Temperature & Rate Constants (OCR A Level Chemistry)

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The Effect of Temperature on Rate Constants

  • The following general reaction and rate equation will be used to discuss the effect of temperature on the rate constant, k:

A + B → C + D

Rate of reaction = k[A][B]

  • The rate equation shows that rate of reaction depends on the rate constant, k, and the concentration of the reactants
    • As the rate of reaction increases the rate constant will increase

  • Increasing the temperature of a reaction increases the rate of a chemical reaction
    • Remember: this does not necessarily increase the yield of a chemical reaction depending on whether a reaction is endothermic or exothermic according to Le Châtelier’s principle

  • Therefore, increasing the temperature also increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged

  • An exponential relationship between the rate of reaction and temperature is observed when seen on a graph:

Relationship between temperature and rate constant k, NH4Cl, and sodium iodide, NaI, with water, hydration enthalpies and enthalpy of solution, downloadable IB Chemistry revision notes

Relationship between temperature and rate constant, k

  • The graph shows that the rate of reaction roughly doubles with an increase of 10 oC
    • This general relationship does not apply to all reactions
    • Also, it is not necessarily every 10 oC, the rate may double every 9 °C or 11 °C
    • The number of degrees needed to double the rate also changes gradually as temperature increases

  • When the temperature of a reaction mixture increases, there are two contributing factors to the increased rate and rate constant:
    • Increasing temperature causes the particles to move around faster resulting in more frequent collisions
    • Furthermore, the proportion of successful collisions increases, meaning a higher proportion of the particles possess the minimum amount of energy (activation energy) to cause a chemical reaction

Reaction Kinetics Boltzmann Distribution Curve at higher Temperature, downloadable AS & A Level Chemistry revision notes

The Maxwell-Boltzmann distribution curve at T oC and when the temperature is increased by 10 oC

  • Therefore, an increase in temperature causes an increased rate of reaction due to:
    • There being more effective collisions as the particles have more kinetic energy, making them move around faster
    • A greater proportion of the molecules having kinetic energy greater than the activation energy

  • The increase in proportion of molecules having kinetic energy greater than the activation has a greater effect on the rate of reaction than the increase in effective collisions

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Richard

Author: Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.