Chemical Equilibrium: Constant, Factors & Le Chatelier

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Questions and Answers

Explain how the choice of indicator is crucial in acid-base titrations and what happens if a wrong indicator is selected.

The indicator must change color close to the equivalence point of the reaction. If the wrong indicator is chosen, the endpoint will not accurately represent the equivalence point, leading to inaccurate concentration determination.

Describe how Le Chatelier's principle applies to reactions in volumetric analysis, particularly when titrating at non-standard temperatures.

Le Chatelier's principle explains how temperature changes can shift equilibrium. In titrations conducted at non-standard temperatures, the equilibrium of any reactions involved (indicator equilibria, complex formation) shifts, potentially affecting endpoint detection and overall accuracy.

How does the strength of an acid or base affect the shape of its titration curve, and what specific feature(s) of the curve are indicative of the strength?

Strong acids/bases have a very sharp change in pH near the equivalence point on their titration curve, while weak acids/bases exhibit a gradual change, creating a buffering region. The sharpness of the equivalence point's pH change and the presence/length of buffering region indicate strength.

Explain the difference between the 'equivalence point' and the 'endpoint' in a titration. Why is it important for these two points to be as close as possible?

The equivalence point is where the titrant and analyte have reacted completely according to stoichiometry. The endpoint is the observed color change of an indicator. They should be close because the endpoint signals when to stop the titration, aiming to stop at (or very near) the equivalence point. Signup and view all the answers

Describe the role of a catalyst in a reversible reaction and explain why adding a catalyst does not affect the equilibrium position.

A catalyst speeds up both the forward and reverse reactions equally. Because it increases the rate of both sides of the reaction to the same degree, the ratio of products to reactants at equilibrium remains unchanged. Adding a catalyst only affects how quickly equilibrium is reached. Signup and view all the answers

Explain how to prepare a standard solution for use as a titrant, including the steps necessary to ensure its concentration is accurately known.

First, a primary standard with high purity is carefully weighed and dissolved in a known volume of solvent. The concentration is then calculated from the mass and volume. Ensuring accuracy involves using high-quality glassware, precise weighing, and accounting for any solution density changes with temperature. Signup and view all the answers

Explain why a strong acid-strong base titration has a sharper end point compared to a weak acid-weak base titration. How does this affect indicator selection?

Strong acid-strong base titrations produce a large change in pH near the equivalence point, leading to a sharper endpoint. Conversely, weak acid-weak base titrations have a gradual pH change, resulting in a less distinct endpoint. This limits indicator options, as not all will provide a clear color change. Signup and view all the answers

Describe the effect of increasing the ionic strength of a solution on the equilibrium of a chemical reaction. How might this impact a titration?

Increasing ionic strength may shift the equilibrium due to changes in activity coefficients of ions. In a titration, this shift can affect the effective concentrations of reactants and products, leading to deviations from expected stoichiometric ratios and potentially inaccurate results. Signup and view all the answers

Explain how the pH of a buffer solution is related to the $pK_a$ of its weak acid component, and describe the conditions under which the buffer is most effective.

According to the Henderson-Hasselbalch equation, pH = $pK_a$ + log([A-]/[HA]). A buffer is most effective when the concentrations of the weak acid (HA) and its conjugate base (A-) are equal or nearly equal, resulting in a pH close to the $pK_a$ value. Signup and view all the answers

What is the purpose of performing a 'blank titration', and how does it help improve the accuracy of volumetric analysis?

A blank titration determines the volume of titrant needed to cause a color change in the absence of the analyte. This corrects for any interference or background reactions, such as titrant reacting with the solvent or indicator itself, leading to more accurate results. Signup and view all the answers

Explain why a phenolphthalein indicator, which has a pH range of 8.3 - 10.0, is not appropriate for titrating a strong acid with a weak base.

The equivalence point of a strong acid-weak base titration is below pH 7. Phenolphthalein changes color in a more basic range. Thus, the endpoint would be reached far after the actual equivalence point, leading to an overestimation of weak base concentration. Signup and view all the answers

Describe the purpose of back titration. In what situations is it particularly useful?

Back titration involves adding an excess of a standard reagent to the analyte, then titrating the excess reagent with another standard solution. It's useful when the reaction between the analyte and titrant is slow, or when the endpoint is difficult to observe directly, or when the analyte is a solid that reacts slowly. Signup and view all the answers

Explain the concept of 'normality' (N) in volumetric analysis, and describe a situation where using normality instead of molarity (M) simplifies calculations.

Normality is the number of equivalents of solute per liter of solution. It simplifies calculations in acid-base or redox titrations where the stoichiometry isn't 1:1. For example, with sulfuric acid ($H_2SO_4$) which has two acidic protons, 1 M $H_2SO_4$ is 2 N, simplifying calculations related to equivalent amounts of acid and base. Signup and view all the answers

Explain the 'common ion effect' and how it affects the solubility of slightly soluble salts. How does this effect relate to complexometric titrations?

The common ion effect describes the decrease in solubility of a sparingly soluble salt when a soluble salt containing a common ion is added to the solution. In complexometric titrations, the addition of a complexing agent sequesters the metal ion, effectively removing it from the solution and increasing the solubility of other salts by shifting the equilibrium. Signup and view all the answers

Describe how you would determine the concentration of acetic acid in vinegar using titration, including the reaction, titrant, indicator, and calculations.

React vinegar with NaOH titrant, using phenolphthalein indicator. The balanced reaction is: $CH_3COOH + NaOH ightarrow CH_3COONa + H_2O$. At the endpoint, calculate moles of NaOH used, which equals moles of $CH_3COOH$. Divide moles of $CH_3COOH$ by the volume of vinegar to get concentration. Signup and view all the answers

Flashcards

Chemical Equilibrium

State where forward and reverse reaction rates are equal, resulting in no net change in reactant/product concentrations.

Equilibrium Constant (K)

Ratio of product to reactant concentrations at equilibrium, each raised to the power of its stoichiometric coefficient.