This method is useful when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder. The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions. The gradient of the line is equal to the rate of reaction:. A few moments later, the instantaneous rate at a specific moment—call it t 1 —would be somewhat slower, as indicated by the speedometer reading at that point in time.
As time passes, the instantaneous rate will continue to fall until it reaches zero, when the car or reaction stops. Like the decelerating car, the average rate of a chemical reaction will fall somewhere between its initial and final rates. The instantaneous rate of a reaction may be determined one of two ways.
If experimental conditions permit the measurement of concentration changes over very short time intervals, then average rates computed as described earlier provide reasonably good approximations of instantaneous rates. Alternatively, a graphical procedure may be used that, in effect, yields the results that would be obtained if short time interval measurements were possible. If we plot the concentration of hydrogen peroxide against time, the instantaneous rate of decomposition of H 2 O 2 at any time t is given by the slope of a straight line that is tangent to the curve at that time Figure 2.
We can use calculus to evaluating the slopes of such tangent lines, but the procedure for doing so is beyond the scope of this chapter. These test strips contain various chemical reagents, embedded in small pads at various locations along the strip, which undergo changes in color upon exposure to sufficient concentrations of specific substances. The usage instructions for test strips often stress that proper read time is critical for optimal results.
This emphasis on read time suggests that kinetic aspects of the chemical reactions occurring on the test strip are important considerations. The test for urinary glucose relies on a two-step process represented by the chemical equations shown here:.
The first equation depicts the oxidation of glucose in the urine to yield glucolactone and hydrogen peroxide. The hydrogen peroxide produced subsequently oxidizes colorless iodide ion to yield brown iodine, which may be visually detected. Some strips include an additional substance that reacts with iodine to produce a more distinct color change.
The two test reactions shown above are inherently very slow, but their rates are increased by special enzymes embedded in the test strip pad. This is an example of catalysis , a topic discussed later in this chapter. A typical glucose test strip for use with urine requires approximately 30 seconds for completion of the color-forming reactions. Reading the result too soon might lead one to conclude that the glucose concentration of the urine sample is lower than it actually is a false-negative result.
Waiting too long to assess the color change can lead to a false positive due to the slower not catalyzed oxidation of iodide ion by other substances found in urine. The rate of a reaction may be expressed in terms of the change in the amount of any reactant or product, and may be simply derived from the stoichiometry of the reaction.
Consider the reaction represented by the following equation:. The stoichiometric factors derived from this equation may be used to relate reaction rates in the same manner that they are used to related reactant and product amounts. The relation between the reaction rates expressed in terms of nitrogen production and ammonia consumption, for example, is:. What is Instantaneous Rate 3. What is Average Rate 4. Instantaneous rate is the rate of a chemical reaction that is measured as the change of the concentration of reactants or the products during a known time period.
In this method, the rate of the reaction during a specific instant in time is measured. It can also be measured as the rate of the reaction at a particular moment. The instantaneous rate is also known as differential rate. The reaction rate of a chemical reaction is often different from one point to another during the progression of the reaction reaction rate changes continuously.
The reaction rate slows when reactants are used up for the reaction. This is because the concentration of reactants decreases with the progression of the reaction reactants are consumed by the chemical reaction.
Above graph shows the decrease of concentration of reactants with time in a chemical reaction; the instantaneous rate is the rate of the reaction at a particular point red colored point ; the average rate is calculated by dividing the total concentration of reactants at the beginning from the total time 50 minutes.
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