Question 1

What is the Beer-Lambert law and how does it relate to absorbance?

Accepted Answer

The Beer-Lambert law states that the concentration of a colored species is proportional to its absorbance, the amount of energy it absorbs at a specific wavelength. Mathematically, absorbance equals the molar attenuation coefficient (epsilon) multiplied by path length (l) and concentration (c). This relationship allows scientists to determine unknown concentrations by measuring absorbance values.

Question 2

How is a calibration curve used to determine unknown concentrations?

Accepted Answer

A calibration curve is created by testing multiple solutions of known concentration and plotting their absorbance values. The resulting line of best fit has a slope equal to the molar attenuation coefficient. Once established, this curve allows researchers to measure absorbance of unknown solutions and determine their concentrations by referencing the calibration line.

Question 3

What is chemical equilibrium and why does the equilibrium constant remain constant?

Accepted Answer

Chemical equilibrium occurs when forward and reverse reactions proceed at the same rate, creating a steady state. The equilibrium constant K, defined as the ratio of product concentrations to reactant concentrations raised to their stoichiometric coefficients, remains constant for a given reaction at a specific temperature, regardless of initial concentrations used in different trials.

Question 4

How does the ICE table help calculate the equilibrium constant?

Accepted Answer

An ICE (Initial-Change-Equilibrium) table organizes concentration data for reactants and products. Initial concentrations are based on known molarities and total volume. The change row shows how concentrations shift as the reaction reaches equilibrium. Equilibrium concentrations are then used to calculate K by substituting values into the equilibrium expression.

Question 5

Why is excess thiocyanate used in the iron thiocyanate reaction procedure?

Accepted Answer

A large excess of thiocyanate ensures that all iron reacts completely, simplifying the analysis. When one reactant is in large excess, its concentration change is negligible, allowing researchers to focus on measuring the colored product's concentration. This approach makes calculations more straightforward and improves the accuracy of equilibrium constant determination.

Question 6

How can spectrophotometry determine concentrations of colorless components indirectly?

Accepted Answer

Spectrophotometry directly measures the concentration of colored species using absorbance. Concentrations of colorless components are then calculated indirectly using the balanced chemical equation and stoichiometric relationships. Since the molar ratios between reactants and products are known, determining one concentration allows calculation of all others.

Question 7

What does the equilibrium constant value indicate about a reaction's extent?

Accepted Answer

A large K value indicates that products form nearly completely, meaning the reaction proceeds forward to a significant extent. Conversely, a very low K value indicates the reaction does not proceed forward significantly and remains dominated by reactants. The magnitude of K provides quantitative information about whether a reaction favors product or reactant formation at equilibrium.

Спектрофотометрическое определение равновесной константы - Оценочный тест