Common misconceptions › Class 12 › Chemistry › Chemical Kinetics Class 12 Chemistry · Chapter 3 · NCERT (lech103)
Chemical Kinetics: common misconceptions
See the tempting mistake students make in Chemical Kinetics, why it feels right at first glance, and a worked visual example that shows the right idea.
Worked visual (2) “If your child says…” (5) Teach & practice 2 worked misconception examples Worked example 1 of 2
Why are the rates for two reactants different? Wrong
In 2A + B → products, A and B disappear at the same rate.
Why it feels right. A and B are part of the same reaction, so I expect a shared rate to mean equal losses.
Right
Each reaction event consumes twice as much A as B. Divide each disappearance rate by its coefficient to obtain the reaction rate.
Worked example
2 mmol of A A A 1 mmol of B B In the same second, two units of A disappear for each one of B. If 2 mmol of A and 1 mmol of B disappear each second in the same vessel, the disappearance rates differ. Dividing by 2 for A and 1 for B gives the same normalised amount rate of 1 mmol/s. Worked example 2 of 2
Can you read reaction order from a balanced equation? Wrong
For an overall reaction written 2A + B → products, the order in A must be 2.
Why it feels right. The equation gives a 2 for A, so I copy it into the rate law.
Right
For an overall reaction, the balanced equation gives the amounts consumed. Rate-law powers come from measurements.
Worked example
0.10 M A 0.20 M A At fixed B and temperature, doubling A doubles the measured initial rate. In a hypothetical experiment on a multistep reaction, hold B and temperature fixed. Increase A from 0.10 M to 0.20 M. The initial rate doubles from 0.020 to 0.040 M/s, consistent with first order in A. If your child says… (5 concept checks in this chapter)
As you work through Chemical Kinetics , listen for these everyday misunderstandings and use the prompt alongside each one.
If your child says…
“Using up a reactant makes the reaction rate negative.”
Ask together What sign results when you put a minus before a negative concentration change? What’s true A falling reactant concentration gives a positive disappearance rate after its sign is reversed. If your child says…
“Doubling concentration leaves the rate unchanged.”
Ask together Which power makes the measured doubling agree with this line? What’s true For this measured first-order relationship, doubling concentration doubles rate while other conditions stay fixed. If your child says…
“Doubling the starting concentration doubles every reaction's half-life.”
Ask together What happens to the concentration ratio when both starting and halfway concentrations double? What’s true Starting concentration affects zero-order half-life, but not first-order half-life at unchanged rate constant. If your child says…
“A catalyst cannot change reaction speed unless it heats the mixture.”
Ask together On the right, did the energy distribution change when the threshold moved left? What’s true A catalyst can increase reaction speed at unchanged temperature by providing a different pathway. If your child says…
“Turning a molecule cannot change whether its collision works.”
Ask together Why does the top row fail even with enough energy? What’s true The same molecules can collide ineffectively when their reacting parts do not meet suitably. Use this chapter three ways Previous chapter Electrochemistry Next chapter The d- and f-Block Elements
In the app, your child answers a short check-in with confidence taps, and you see which of these misunderstandings showed up—and which ones are gone.
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