why should the maximum percent yield never be above 100%
because of both controllable and uncontrollable factor such as the during the experiment is conducted certain proportion of the reactant may lost or not fully reacted. It is also because the maximum percent yield is influence by the limiting reagent, the one that decides the amount of product.
I disagree with the above answer since it doesn't address the question. You can NEVER get more than you started with. A yield greater than 100% means you've discovered the secret of making something from nothing. If you grind 15 g apples and 15 g oranges you can get no more than 30 g of product unless impurities were introduced, either accidentally or on purpose.
The maximum percent yield should never be above 100% because it is a measure of efficiency in a chemical reaction. Percent yield is calculated by comparing the actual amount of product obtained to the maximum amount of product that could theoretically be obtained, known as the theoretical yield. It is expressed as a percentage.
Theoretical yield is calculated using stoichiometry, which is the study of the quantitative relationships between reactants and products in a chemical reaction. It is based on the balanced chemical equation for the reaction and assumes that all reactants are completely converted into products.
In reality, due to various factors such as side reactions, incomplete reactions, impurities, and losses during separation or purification, the actual yield of a reaction is often less than the theoretical yield. This means that the percent yield is typically less than 100%.
If the percent yield were to be above 100%, it would imply that you have obtained more product than is theoretically possible. This violates the principle of conservation of mass, which states that mass is neither created nor destroyed during a chemical reaction. It is important to note that such a situation would not be possible according to the laws of chemistry.
Therefore, percent yield serves as a practical measure of the efficiency of a reaction, and it is expected to be less than or equal to 100%.