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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. 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. RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone react in the presence of acid CH3COCH3 + I2 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? CH3COCH2I + HI RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone react in the presence of acid CH3COCH3 + I2 CH3COCH2I + HI The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in the rate equation? The reaction is catalysed by acid [H+] affects the rate but is unchanged overall Why does I2 not appear in the rate equation? The rate determining step doesn’t involve I2 RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone react in the presence of acid CH3COCH3 + I2 CH3COCH2I + HI The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in the rate equation? The reaction is catalysed by acid [H+] affects the rate but is unchanged overall Why does I2 not appear in the rate equation? The rate determining step doesn’t involve I2 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 In pairs: remember the two mechanisms for the hydrolysis of haloalkanes. Discuss how can you make the reaction faster in each case. RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES OH- Haloalkanes (general formula RX) are hydrolysed by hydroxide ion to give alcohols. RX + With many haloalkanes the rate equation is... This is because both the RX and OH- must collide for a reaction to take place in ONE STEP r = k [RX][OH-] ROH + X- SECOND ORDER RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES OH- RX + With many haloalkanes the rate equation is... This is because both the RX and OH- must collide for a reaction to take place in ONE STEP r = k [RX][OH-] SECOND ORDER 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 - the second step involves the two ions joining i) ii) ROH + X- Haloalkanes (general formula RX) are hydrolysed by hydroxide ion to give alcohols. RX R+ + XR+ + OHROH Slow Fast 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. Worksheet 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? RATE DETERMINING STEP The reaction H2O2 + 2H3O+ + 2I¯ Step 1 H2O2 + I¯ Step 2 IO¯ + H3O+ Step 3 HIO + H3O+ + I¯ I2 + 4H2O IO¯ + H2O HIO + H2O I2 + 2H2O The rate determining step is STEP 1 as it is the slowest takes place in 3 steps SLOW FAST FAST RATE DETERMINING STEP The reaction H2O2 + 2H3O+ + 2I¯ Step 1 H2O2 + I¯ Step 2 IO¯ + H3O+ Step 3 HIO + H3O+ + I¯ I2 + 4H2O IO¯ + H2O HIO + takes place in 3 steps SLOW H2O FAST 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 Step 2 NO2 + NO3 NO + NO2 + O2 FAST Step 3 NO + 2NO2 FAST NO3 The rate determining step is STEP 1 SLOW from another Step 1 rate = k [N2O5]