Final answer:
Altering the order of reactant addition in a chemical reaction, such as adding Na2S before KI, can influence the reaction pathway, affect the yield, or possibly prevent the reaction from occurring, depending on the specific reaction and mechanisms involved.
Step-by-step explanation:
If the procedure in a chemical experiment is changed and sodium sulfide (Na2S) is added before potassium iodide (KI), several outcomes are possible depending on the nature of the reaction and the compounds involved. Generally, switching the order of addition of reactants can affect the pathway of the reaction, possibly leading to different intermediates or sidestepping certain kinetic barriers. In the context of equilibrium, such a change could alter the concentrations of products and reactants and hence, potentially influence the yield of the product.
Depending on the specific reaction, adding Na2S before KI may have no significant effect, or it could either increase or decrease the yield of the desired product. In some cases, this could even prevent the reaction from occurring if, for example, a necessary intermediate is not formed in the correct sequence. Without exact details of the reaction mechanisms, it is not possible to determine the precise impact on the yield or the progression of the reaction.