How to Calculate Initial Rate of Reaction
Nov 21, †Ј In this video I will teach you how to calculate the initial rate of reaction from a graph quickly and easily using the tangent method. This is a required ski. The method of initial rates is a commonly used technique for deriving rate laws. As the name implies, the method involves measuring the initial rate of a reaction. The measurement is repeated for several sets of initial concentration conditions to see how the reaction rate varies.
The rate of a chemical reaction can be thought of as the speed at which a chemical reaction happens and this can be expressed as the change in concentration of a substance over time:. The concentration vs time graph shows the amount of reactant being used up over time or the amount of product being produced over time. The graphs will have slopes going in opposite directions to each other as illustrated below. Click on the tabs below to learn more. It can be determined by the slope of the line.
The initial rate of a reaction is the instantaneous rate at the start of the reaction. The instantaneous rate is the rate at some instant in time eg. To calculate the instantaneous rate, measure the negative slope of the line fate to the curve at the time t of the reactant vs time curve. You can also use the product vs time curve to calculate instantaneous rate of reaction.
Change in concentration is calculated using [A]final Ч [A]initial and change in time is calculated using T final Ч T initial. Speed cannot be negative so the reactant rates are given - sign in front in order to cancel out the negative values. Video credit: Dr. To learn about reaction rate, a simple experiment using magnesium hos hydrochloric acid can be performed.
In this experiment a small piece of magnesium ribbon is placed in HCl solution and the reaction produces hydrogen gas. The gas is collected in a gas syringe and the volume produced is measured every 10 seconds. Using the data collected, a graph can be generated using an excel spreadsheet. From the graph, the initial, average, and instantaneous rate of reaction can be determined using the methods described above.
The change in volume must first be converted to change in concentration in order to plot the concentration vs time graph.
It will look like something below. As time proceeds, the reaction slows down and eventually stops when all of the Mg is determime used up. Your email address will not be published.
Facebook Twitter RSS. Rate of reaction. Rate of reaction graphs The concentration vs time graph what is masonry sand used for the amount of reactant being used up over time or the amount of product being produced over time.
The product is increasing over time, the graph has an ascending curve. Submit a Comment Cancel reply Your email address will not be published. Chemical kinetics 1. Introduction to Chemical Kinetics off. Rate of reaction 3. Differential Rate Laws 4. Integrated Rate Laws 5. Mechanisms and Intermediates 6. Reaction Coordinate Diagram.
Contributors and Attributions
The initial rate of a reaction is the instantaneous rate at the start of the reaction (i.e., when t = 0). The initial rate is equal to the negative of the slope of the curve of reactant concentration versus time at t = 0. To determine the rate law of a chemical reaction using the Method of Initial Rates. To determine the activation energy of the reaction by finding the value of the rate constant, \(k\), at several temperatures. To observe the effect of a catalyst on the reaction rate. Instantaneous Rate of Reaction The average rate of reaction is the change in concentration over the period of time. It can be determined by the slope of the line.
Kinetics is the branch of physical chemistry that studies the rate of chemical reactions. In contrast, thermodynamics tells us which direction of the reaction is favored, without revealing its reaction rate. Some reactions can be thermodynamically favored but kinetically unfavored. For example, in the conversion of diamond to graphite, graphite has a lower free energy than diamond, so the conversion is thermodynamically favored.
However, there is a large activation barrier for diamond to break and reform all the bonds to the more stable graphite configuration, thus this reaction is kinetically unfavored and will not actually occur. The rate of reaction is a measure of how fast the products are formed and the reactants are consumed, so you can determine it by measuring the change in the concentration of products or reactants, over a period of time.
Consider a general chemical reaction:. To determine the rate of this reaction by experiment, you can measure the concentration of H 2 at different times of the reaction, and plot it against time as follows:. The average rate of reaction is an approximation of the reaction rate in a time interval and can be denoted by:. The instantaneous rate of reaction is defined as the rate of reaction at some instant in time.
It is a differential rate and can be expressed by:. In most cases, the reaction rate is dependent on the concentration of the various reactants at time t. For instance, in a higher concentration of all reactants, reactants collide more frequently and result in a faster reaction. Where k is the rate constant, and the power x and y is the order of the reaction with respect to reactant A and B. The rate law must be determined experimentally and cannot be deduced from just the stoichiometry of a balanced chemical reaction.
The rate law can be determined by the method of initial rates. In this method, the experiment is performed multiple times, only changing the concentration of one reactant for each run while keeping other variables constant.
The rate of the reaction is measured for each run to determine the order of each reactant in the rate law. For trial 1 and 3, the concentration of NO is kept constant while the concentration of H 2 is doubled. For trial 1 and 2, the concentration of NO is doubled while the concentration of H 2 remains constant. The result of this change is that the initial rate quadrupled think of it as 2 2.
And the reaction is first order in H 2 and second order in NO. She has years of research experience in asymmetric catalysis, natural product synthesis, drug discovery and drug delivery. She has served as a contributor for Synfacts and a reviewer for journal articles. For example, consider the following initial rate data for the reaction:.
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