That varying reactant concentrations or proportions can alter reaction pathways and product distributions is a foundational principle of chemical kinetics and stoichiometry; no citation is needed.
The claim that reactant concentrations alter resulting products in certain chemical reactions is a basic, definitional aspect of chemical equilibrium, kinetics, and reaction stoichiometry (e.g., different initial concentrations shifting equilibria or favoring different competing pathways). Under the instructions, statements that are trivially true by direct chemical definition and everyday observation are classified as common knowledge and require no citation.