Preparation of Gases
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flagWhat you'll discover
- arrow_forwardPrepare H₂ from zinc and dilute HCl, O₂ from H₂O₂ with MnO₂, and CO₂ from marble chips and dilute HCl
- arrow_forwardChoose the collection method from the gas's density and solubility
- arrow_forwardPerform the identifying test for each gas and state the expected result
- arrow_forwardWrite the balanced equation for each preparation and test
- arrow_forwardExplain the role of MnO₂ as a catalyst
Three generators, one design
All three preparations use the same apparatus logic: a flask holding the solid, a thistle funnel to add the liquid from above (its stem dipping below the liquid surface so gas cannot escape up it), and a delivery tube leading to the collection vessel. Hydrogen: granulated zinc + dilute HCl → ZnCl₂ + H₂. Carbon dioxide: marble chips (CaCO₃) + dilute HCl → CaCl₂ + H₂O + CO₂.
Oxygen is the odd one out — no acid needed, just decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂, with a pinch of black MnO₂. The MnO₂ is a catalyst: it speeds the decomposition enormously but is chemically unchanged and can be recovered at the end with its mass intact. (The older method, heating KClO₃ with MnO₂, gives the same gas: 2KClO₃ → 2KCl + 3O₂.)
Collection: let the gas's properties decide
How you collect a gas follows from two properties — solubility in water and density relative to air. Hydrogen and oxygen are (almost) insoluble in water, so both are collected over water: the gas bubbles up into an inverted water-filled jar, visibly displacing the water downward. This method has the bonus of showing exactly how full the jar is.
Carbon dioxide is noticeably soluble in water, so water collection wastes it. Instead, being about 1.5 times denser than air, it is collected by upward displacement of air: the jar stands mouth-up and CO₂ sinks in, pushing the lighter air out of the top. (If you ever collect hydrogen by air displacement, the jar must be mouth-down, since H₂ is 14 times lighter than air.) Exam logic: insoluble → over water; soluble and denser than air → upward displacement of air.
The identifying tests
Each gas has a one-line confirmatory test. Hydrogen: bring a burning splint to the jar mouth — the gas burns with a squeaky pop, the mini-explosion of 2H₂ + O₂ → 2H₂O. Oxygen: a glowing (not burning) splint thrust into the jar relights, because combustion accelerates dramatically in pure O₂. Carbon dioxide: bubble the gas through lime water, which turns milky as insoluble CaCO₃ forms — the same reaction as in the carbonate anion test.
Precautions worth quoting: keep all flames away from the hydrogen generator (a flame at the delivery tube can flash back into the flask), use dilute acid and add it gradually through the funnel, and never let the thistle funnel's stem rise above the liquid or gas escapes unnoticed. These small details are exactly what practical-exam vivas probe.