Summary of Electrolysis and Potometer Experiments
Electrolysis and Potometer Experiments: Your Study Guide
Introduction
Setting up experimental apparatus correctly is a fundamental lab skill. Proper setup ensures accurate results, safe working practice, and clear observations. This guide covers two common experimental set-ups: collecting gases from electrolysis of aqueous solutions and assembling a potometer to measure plant water uptake. Each section breaks steps into manageable parts, gives practical tips, and explains why each action matters.
Electrolysis: Predicting and Collecting Products
Electrolysis of an aqueous solution involves passing electric current through the solution to force non-spontaneous chemical reactions at the electrodes.
Definition: Electrolysis is the chemical decomposition of a substance produced by passing an electric current through an ion-containing solution or molten compound.
How products depend on reactivity
- In aqueous electrolysis, water also contributes ions: $\ce{H+}$ and $\ce{OH-}$. The species that are discharged at each electrode depend on their reactivity (tendency to be reduced or oxidized) relative to the water-derived ions.
- At the cathode (reduction): either a pure metal plates onto the electrode (if the metal ions are less reactive than hydrogen) or hydrogen gas is produced (if metal ions are more reactive than hydrogen).
- At the anode (oxidation): usually oxygen gas is produced from water oxidation unless a halide ion (e.g., $\ce{Cl-}$, $\ce{Br-}$, $\ce{I-}$) is present; then the corresponding halogen (e.g., $\ce{Cl2}$) is released instead.
Definition: Cathode is the electrode where reduction occurs; anode is the electrode where oxidation occurs.
Practical setup to collect gases
- Use inert electrodes (e.g., graphite) unless a metal deposition is required.
- Place electrodes in the aqueous solution and position open-ended test tubes (or gas collection tubes) over each electrode to trap any gas produced.
- Ensure the test tube is filled with water and inverted over the electrode before electrolysis begins so gas displaces the water and can be observed as bubbles rising into the tube.
- Run the current and monitor which gas appears at each electrode: hydrogen forms bubbles at the cathode and oxygen (or halogen) at the anode.
Table: Typical products for aqueous electrolysis
| Solution contains | At cathode | At anode |
|---|---|---|
| Metal ions less reactive than H (e.g., Cu^{2+}) | Metal plates | Oxygen (from water) |
| Metal ions more reactive than H (e.g., Na^{+}) | Hydrogen gas | Oxygen (from water) |
| Halide ions present (e.g., Cl^{-}) | Hydrogen gas (if metal is reactive) or metal plating | Halogen gas (e.g., Cl_{2}) |
Practical example: Electrolyse aqueous $\ce{CuSO4}$. Copper ions are less reactive than hydrogen, so copper metal plates at the cathode and oxygen is produced at the anode. Represented with mhchem:
$$\ce{Cu^{2+}(aq) + 2e- -> Cu(s)}$$ $$\ce{2H2O(l) -> O2(g) + 4H+(aq) + 4e-}$$
Potometer: Measuring Plant Water Uptake
A potometer is an apparatus that estimates the rate of water uptake by a plant shoot, which is closely related to transpiration.
Definition: A potometer measures the volume of water taken up by a plant shoot over time; this is used as an indirect measure of transpiration rate.
Why set up underwater
- Cutting and assembling underwater prevents air entering the xylem. Air bubbles in the xylem (embolism) interrupt continuous water columns and invalidate measurements.
Step-by-step assembly and use
- Cut the shoot underwater: Use sharp scissors and cut at a slant to increase water-absorbing surface area.
- Assemble the potometer in water: Insert the cut shoot into the apparatus while submerged so no air enters the system.
- Keep the capillary end submerged: After assembly, remove the apparatus from the
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Laboratory Experimental Setups
Klíčové pojmy: Electrolysis products depend on ion reactivity vs water ions, At the cathode: metal plates if ion is less reactive than H, else hydrogen gas, At the anode: oxygen unless halide ion present, then halogen gas, Collect gases using inverted water-filled test tubes over electrodes, Assemble potometer and cut shoot underwater to avoid xylem embolism, Cut shoot at a slant to increase surface area for uptake, Form a single air bubble in the potometer capillary to measure uptake distance, Volume uptake = $Ad$ and rate = $Ad/t$ for capillary area $A$, distance $d$, time $t$, Check and seal all joints to prevent leaks and air entry, Use low voltages and PPE for electrolysis safety, Let the shoot acclimatise and dry leaves before measurement, Re-cut underwater if xylem is blocked or no movement observed