, toxicity, or safety make excess impractical.