rate-determining step

Examples of rate-determining step in the following topics:

• Rate-Determining Steps

  • The rate of a multi-step reaction is determined by the slowest elementary step, which is known as the rate-determining step.
  • In kinetics, the rate of a reaction with several steps is determined by the slowest step, which is known as the rate-determining, or rate-limiting, step.
  • Since the first step is the slowest, and the entire reaction must wait for it, it is the rate-determining step.
  • If the second or a later step is rate-determining, determining the rate law is slightly more complicated.
  • Describe the relationship between the rate determining step and the rate law for chemical reactions

• Molecularity

  • If a chemical reaction proceeds by more than one step or stage, its overall velocity or rate is limited by the slowest step, the rate-determining step.
  • Once a group gathers at the door, the speed at which other people leave their seats and move along the aisles has no influence on the overall exit rate.
  • When we describe the mechanism of a chemical reaction, it is important to identify the rate-determining step and to determine its "molecularity".
  • The molecularity of a reaction is defined as the number of molecules or ions that participate in the rate determining step.
  • A mechanism in which two reacting species combine in the transition state of the rate-determining step is called bimolecular.

• Overall Reaction Rate Laws

  • Rate laws for reactions are affected by the position of the rate-determining step in the overall reaction mechanism.
  • Since the first step is the rate-determining step, the overall reaction rate for this reaction is given by this step: $\text{rate}=k[H_2][ICl]$.
  • Step two is the slow, rate-determining step, so it might seem reasonable to assume that the rate law for this step should be the overall rate law for the reaction.
  • We can now substitute this expression into the rate law for the second, rate-determining step.
  • How to determine the rate law for a mechanism with a fast initial step.

• Steady-State Approximation

  • The steady state approximation can be used to determine the overall rate law when the rate-determining step is unknown.
  • This slowest step determines the rate of the entire reaction, and as such, it is called the rate-determining step.
  • We will now consider cases in which the rate-determining step is either unknown or when more than one step in the mechanism is slow, which affects the overall reaction rate.
  • Before, we assumed that the first step was fast, and that the second step was slow, thereby making it rate-determining.
  • We will now proceed as if we had no such prior knowledge, and we do not know which, if either, of these steps is rate-determining.

• Rate Laws for Elementary Steps

  • The rate law of the rate-determining step must agree with the experimentally determined rate law.
  • The rate-determining step is the slowest step in a reaction mechanism.
  • The molecularity of the elementary step, and the reactants involved, will determine what the rate law will be for that particular step in the mechanism.
  • For now, just keep in mind that the rate laws for each elementary step are determined by the molecularity of each step only.
  • The molecularity of an elementary step in a reaction mechanism determines the form of its rate law.

• The Rate Law

  • For the general reaction$aA + bB \rightarrow C$ with no intermediate steps in its reaction mechanism, meaning that it is an elementary reaction, the rate law is given by:
  • The exponents x and y vary for each reaction, and they must be determined experimentally; they are not related to the stoichiometric coefficients of the chemical equation.
  • To reiterate, the exponents x and y are not derived from the balanced chemical equation, and the rate law of a reaction must be determined experimentally.
  • A certain rate law is given as $Rate=k[H_2][Br_2]^\frac{1}{2}$.
  • Note that the reaction order is unrelated to the stoichiometry of the reactions; it must be determined experimentally.

• Foreign Exchange Rates

  • They calculate the cross rate to determine the exchange rate for these currencies.
  • It does not matter which exchange rates we calculate the cross rate from.
  • Step 1: Trader converts the U.S. dollars to British pounds at Citibank, yielding 64,935.06 £.
  • Step 2: Trader converts the British pounds into euros at Credit Suisse, yielding 103,896.10 €.
  • Step 3: Finally, the trader converts the euros into U.S. dollars at Deutsche bank.

• Activation Energy

  • Activation energy is the energy required for a reaction to occur, and determines its rate.
  • The reason lies in the steps that take place during a chemical reaction.
  • For this reason, reactant molecules don't last long in their transition state, but very quickly proceed to the next steps of the chemical reaction.
  • The activation energy of a particular reaction determines the rate at which it will proceed.
  • It is determined experimentally.

• Confidence Interval for a Population Mean, Standard Deviation Not Known

  • Step By Step Example of a Confidence Interval for a Mean—Standard Deviation Unknown
  • Suppose you do a study of acupuncture to determine how effective it is in relieving pain.
  • You measure sensory rates for 15 random subjects with the results given below:
  • We have a sample of 15 rates.
  • We are 95% confident that the interval from 7.30 to 9.15 contains the true mean score of all the sensory rates. 95% of all confidence intervals constructed in this way contain the true mean sensory rate score.

• Measuring Vendor Performance

  • Firms can measure vendor quality, service, availability, and overall reliability to determine future engagement with the vendor.
  • Decision makers complete five steps when making a business buying decision:
  • Vendor performance measurement plays a role in Steps 3 and 5.
  • Step 3 requires searching for and evaluating possible products and suppliers.
  • Supplier performance evaluation teams are used to monitor activity and performance data, and to rate vendors.