Europium (Ar=152.0A_r = 152.0Ar=152.0) reacts with dilute hydrochloric acid in a redox reaction, producing a solution and hydrogen gas. A chemist proposes three possible ionic equations for this reaction, representing different oxidation states of europium:
Equation 1: 2Eu(s)+2H+(aq)→2Eu+(aq)+H2(g)2\text{Eu(s)} + 2\text{H}^+\text{(aq)} \rightarrow 2\text{Eu}^+\text{(aq)} + \text{H}_2\text{(g)}2Eu(s)+2H+(aq)→2Eu+(aq)+H2(g)
Equation 2: Eu(s)+2H+(aq)→Eu2+(aq)+H2(g)\text{Eu(s)} + 2\text{H}^+\text{(aq)} \rightarrow \text{Eu}^{2+}\text{(aq)} + \text{H}_2\text{(g)}Eu(s)+2H+(aq)→Eu2+(aq)+H2(g)
Equation 3: 2Eu(s)+6H+(aq)→2Eu3+(aq)+3H2(g)2\text{Eu(s)} + 6\text{H}^+\text{(aq)} \rightarrow 2\text{Eu}^{3+}\text{(aq)} + 3\text{H}_2\text{(g)}2Eu(s)+6H+(aq)→2Eu3+(aq)+3H2(g)
An experiment is conducted using excess dilute hydrochloric acid under room temperature and pressure (RTP) conditions where 1 mol1\text{ mol}1 mol of gas occupies 24000 cm324000\text{ cm}^324000 cm3. The following measurements are recorded:
Analyse these experimental results to determine which of Equation 1, 2, or 3 is correct.
Explain how the measured gas volume and the chemist's conclusion in (a) would be affected if the gas syringe was not allowed to cool back to room temperature (remaining hot from the exothermic reaction) before the volume was recorded.
The chemist proposes an alternative gravimetric method: 1.52 g1.52\text{ g}1.52 g (0.01 mol0.01\text{ mol}0.01 mol) of europium is reacted with excess dilute hydrochloric acid, followed by the addition of excess aqueous sodium hydroxide. A precipitate forms, which is filtered, thoroughly dried, and weighed. Explain how the final dry mass of this precipitate would allow the chemist to determine which of the three proposed equations is correct.
199 exam-style questions on OCR A A Level Chemistry Atoms and reactions, covering Atomic structure and isotopes, Compounds, formulae and equations, Amount of substance, Acids, and Redox. Each one has a worked solution and a mark scheme showing where the marks go.