Question

1) Give a detailed explanation of how you can use the laboratory technique of titration to determine the concentration of an acetic acid solution.

2) When a reaction is reversible, eventually the rates of the forward and reverse actions will be equal. Which of the following best explains what this means?


The concentrations of the reactants and products have become equal.

The concentrations of the reactants have become constant.

The concentrations of the reactants have not reacted well.

The concentrations of the reactants have completed their reaction.

3) Balance the following redox equation and identify the element oxidized, the element reduced, the oxidizing agent, and the reducing agent. Show all of the work used to solve the problem. (5 points)

NO2- + Al yields AlO2- + NH3

Answer 1

1) Answer is: Determine the endpoint of titration.

The endpoint is the point at which the indicator changes colour in a colourimetric titration and that is point when titration must stop.

Equivalence point is the point which there is stoichiometrically equivalent amounts of acid and base.  

Chemist can draw pH curve (graph showing the change in pH of a solution, which is being titrated) for titration and determine equivalence point.

This is example of acid-base titration.

a) measure volume and molarity of some strong base, for example sodim hydroxide (NaOH).

b) measure the volume of acetic acid into a beaker.  

c) titrate until the endpoint is reached, when the colour of indicator, for example phenolphthalein changes color.

d) Use balanced chemical reaction of acetic acid and sodium hydroxide:

CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l).  From molarity and volume of base determine the concentration of acetic acid.

2) Answer is: The concentrations of the reactants have become constant.

For example, balanced chemical reaction: 2CH₄(g) ⇄ C₂H₂(g) + 3H₂(g).

a) In a chemical reaction, chemical equilibrium is the state in which both reactants (methane CH₄) and products (ethyne C₂H₂ and hydrogen H₂) are present in concentrations which have no further tendency to change with time.  

b) At equilibrium, both the forward and reverse reactions are still occurring.

c) Reaction rates of the forward and backward reactions are equal and there are no changes in the concentrations of the reactants and products.

3) Chemical equation: NO₂⁻ + Al → AlO₂⁻ + NH₃.

Oxidation half reaction: Al⁰ + 4OH⁻ → AlO₂⁻ + 2H₂O + 3e⁻ /×2.

Reduction half reaction: NO₂⁻ + 5H₂O + 6e⁻ → NH₃ + 7OH⁻.

Oxidation half reaction: 2Al⁰ + 8OH⁻ → 2AlO₂⁻ + 4H₂O + 6e⁻.

Reduction half reaction: NO₂⁻ + 5H₂O + 6e⁻ → NH₃ + 7OH⁻.

Balanced chemical equation:

2Al + 8OH⁻ + NO₂⁻ + 5H₂O → 2AlO₂⁻ + 4H₂O + NH₃ + 7OH⁻.

The same species on opposite sides of the arrow can be canceled:

2Al + OH⁻ + NO₂⁻ + H₂O → 2AlO₂⁻ + NH₃.

Aluminium is oxidized (change oxidation number from 0 to +3) and nitrogen is reduced (change oxidation number from +3 to -3).

Aluminium is reducing agent (lost electrons) and nitrogen is oxidizing agent (gain electrons).