Effect of Reactant Concentration on Reaction Rate

Reaction of Sodium Thiosulphate with Sulphuric Acid. Pour 10 ml of a 2.5% sulphuric acid solution into each of four small heakers, and a 5% sodium thiosulphate solution and water into four other beakers in the following proportions  

Measure the volumes of the solutions as accurately as possible and pour them together in pairs. Use a stop watch or a metronome to note the time interval after which turbidity appears. What is the turbidity of the solution caused by Write the equations of the reactions. Enter the results in your laboratory notebook using Form 7. 

REACTION OF SODIUM THIOSULPHATE WITH SULPHURIC ACID 

Experiment No. Volume of solution, ml Volume of water, ml Total volume, ml Time, s 

The concentration of which substance and how many times changes from one experiment to another What conclusion should be made from the data of the experiments How does the reaction rate depend on the reactant concentration  

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The effect of reactant concentrations on reaction rate was studied using unpromoted skeletal copper catalysts initially leached at 278 K and then... 

Effect of Reactant Concentration on Reaction Rate (70). Effect... 

One way to study the effect of reactant concentration on reaction rate is to determine how the initial rate depends on the starting concentrations. It is preferable to measure the initial rates because as the reaction proceeds, the concentrations of the reactants decrease and it may become difficult to measure the changes accurately. Also, there may be a reverse reaction of the type... 

In chemical equilibria, the energy relations between the reactants and the products are governed by thermodynamics without concerning the intermediate states or time. In chemical kinetics, the time variable is introduced and rate of change of concentration of reactants or products with respect to time is followed. The chemical kinetics is thus, concerned with the quantitative determination of rate of chemical reactions and of the factors upon which the rates depend. With the knowledge of effect of various factors, such as concentration, pressure, temperature, medium, effect of catalyst etc., on reaction rate, one can consider an interpretation of the empirical laws in terms of reaction mechanism. Let us first define the terms such as rate, rate constant, order, molecularity etc. before going into detail. 

So far only the reactants directly involved in a reaction have been considered in their contribution to the rate law. Added inert cations and anions can sometimes contribute in a profound way by modifying the major reactants (e. g. by ion pairing) but usually the effects of their concentrations on the rates of reactions are best accommodated by the general theories of the effect of ionic strength on the reaction rate (Sec. 2.9.1). 

Steven W. Wright, "Tick Tock, a Vitamin C Clock/ J. Chem. Educ., Vol. 79, 2002,40A-40B. This article presents an activity that uses supermarket chemicals to perform a clock reaction in which the endpoint is signaled by an abrupt change in appearance from colorless to blue-black. This activity can be used to explore reaction kinetics and the effect of reactant concentrations on the apparent rate of reaction. 

Equation (3.40) is quite general and describes the effect of reactant concentration on rate, not only at the start of the reaction in the complete absence of P (in the forward direction) or A (in the reverse direction), but also at any time during the approach to equilibrium (Alberty, 1959 Qeland, 1963, 1977). 

The order of a reaction can be determined only by experiment. A common way to determine reaction order is the method of initial rates. In this method, the initial rate— the rate for a short period of time at the beginning of the reaction—is measured by running the reaction several times with different initial reactant concentrations to determine the effect of the concentration on the rate. For example, let s return to our simple reaction in which a single reactant. A, decomposes into products ... 

FIGURE 12.1 Effects of substrate (reactant) concentration on the rate of enzymatic reactions (a) simple Michaelis-Menten kinetics (b) substrate inhibition (c) substrate activation. 

In Lab 17.1, you learned about the effect of temperature and concentration on reaction rate. Another factor that affects reaction rate is the amount of surface area of the reactants. If a chemical reaction is to take place, the molecules of reactants must collide. Changing the amount of surface area modifies the rate of collision, and, thus, the rate of reaction. If surface area increases, collision frequency increases. If surface area decreases, so does the number of collisions. In this lab, you will examine the effect of surface area on rate of reaction. You will also determine how a combination of factors can affect reaction rate. 

The effect of external pressure on the rates of liquid phase reactions is normally quite small and, unless one goes to pressures of several hundred atmospheres, the effect is difficult to observe. In terms of the transition state approach to reactions in solution, the equilibrium existing between reactants and activated complexes may be analyzed in terms of Le Chatelier s principle or other theorems of moderation. The concentration of activated complex species (and hence the reaction rate) will be increased by an increase in hydrostatic pressure if the volume of the activated complex is less than the sum of the volumes of the reactant molecules. The rate of reaction will be decreased by an increase in external pressure if the volume of the activated complex molecules is greater than the sum of the volumes of the reactant molecules. For a decrease in external pressure, the opposite would be true. In most cases the rates of liquid phase reactions are enhanced by increased pressure, but there are also many cases where the converse situation prevails. 

Besides the already mentioned techniques, a low-temperature plasma has been adopted to enhance the reaction in CVC. Through the synthesis of AIN UFPs by an RF-plasma-enhanced CVC using trimethylaluminum [A1(CH3)3] and NH3 as reactants, the effect of experimental parameters on the rate of powder formation, particle size, and structure was examined (60). A high RF current was primarily connected to a high electron density, which activated the gas-phase reaction to promote the powder formation rate. The increase of both susceptor temperature and A1(CH3)3 concentration also increased the powder formation rate and enhanced the grain growth, where both mechanisms—coalescence by particle collision and vapor deposition on to particle surfaces—were believed to occur. 

The effect of reactant concentration can be divided into two separate influences. The simplest is obvious Lower overall concentrations result in a slower rate. This does not necessarily mean a thinner fihn, however—sometimes the opposite. The reason for this is clear if we return to our introductory discussion on the CD process—rapid precipitation. It is clear that if the reaction is too fast, it will terminate with most of the product precipitating homogeneously in solution rather than depositing on the substrate (which requires time to occur). This results in a very thin film, if any fihn at all. Similarly, for the less extreme case of a CD reaction that terminates, not within a second, but still in a short time, the final fihn thickness will be small. At the other extreme, if the reaction is extremely slow, a thick fihn can be built up, but it may take a very long time for this to occur (weeks, even months). It is therefore evident that there is an optimum rate for the reaction, which can be controlled by a combination of reactant concentrations, temperature, and pH. 

Kinetic analysis is one of the most basic topics of en-zymology. Such studies reveal not only how fast an enzyme can function, but also its preferences for various reactants (or substrates as they usually are called), the effect of substrate concentration on the reaction rate, and the sensitivity... 

The relationship between the concentration of reactants and the reaction rate is described by a factor known as the reaction order. In the previous example, the relationship between the reactants and the reaction rate was directly proportional, meaning that an increase in the concentration of one reactant caused proportionally the same increase in the rate. Doubling the concentration of a reactant doubled the rate of the reaction. This directly proportional relationship is known as a first-order relationship. If changing the concentration of a reactant had no effect on the reaction rate, the relationship would be described as a zero-order relationship. A second-order relationship is exponential in other words, doubling the concentration of a reactant will increase the rate by 4. The reaction order for a particular reactant is written as an exponent next to the concentration of that reactant. For instance, because the previous reaction was first order for substance A, we could represent this symbolically as [A] (the exponent 1 is understood). If A had a zero-order or second-order relationship, the symbols would be written [A]0 and [A]2, respectively. 

Figure 9.2 shows the concentration profile for various values of Ly/kv/De. This solution shows that the diffusion resistance causes a concentration profile to exist in the pellet when the reactant cannot diffuse in from the bulk sufficiently rapidly. If the resistance is small due to a large value of De, then the concentration profile becomes flat, while it will behave conversely for a large diffusion resistance. In practice, however, the possible adverse effect of diffusion resistance on the rate of reaction is highly compensated by the enormous increase in surface area of the pores. 

In liquid-phase reactions, the concentration of reactants is insignificantly affected by even relatively large changes in the total pressure. Consequently, we can totally ignore the effect of pressure drop on the rate of reaction when sizing liquid-phase chemical reactors. However, in gas-phase reactions, the concentration of the reacting species is proportional to the total pressure and consequently, proper accounting for the effects of pressure drop on the reaction system can, in many instances, be a key factor in the success or failure of the reactor operation. 

A major point was made after a critical review of numerous reports on reaction rates in frozen systems (50). Kinetic-mechanistic surprises in frozen systems may not require exceptional hypotheses. Concentration effects may account for them. Even if a system appears to be completely solidified and, therefore, not amenable to analysis in terms of unfrozen liquid puddles (51), a liquid phase should still be considered a possibility. A case in point is provided by the efficient electron transfer observed between ferrous and ferric ions in an aqueous system frozen below its putative eutectic point (52). This seemed to require an ice structure in order to bridge the distance between reactants that were calculated to be too far apart for significant reactivity. However, it was pointed out that the assumed eutectic point was based only on the major... 

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