Investigate aqueous copper sulfate with inert graphite electrodes, then copper electrodes. For copper electrodes, measure how current affects electrode mass change over a controlled time.Apparatus and method schematic; not to scale. Use the stated controls and measurements.
Use a low-voltage DC supply, ammeter, connecting wires, beaker, electrode supports, timer and balance. Electrodes must be immersed but not touching; a voltmeter, if used, connects in parallel.
The cathode is negative and the anode positive in this electrolytic cell. Keep electrode area, immersion depth/separation, solution composition/volume and time matched when changing current.
Wear eye protection and avoid copper sulfate contact; follow disposal instructions. Use low voltage, turn off before adjusting the circuit and keep liquid away from the supply.
Inert-electrode observations
Place graphite electrodes in copper sulfate solution and apply DC. Copper forms at the negative cathode; bubbles of oxygen form at the positive anode.
Cu²⁺ + 2e⁻ → Cu at the cathode is reduction. At an inert anode, oxygen forms from water/hydroxide rather than sulfate being discharged; oxidation loses electrons.
Collect and test sufficient gas only using the approved school method. Oxygen relights a glowing splint; an observed bubble alone does not identify a gas.
Copper ions leave the solution as copper is deposited, so the blue colour can weaken. Inert graphite does not supply replacement copper ions.
Copper electrodes and quantitative method
Clean copper electrodes, dry consistently and record initial masses. Set up the same solution, electrode spacing and immersion depth, set a measured current and run for a fixed time.
Switch off, remove electrodes, rinse gently to remove solution without scraping deposits, dry and reweigh. Use the same drying criterion before and after.
Copper transfers from anode to cathode: Cu → Cu²⁺ + 2e⁻ at the anode, while cathode Cu²⁺ gains electrons. The anode loses mass and cathode gains it.
With pure copper electrodes under suitable conditions, solution copper concentration stays approximately constant because copper ions are replaced. Ideal mass loss and gain are similar, but losses and side reactions can cause differences.
Repeat for several currents, retaining the same time. Cathode mass gain = final − initial; anode mass loss = initial − final.
At a fixed time, plot mass change against current. If both current and time vary, plot against charge, Q = It. You expect approximately direct proportionality if a similar fraction of the current produces the desired electrode reaction in each run (similar current efficiency).
Precision and evaluation
Control the measured current throughout, not just supply voltage: resistance and conditions can change. Record actual current and duration accurately.
Dry consistently; residual solution inflates mass. Avoid brushing off deposited copper, which underestimates gain. Use a suitable balance because changes can be small.
Use fresh equivalent conditions or maintain solution concentration across runs. Changing electrode surface area or spacing can affect current distribution.
A rough porous deposit may trap solution or fall off. A difference between anode loss and cathode gain requires investigation, not invented balancing values.
Repeats reduce the effect of random variation but cannot correct a consistently late timer start, systematic deposits lost during rinsing or an uncalibrated balance.
Explain electron gain/loss at the correct electrode and distinguish copper metal from copper ions. Ion motion in solution and electron motion in wires are different.
Exam skills: planning, precision and evaluation
State what you change (the independent variable), what you measure (the dependent variable) and what you keep the same (control variables). Explain how you keep each control variable constant, rather than just saying “make it fair”.
Accuracy means how close a result is to the true value. Precision means how close repeated measurements are to each other. Resolution is the smallest change an instrument can show. More digits on a display do not automatically mean a more accurate result.
Repeat measurements for each condition, calculate a mean and describe how spread out the results are. This helps assess and reduce the effect of random errors. Repeating cannot fix an error that pushes results consistently in one direction (a systematic error), such as wet electrodes at final weighing.
Repeatability means getting similar results when the same person repeats the same method with the same equipment. Reproducibility means getting similar results when someone else, or different suitable equipment, repeats the experiment. Results can be consistent but still inaccurate.
Check that instruments read zero correctly and are calibrated where needed. Read scales at eye level: looking from an angle can give a wrong reading (parallax error). Choose suitable ranges, measurement intervals and scale divisions (resolution).
Write down the original readings straight away in a table, with units in the headings. Use decimal places that match the instrument’s resolution. Keep the original data and round only when needed. Do not discard a result just because it differs from your prediction.
An anomalous result does not fit the pattern of the other results. Repeat that measurement and check the method. Only leave it out of a mean if you have a clear reason; state which result you excluded and why.
For continuous variables, plot the independent variable on the horizontal axis and the dependent variable vertically. Use sensible scales, units and a best-fit line or curve; do not automatically join every point or force the graph through zero.
Find the gradient of a straight best-fit line using a large triangle: vertical change ÷ horizontal change. For a curve, draw a tangent to estimate the gradient at one point. Explain what the gradient shows in this experiment, include its units and use measured values to support your conclusion.
Uncertainty describes the possible range around a measurement. For one reading on a scale, half the smallest division is a useful classroom estimate unless the question says otherwise. If you subtract two readings, both have uncertainty. Percentage uncertainty = absolute uncertainty ÷ measured value × 100. Follow the method specified in the question.
Use results as evidence and then explain what they mean. A pattern linking variables (a correlation) does not prove that one causes the other. If the ranges of repeat results overlap, a claimed difference may be less convincing. Keep conclusions within the range tested and suggest an improvement that tackles a specific error.
Mr Kidman's Classroom