Reactant concentration changes

The specific rate of an electrode reaction depends not only on electrode polarization but also on tfie reactant concentrations. Changes in reactant concentrations affect not only reaction rates but also the values of equilibrium potentials. To differentiate both these influences, kinetic equations are generally used (especially at high values of polarization), relating the current density not with the value of polarization AE but with the potential of the electrode E ... 

The half-life of a first-order reaction is a constant because it depends only on the rate constant and not on the reactant concentration. This point is worth noting because reactions that are not first order have half-lives that do depend on concentration that is, the amount of time in one half-life changes as the reactant concentration changes for a non-first-order reaction. 

The quasi-steady approximation is generally valid when the amount that enzyme complex concentrations change is less than the amount that reactant concentrations change over the timescale of interest. This is true, for example, in Section 3.1.3.2 as long as dc/dt <reactant concentrations are large compared to enzyme concentrations (a condition that is by no means universally true in vivo) is not necessarily required to apply the approximation. 

If) Example 10-6. What if. .. space time were changed How would the minimum reactant concentration change Compare your results w ith the case when the reactor is full of inerts at time r = 0 instead of 805i- reactant. Is your catalyst lifetime longer or shorter (2) What if the tentperature were increased so that the specific rate constants increase to k= 120 and = 12 Would your catalyst lifetime be longer or shorter than at ihe lower temperature (3) Describe how the minimum in reactant concentration changes as the space lime r changes Whai is the minimum if r = 0.005 h If/ = 0.01 h ... 

Because of the volume change of the reaction mixture, the reactant concentration changes not only by chemical transformation, but also by expansion. Supposing a linear dependency between reaction volume and conversion (Equation 2.64), the concentration of reactant at any point of the reactor is given by ... 

In order to get the current—potential relationship on the RDE, particularly the expression of limiting current density as the function of the electrode rotating rate and the reactant concentration, Pick s second law has to be used to give the equations of reactant concentration change with time at the steady-state situation of diffusion—convection. When the surface concentration of oxidant reaches zero during the reaction at the steady-state situation, the concentration distribution within the diffusion—convection layer is not changing with time anymore, meaning that the diffusion rate is... 

If the essential experimental requirement of a very low enzyme concentration is fulfilled, a reaction can be studied over a period in which the reactant concentrations change so little that they can be regarded as constants, so there is no need to distinguish between their initial and instantaneous values. In the case of a reaction involving the conversion of a single substrate A, the equation that describes the steady-state behaviour is the Michaelis-Menten equation ... 

In both oxidation and hydrolysis, the polymer matrix reacts with a small molecule coming from the environment (oxygen or water, for instance). In a thin elementary layer at a distance z from the sample surface, the reactant concentration balance can be ascribed as follows reactant concentration change = rate of reactant supply by diffusion - rate of reactant consumption by reaction. 

To find the values of m and n, we run a series of experiments in which one reactant concentration changes while the other is kept constant, and we measure the effect on the initial rate in each case. Table 16.2 on the next page shows the results. 

A rate law tells you how the rate of a reaction depends on reactant concentrations at a particular moment. But often you would like to have a mathematical relationship showing how a reactant concentration changes ovct a period of time. Such an equation would be directly comparable to the experimental data, which are usually obtained as concentrations at various times. In addition to summarizing the experimental data, this equation would predict concentrations for all times. Using it, you could answer questions such as How long does it take for this reaction to be 50% complete to be 90% complete ... 

In the genera] case of constant current i electrolysis, the reactant concentration change Ac is given by Faraday s laws of electrolysis as ... 

The factors that control the rates, or speeds, at which chemical reactions occur, summarized in the branch of chemistry known as chemical kinetics, were discovered, and chemists learned how to control the rates of reactions by altering reactant concentrations, changing the temperature, or introducing a catalyst, a substance that speeds up a chemical reaction without itself being altered in the process. 

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