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The rate-determining step

The rate-determining step. L.O.: What is the rate-determining step of a reaction? What is the connection between the rate equation for a reaction and the mechanism?. Discuss in pairs any of these two situations. Think what are the steps involved and which one is the slowest step.

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The rate-determining step

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  1. The rate-determining step L.O.: What is the rate-determining step of a reaction? What is the connection between the rate equation for a reaction and the mechanism?

  2. Discuss in pairs any of these two situations. Think what are the steps involved and which one is the slowest step.

  3. RATE DETERMINING STEP Many reactions consist of a series of separate stages. Each step has its own rate and rate constant. The overall rate of a multi-step process is governed by the slowest step (like a production line where overall output can be held up by a slow worker). This step is known as the RATE DETERMINING STEP. If there is more than one step, the rate equation may not contain all the reactants in its format.

  4. RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in the rate equation? Why does I2 not appear in the rate equation?

  5. RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in The reaction is catalysed by acid the rate equation? [H+] affects the rate but is unchanged overall Why does I2 not appear The rate determining step doesn’t involve I2 in the rate equation?

  6. RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in The reaction is catalysed by acid the rate equation? [H+] affects the rate but is unchanged overall Why does I2 not appear The rate determining step doesn’t involve I2 in the rate equation? The slowest step of any multi-step reaction is known as the rate determining step and it is the species involved in this step that are found in the overall rate equation. Catalysts appear in the rate equation because they affect the rate but they do not appear in the stoichiometric equation because they remain chemically unchanged

  7. In pairs: remember the two mechanisms for the hydrolysis of haloalkanes. Discuss how can you make the reaction faster in each case.

  8. RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES Haloalkanes (general formula RX) are RX + OH- ROH + X- hydrolysed by hydroxide ion to give alcohols. With many haloalkanes the rate equation is...r = k [RX][OH-] SECOND ORDER This is because both the RX and OH- must collide for a reaction to take place in ONE STEP

  9. RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES Haloalkanes (general formula RX) are RX + OH- ROH + X- hydrolysed by hydroxide ion to give alcohols. With many haloalkanes the rate equation is... r = k [RX][OH-] SECOND ORDER This is because both the RX and OH- must collide for a reaction to take place in ONE STEP but with others it only depends on [RX]...r = k [RX] FIRST ORDER The reaction has taken place in TWO STEPS... - the first involves breaking an R-X bond i) RX R+ + X-Slow - the second step involves the two ions joining ii) R+ + OH- ROHFast The first step is slower as it involves bond breaking and energy has to be put in. The first order mechanism is favoured by tertiary haloalkanes because the hydroxide ion is hindered in its approach by alkyl groups if the mechanism involves the hydroxide ion and haloalkane colliding.

  10. Worksheet

  11. The rate-determining step L.O.: What is the rate-determining step of a reaction? What is the connection between the rate equation for a reaction and the mechanism?

  12. RATE DETERMINING STEP The reaction H2O2 + 2H3O+ + 2I¯ I2 + 4H2O takes place in 3 steps Step 1 H2O2 + I¯ IO¯ + H2O SLOW Step 2 IO¯ + H3O+ HIO + H2O FAST Step 3 HIO + H3O+ + I¯ I2 + 2H2O FAST The rate determining step is STEP 1 as it is the slowest

  13. RATE DETERMINING STEP The reaction H2O2 + 2H3O+ + 2I¯ I2 + 4H2O takes place in 3 steps Step 1 H2O2 + I¯ IO¯ + H2O SLOW Step 2 IO¯ + H3O+ HIO + H2O FAST Step 3 HIO + H3O+ + I¯ I2 + 2H2O FAST The rate determining step is STEP 1 as it is the slowest The reaction 2N2O5 4NO2 + O2 takes place in 3 steps Step 1 N2O5 NO2 + NO3 SLOW Step 2 NO2 + NO3 NO + NO2 + O2 FAST Step 3 NO + NO3 2NO2from another Step 1 FAST The rate determining step is STEP 1 rate = k [N2O5]

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