Acids, Bases and Rates
Acids and Bases
Acids produce hydrogen ions in solution; they turn litmus red and have pH below 7. Alkalis are soluble bases, turn litmus blue and have pH above 7. A pH of 7 is neutral. The pH scale is logarithmic in effect: each step of 1 represents a tenfold change in hydrogen ion concentration. Indicators (litmus, universal indicator, methyl orange, phenolphthalein) show acidity or alkalinity.
Reactions of Acids
Acid + metal → salt + hydrogen (test hydrogen with a lighted splint - a squeaky pop). Acid + metal carbonate → salt + water + carbon dioxide (test with limewater, which turns milky). Acid + base → salt + water. The salt name comes from the acid: hydrochloric → chloride, sulfuric → sulfate, nitric → nitrate.
Measuring Rates
Rate can be measured by gas volume collected over time, mass lost over time, or time for a cross to disappear. Plot volume against time: the gradient is the rate; the curve flattens as reactants are used up. Higher temperature, concentration or surface area steepens the early part of the curve. A catalyst raises the rate but does not change the final amount of product.
Traps to Watch
- Naming the salt from the metal rather than the acid.
- Confusing "strong" acid (fully ionised) with "concentrated" (many particles per volume).
- Saying a catalyst changes the final volume of gas produced.
Worked Example
Zinc reacting with hydrochloric acid gives zinc chloride and hydrogen: Zn + 2HCl → ZnCl₂ + H₂. The salt is a chloride because the acid is hydrochloric, not because zinc is a metal.