Module 5
What does it mean when a chemical reaction is at dynamic equilibrium?
The forward and reverse reactions occur at equal rates, so concentrations remain constant.
A solution has a pH of 3. What is its hydrogen ion concentration, [H⁺]?
1 × 10⁻³ mol L⁻¹
What functional group is present in an alcohol?
OH
What is a complex ion?
A metal ion bonded to one or more ligands.
What is the conjugate base of H₂CO₃?
HCO₃⁻
What happens to an equilibrium system if the concentration of a reactant is increased?
The equilibrium shifts towards the products to partially oppose the change.
A student adds a small amount of HCl to a buffer containing a weak acid and its conjugate base. What happens to the pH, and why doesn't it change dramatically?
The pH decreases only slightly because the conjugate base reacts with the added H⁺, reducing its effect on the pH.
What type of reaction occurs when an alcohol is converted into an alkene?
Elimination reaction (dehydration) — water is removed from the alcohol to form an alkene.
Example:
ethanol → ethene + water
Why are samples often diluted before AAS analysis?
To bring their concentration into the range of the calibration curve.
What is the difference between qualitative and quantitative analysis?
Qualitative identifies what substances/ions are present; quantitative determines how much is present.
What is the difference between Q and K?
Q describes the reaction at any point; K describes the reaction at equilibrium.
In a titration, what is the purpose of an indicator?
To show when the endpoint has been reached through a colour change.
What is the difference between an addition polymer and a condensation polymer?
Addition polymers form without producing a small molecule; condensation polymers form with the loss of a small molecule such as water.
A calibration curve for AAS has concentration on the x-axis and absorbance on the y-axis. An unknown has an absorbance of 0.45. What would you use the graph to determine?
The concentration of the unknown sample.
What does Ksp describe?
The equilibrium constant for the dissolution of a sparingly soluble ionic compound.
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens to the equilibrium position when pressure is increased?
It shifts right, towards the side with fewer moles of gas.
A 25.00 mL sample of HCl is titrated with 0.1000 mol L⁻¹ NaOH. The average titre is 18.60 mL. Calculate the concentration of the HCl.
0.07440 mol L⁻¹. Since HCl + NaOH → NaCl + H₂O is 1:1: n(NaOH) = 0.1000 × 0.01860 = 0.001860 mol. Therefore n(HCl) = 0.001860 mol, and c = 0.001860 ÷ 0.02500 = 0.07440 mol L⁻¹.
A student says, "A ¹H NMR spectrum with four peaks means the molecule contains four hydrogen atoms."
Explain why this is incorrect.
The number of peaks represents the number of different proton environments, not the total number of hydrogen atoms. The integration gives the relative number of hydrogens in each environment.
A precipitate contains AgCl. NH₃ is added and the precipitate dissolves. What has happened?
NH₃ forms a complex ion with Ag⁺, reducing the concentration of free Ag⁺ and causing more AgCl to dissolve.
What does a larger K value generally indicate?
Products are favoured at equilibrium.
A reaction has Q < K. Is the reaction currently at equilibrium? If not, which direction will it proceed?
No. It will proceed in the forward direction to produce more products until Q = K.
A student calculates 0.0050 mol of excess HCl remaining after a reaction. The original amount was 0.0200 mol. How many moles of HCl actually reacted?
0.0150 mol
A compound produces two signals in its ¹H NMR spectrum with an integration ratio of 3:2. What does this tell you?
There are two different proton environments, containing relative numbers of H atoms in a 3:2 ratio.
Why might an industrial process not operate at the temperature that gives the maximum equilibrium yield?
A lower temperature may give a better equilibrium yield but produce the product too slowly, so industry balances yield, rate, energy use and economic factors.
A 25.00 mL sample of a solution containing Ag⁺ ions is treated with excess Cl⁻ ions, causing AgCl(s) to precipitate.
The precipitate is filtered, dried and found to have a mass of 0.3588 g.
Calculate the concentration of Ag⁺ ions in the original solution.
0.100 mol L⁻¹ Ag⁺