Diagrams and Graphs Worth Practising for the Class 12 Chemistry Paper
A handful of drawings recur every year, and each carries marks that are lost only for lack of practice.
Why these carry easy marks
A diagram question has a defined answer with defined labels. Unlike a long answer, there is no ambiguity about what earns the mark, so a well-practised diagram is close to guaranteed marks.
The diagrams that recur
| Diagram | Must show |
|---|---|
| Unit cells | Atom positions at corners, faces and body centre; the type named |
| Galvanic cell | Both electrodes, both solutions, salt bridge, electron flow direction, anode and cathode labelled |
| Potential energy profile | Reactants, products, activation energy, transition state, and the catalysed path if asked |
| Adsorption isotherms | Correct axes, and the plateau for Langmuir |
| Conductivity versus concentration | Different shapes for strong and weak electrolytes |
| Tetrahedral and octahedral voids | Position relative to the close-packed layers |
The galvanic cell
The commonest omissions are the direction of electron flow and the identification of anode and cathode. Both carry marks and both take a second to add.
Remember the convention: oxidation at the anode, reduction at the cathode, electrons flowing through the external wire from anode to cathode. The salt bridge completes the circuit internally, and stating its purpose is often a separate mark.
Potential energy profile
Show the energy of reactants and products, the peak representing the transition state, and the activation energy measured from reactants to peak. If a catalysed path is required, draw it as a second curve with a lower peak, and note that the reactant and product energies are unchanged.
Drawing the catalysed curve with a different product energy is a common error and contradicts the fact that a catalyst does not affect the enthalpy of reaction.
Conductivity plots
Molar conductivity against the square root of concentration is a straight line for a strong electrolyte and a steeply rising curve for a weak one near infinite dilution. Drawing both on the same axes, correctly shaped, is a standard question, and the difference in shape is the whole answer.
Practising them
Draw each from memory, then compare against the textbook and note exactly what was missing. Repeat after a few days. Three passes spread over a fortnight is usually enough to make them automatic.
Keep one page with all of them, and use it as a final-week revision sheet. Diagrams decay from memory faster than reasoning does, so they benefit particularly from a late pass.
General points that earn marks
- Label both axes on any graph, with units where they apply.
- Mark any significant intercept or plateau explicitly.
- Use a ruler for axes and straight lines — a curve drawn where a straight line is expected suggests the wrong relationship.
- Where two curves are compared, label which is which.
- Keep diagrams large enough to label clearly.
Frequently asked questions
Do marks depend on drawing quality?
Only to the extent that the diagram must be readable and the features distinguishable. Accuracy of labelling matters far more than neatness.
Should axes always start at zero?
Where the origin is meaningful, yes, particularly for plots whose intercept carries information. Otherwise a clearly marked scale is sufficient.
How much detail is needed in a cell diagram?
Enough to identify every component and the direction of flow. Elaborate apparatus drawing is not required and wastes time.
Is the catalysed curve drawn on the same axes?
Yes, and with the same reactant and product energies. Only the peak height changes, which is the whole point of the comparison.
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