Because → Therefore: How to Explain Trends in Chemistry
Many “explain” marks are lost for one reason: the answer gives facts, but not a cause → effect chain.
This template fixes that.
Because [driver / cause], therefore [what changes at particle/ionic level], so [the observation / trend].
Quick Reference (what your chain must contain)
| Part | What to write | Examples of good words |
|---|---|---|
| Because | the real driver (cause) | effective nuclear charge, shielding, collision frequency, fraction with energy ≥ Eₐ, equilibrium shift, oxidation/reduction tendency |
| Therefore | the mechanism at particle/ionic level | stronger attraction, more effective collisions, greater extent of ionisation, more product-favoured, electrons flow from… to… |
| So | what the question asked for (observable trend) | higher boiling point, faster rate, higher pH, more reactive, larger E°_cell |
1) The #1 mark-loser to delete
If your explanation contains any of these, rewrite it:
- “Because it increases…” (what increases?)
- “Because it is stronger…” (what is stronger and why?)
- “Because it is more reactive…” (why is it more reactive?)
- “Because it has more energy…” (what energy? where does it act?)
Replace vague words with a named driver: particles, ions, electrons, attraction, shielding, effective nuclear charge, activation energy, equilibrium position.
2) Driver library (high-frequency drivers you can reuse)
A. Kinetics (rate)
- Driver: temperature increases average kinetic energy.
- Mechanism: larger fraction of particles have energy ≥ Eₐ → more effective collisions per second.
- Result: rate increases.
Helpful hubs:
- O Level: Collision Theory, Factors Affecting Rate
- A Level: Reaction Kinetics, Activation Energy & Boltzmann
B. Periodicity (trends)
- Driver: effective nuclear charge increases across a period (while shielding is similar).
- Mechanism: stronger attraction between nucleus and outer electrons.
- Result: smaller atomic radius / higher ionisation energy (with known anomalies).
Helpful hubs:
- O Level: The Periodic Table
- A Level: The Periodic Table (A Level), Atomic Structure
C. Equilibria (position changes)
- Driver: a change in concentration/pressure/temperature disturbs equilibrium.
- Mechanism: system shifts to oppose the change (Le Chatelier).
- Result: equilibrium composition changes (yield up/down).
Helpful hubs:
D. Electrochemistry (feasibility)
- Driver: difference in electrode potentials (tendency to be reduced).
- Mechanism: electrons flow from the species more easily oxidised to the species more easily reduced.
- Result: positive E°_cell → feasible under standard conditions (and Δ G° < 0).
Helpful hubs:
3) Worked Examples (use the structure, not the wording)
Attempt each in 2–4 lines. Then open the answer and check whether your chain had a named driver, a mechanism, and the final “so”.
Worked Example 1 (Kinetics)
Explain why increasing temperature increases the rate of reaction.
Show Answer
Because particles have higher kinetic energy at higher temperature, therefore a larger fraction of collisions have energy ≥ Eₐ, so there are more effective collisions per second and the rate increases.
Worked Example 2 (Periodicity)
Explain why atomic radius decreases across Period 3.
Show Answer
Because the proton number increases across Period 3 while shielding is similar (electrons are added to the same shell), therefore effective nuclear charge increases and attraction between nucleus and outer electrons is stronger, so atomic radius decreases.
Worked Example 3 (Equilibria: temperature)
In an exothermic equilibrium, explain why increasing temperature decreases the equilibrium yield of products.
Show Answer
Because increasing temperature adds heat, therefore the equilibrium shifts in the endothermic direction to absorb the added heat (Le Chatelier), so the position shifts towards reactants and the product yield decreases.
Worked Example 4 (Electrochemistry)
Explain why a cell with E°_cell > 0 is feasible under standard conditions.
Show Answer
Because E°_cell measures the driving force for electron transfer, therefore E°_cell > 0 means the overall redox reaction is spontaneous in the forward direction, so the cell reaction is feasible under standard conditions (and Δ G° is negative).
Worked Example 5 (Explain, using the data)
The volume of gas produced increases quickly at first, then levels off. Explain this pattern using the data trend.
Show Answer
Because the curve is steep at the start (high initial gradient), therefore the initial rate is high when reactant concentration is highest, so gas forms quickly at first. As the curve levels off, reactants are used up and concentration falls, therefore there are fewer effective collisions per second, so the rate decreases and the total volume stops increasing.
Top Exam Traps
- Writing the observation twice (describe → describe), with no driver.
- Using vague causes (“it increases”, “it is stronger”, “it is more reactive”).
- Skipping the particle/ionic mechanism (jumping from cause straight to result).
- For “using the data”: giving a correct explanation but never referencing the trend/values.
Next steps
- Exam Skills Hub: command words, definitions, data answers, and practical writing.
- Explain vs Describe vs Suggest: don’t do the wrong job.