Back to the full dot-point answer
QLDChemistryQuick questions
Unit 2: Molecular interactions and reactions
Quick questions on The pH scale and introduction to acid-base chemistry (QCE Chemistry Unit 2)
13short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is acids and bases (Arrhenius framing)?Show answer
$$HCl_{(g)} \xrightarrow{\text{water}} H^+_{(aq)} + Cl^-_{(aq)}$$
What is strong vs weak acids and bases?Show answer
Strong acid or base: ionises essentially completely in water. Written with a single arrow. The concentration of dissolved H+ (or OH-) equals the concentration of acid (or base) added.
What is the pH scale and K_w?Show answer
$$2H_2O_{(l)} \rightleftharpoons H_3O^+_{(aq)} + OH^-_{(aq)}$$
What is calculating pH of a strong acid solution?Show answer
Strong acid fully ionises, so [H_3O+] equals the formal acid concentration.
What is calculating pH of a strong base solution?Show answer
Strong base fully ionises, so [OH-] equals the formal base concentration. Then either:
What is reactions of acids?Show answer
Three reaction types are required at QCE Unit 2 level. All produce a salt; two also produce a gas; one also produces only water (neutralisation).
What is indicators?Show answer
pH indicators are weak acids or bases whose protonated and deprotonated forms have different colours. Common ones for QCE level:
What is strong acid or base: ionises essentially completely in water?Show answer
Written with a single arrow. The concentration of dissolved H+ (or OH-) equals the concentration of acid (or base) added.
What is weak acid or base: ionises only partially in water?Show answer
Written with a double arrow. Only a small fraction donates or accepts a proton at any instant.
What is confusing strength and concentration?Show answer
Strong/weak refers to extent of ionisation; concentrated/dilute refers to mol/L. A 0.001 mol/L HCl solution is dilute but the acid is strong.
What is forgetting that base concentration gives [OH-], not [H+]?Show answer
Calculate pOH first or convert via K_w.
What is reporting pH to too many decimal places?Show answer
pH is a logarithm; only the digits after the decimal count as significant figures. pH = 1.60 has 2 sig fig (the leading 1 is the order-of-magnitude digit).
What is predicting gas evolution from neutralisation?Show answer
Acid + hydroxide -> salt + water only. No gas. Carbon dioxide comes only from carbonates and hydrogencarbonates; hydrogen comes only from active metals.