Stœchiométrie, rendement du produit et réactifs limitants
Se connecter ou pour accéder au contenu complet. En savoir plus sur l’accès de votre institution au contenu JoVE ici
View the full transcript and gain access to JoVE Lab Manual videos
Q1: What is stoichiometry and why does it matter in chemical reactions?
Stoichiometry describes the relative quantities of reactants and products in a chemical reaction, based on the Law of Conservation of Mass. Atoms are rearranged but never created or destroyed, so the number and identity of reactant atoms must equal those on the product side. This relationship allows chemists to predict how much product forms from given reactant amounts.
Q2: How do you balance a chemical equation?
Balancing requires adding coefficients to each side until atom counts are equal for every element. Count atoms by multiplying coefficients by subscripts, then add atoms across all molecules containing that element. Use trial and error or systematic approaches like tables to adjust coefficients until both sides match. A balanced equation ensures the molar relationship between reactants and products is accurate.
Q3: What is a limiting reactant and how does it affect product formation?
The limiting reactant is the reactant completely consumed first, stopping the reaction and limiting total product formation. Using the balanced equation's coefficients, calculate how much product each reactant produces. The reactant yielding the least product is the limiting reactant. Once entirely consumed, no additional product can form regardless of excess reactants remaining.
Q4: What is the difference between theoretical yield and actual yield?
Theoretical yield is the maximum possible product amount based on the limiting reactant and balanced equation. Actual yield is what you measure experimentally. Differences occur due to side reactions, purification losses, or human error. Comparing these values reveals reaction efficiency and helps identify experimental problems or improvements.
Q5: How do you calculate percent yield?
Percent yield is calculated by dividing actual yield by theoretical yield and multiplying by 100. A 100% yield means maximum product was produced from the reactants used. Yields below 100% are common and indicate product loss during the reaction. Percent yield never exceeds 100%; higher values suggest experimental or calculation errors.
Q6: Why must a chemical equation be balanced before determining product amounts?
A balanced equation ensures equal atom counts on both sides, satisfying the Law of Conservation of Mass. The coefficients in a balanced equation represent the molar relationship between reactants and products, which is essential for calculating how much product each reactant produces. Without balancing, stoichiometric calculations would be inaccurate and unreliable.
Q7: What role do coefficients play in stoichiometric calculations?
Coefficients indicate how many molecules participate in the reaction and define the molar relationship between all reactants and products. They are multiplied by subscripts to count total atoms per element during balancing. These same coefficients are used to calculate how much product forms from each reactant, determining which is the limiting reactant and the theoretical yield.