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Chemistry · Grades 9–12

Reaction rates and catalysts

Separate a rate change from a change in equilibrium or reaction enthalpy.

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Learn the idea

Rate

A rate measures concentration or amount change per time under a specified definition.

Collisions

Suitable encounters and sufficient energy help explain rates in an introductory model.

Catalyst

A catalyst provides an alternative pathway with a lower activation barrier and is regenerated overall.

Equilibrium

A catalyst speeds approach to equilibrium without changing the equilibrium constant at fixed temperature.

Worked examples

A reactant concentration falls from 0.80 to 0.50 mol/L in 30 s. What is its average disappearance rate?

The positive disappearance rate is (0.80 − 0.50)/30 = 0.010.

0.01 mol/(L·s)

What does a catalyst lower in the simple energy-pathway description?

It provides an alternative pathway.

The activation barrier.

Check your understanding

  1. At fixed temperature, does a catalyst change the equilibrium constant?

    1. Yes, always doubles it.
    2. No.
    3. Only by making matter vanish.

  2. Why should the surface area of solid particles be controlled in a rate experiment?

  3. A faster reaction necessarily has a more negative ΔH. Is that valid?

    1. No; rate and reaction enthalpy are different properties.
    2. Yes, unless it is blue.
    3. Yes, by definition.

Answers and marking notes

Answers and marking notes
  1. No.

    The catalyst changes rates, not the thermodynamic equilibrium constant.

  2. Different exposed surface areas can change reaction rate and confound the intended comparison.

    Identify it as a potential variable affecting contact.

  3. No; rate and reaction enthalpy are different properties.

    A barrier controls rate separately from the net enthalpy change.

Further reading and review references

These are reference links, not copied exercises, institutional endorsement or proof of independent review.

Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.