Chemical Systems and Equilibrium: Test Review

Key facts

Where marks get lost

• ×Thinking equilibrium means equal concentrations of reactants and products. It means equal forward and reverse rates. (Dynamic Equilibrium and the Equilibrium Constant)
• ×Forgetting to raise concentrations to their coefficients, e.g. writing $[{\text{NO}}_2]$ instead of $[{\text{NO}}_2{]}^2$. (Dynamic Equilibrium and the Equilibrium Constant)
• ×Including a pure solid or liquid, such as ${\text{CaCO}}_3(s)$ or ${\text{H}}_2\text{O}(l)$, in the K expression. (Dynamic Equilibrium and the Equilibrium Constant)
• ×Putting moles instead of mol/L into an ICE table, or using initial concentrations in the K expression instead of equilibrium ones. (Dynamic Equilibrium and the Equilibrium Constant)
• ×Writing the Change row as $-x$ for every species instead of scaling by the coefficients ($-x$, $-x$, $+2x$). (Dynamic Equilibrium and the Equilibrium Constant)
• ×Reversing the Q rule: when $Q<K$ the reaction shifts right (toward products), not left. (Le Chatelier's Principle and the Reaction Quotient)
• ×Saying a catalyst increases the yield at equilibrium. It only makes the system reach equilibrium faster. (Le Chatelier's Principle and the Reaction Quotient)
• ×Claiming that adding more reactant changes K. Only a temperature change alters K; concentration changes alter Q. (Le Chatelier's Principle and the Reaction Quotient)
• ×Counting all moles, including solids and liquids, when predicting a pressure change. Only moles of gas matter. (Le Chatelier's Principle and the Reaction Quotient)
• ×Assuming a shift completely undoes a stress. The system only partly counteracts it, so an added reactant stays higher than its original value. (Le Chatelier's Principle and the Reaction Quotient)
• ×Using $x=\sqrt{K_{b}c}$ as $[{\text{H}}_3{\text{O}}^{+}]$ for a weak base. That value is $[{\text{OH}}^{-}]$; you must find pOH first and then pH. (Acid-Base Equilibria)
• ×Treating a weak acid like a strong acid by setting $[{\text{H}}_3{\text{O}}^{+}]$ equal to the initial acid concentration. (Acid-Base Equilibria)
• ×Using the small-$x$ approximation without checking that percent ionization is below 5%. (Acid-Base Equilibria)
• ×Confusing strong with concentrated: strength is the fraction ionized, concentration is the amount dissolved per litre. (Acid-Base Equilibria)
• ×Writing a conjugate base by removing a hydrogen atom from the wrong place or forgetting to lower the charge by one, e.g. writing ${\text{NH}}_3^{-}$ as the conjugate base of ${\text{NH}}_4^{+}$. (Acid-Base Equilibria)
• ×Writing $K_{sp}=s\times 2s=2s^2$ for an AB2 salt. The $[{\text{X}}^{-}]$ term is ${(2s)}^2$, so $K_{sp}=4s^3$. (Solubility Equilibria, Buffers and Titration)
• ×Calculating Q for precipitation with the concentrations before mixing instead of after dilution. (Solubility Equilibria, Buffers and Titration)
• ×Assuming every titration has its equivalence point at pH 7. That is only true for a strong acid with a strong base. (Solubility Equilibria, Buffers and Titration)
• ×Comparing Ksp values directly for salts that release different numbers of ions. (Solubility Equilibria, Buffers and Titration)
• ×Thinking a buffer keeps pH perfectly constant or works for any amount of added acid. It resists small changes, and it fails once one component is used up. (Solubility Equilibria, Buffers and Titration)
• ×Saying the contact process uses 450 °C because it gives the highest yield. Low temperature gives the highest yield for this exothermic reaction; 450 °C is chosen for rate, with a catalyst. (Equilibrium at Work: Industry, Body and Environment)
• ×Thinking the catalyst increases the amount of ${\text{SO}}_3$ at equilibrium. ${\text{V}}_2{\text{O}}_5$ only speeds up reaching equilibrium. (Equilibrium at Work: Industry, Body and Environment)
• ×Comparing Q with Ksp using concentrations before dilution or mixing. Always use the concentrations actually present. (Equilibrium at Work: Industry, Body and Environment)
• ×Saying CO poisoning is treated by removing CO from the blood directly. Treatment raises $[{\text{O}}_2]$, which shifts the hemoglobin equilibrium to release CO. (Equilibrium at Work: Industry, Body and Environment)
• ×Thinking ocean acidification means the ocean has become acidic. Its pH is still above 7; it has become less basic, and the lower carbonate ion concentration is what harms shell builders. (Equilibrium at Work: Industry, Body and Environment)
• ×Forgetting to account for dilution when two solutions are mixed. Initial concentrations in the ICE table are the values after mixing. (Measuring K in the Lab)
• ×Assuming the equilibrium concentration of FeSCN²⁺ in a test mixture equals the SCN⁻ added. That assumption is only valid for standards made with a large excess of Fe³⁺. (Measuring K in the Lab)
• ×Using $[{\text{Ca}}^{2+}]=[{\text{OH}}^{-}]$ for calcium hydroxide. Each formula unit releases two ${\text{OH}}^{-}$, so $[{\text{Ca}}^{2+}]$ is half of $[{\text{OH}}^{-}]$. (Measuring K in the Lab)
• ×Titrating an unfiltered saturated solution. Suspended solid dissolves as acid is added, so too much acid is used and Ksp comes out too high. (Measuring K in the Lab)
• ×Reporting K with more significant figures than the data allow, especially from a pH reading, which gives only as many significant figures as it has decimal places. (Measuring K in the Lab)

Practice test

24 questions from across the unit. Each one shows the lesson it comes from.

Endless drill

Fresh numbers every time, for the calculation questions in this unit.