The reaction rate of a chemical reaction is determined as the change in the concentration of a reactant or product over the change in time. [1] The rate of a reaction is determined by experiment. Many factors influence the rate of a reaction: the nature of the reaction, concentration, pressure, temperature, and surface area, presence of catalyst and intensity of light. [2] For a chemical reaction, the rate law or rate equation is a mathematical expressed equation that links the reaction rate with the concentration or pressures of each reactant. There are two types of rate laws: differential rate law and integrated rate law.
The differential rate law, often simply called the rate law, expresses how the rate depends on the concentration whereas the integrated rate law indicates how the concentration of solution depends on time. [3] Determining the rate law of a reaction helps to conclude the individual steps by which the reaction occurs from the specific form of the rate law. [3] Rate laws are related to each other, i. e. the experimental determination either the rate laws is enough. Whether the differential or the integrated rate law is going to be defined depends on the data that can be assembled easily and accurately.
The method of initial rates is the most common method that is used to determine the differential rate law experimentally. [5] In this method several experiments are conducted using different initial concentrations and the initial rates are determined for each run. The dependence of the rate on the concentrations of different reactants can be received by comparing the initial rates and the initial concentration, thereby allowing us to estimate the order in each reactant. [5] This method is used in the method 2 of our practical to determine the order of reaction by calculating initial rates besides the color change during the reaction.
Another method that we use in method 1 of our practical is described as the concentrations of solutions are measured at different values of time at which the reaction occurs in order to determine the integrated rate law for a reaction. After that, the data is observed on which of the integrated rate law correctly corresponds the obtained data. Usually this is done by plotting the graph and drawing a straight-line. As soon as the accurate straight-line plot is found, the correct integrated rate law can be obtained. The form of integrated rate law depends on the order of reaction.
The order of reaction in regard to a given substance is determined as the exponent to which the concentration of reacting substances is related. [6] The order of reaction can be found by carrying out experiments. If the rate of reaction is independent of the concentration of the reactant then this type of reaction is called zero-order reaction. For the first-order reactions, the reaction rate is dependent on a single reactant and the value of exponent is equal to one. If the rate of reaction is proportional to the square of concentration then the reaction is defined as the second-order reaction.
In this practical the order of a reaction with respect to iodine will be determined using the substitution reaction between iodine and propanone: CH3COCH3(aq) + I2(aq) ? CH3COCH2(aq) + H+(aq) + I-(aq) Aim The aim of this practical is to determine the order of a reaction with respect to iodine through plotting a graph of titres against time from the start of the reaction, and to measure and compare initial rates of reactions. Procedure Method 1. Determining the order of a reaction with respect to iodine First, seven conical flasks were labelled as 1, 2, T3, T4, T5, T6 and T7.
Using a measuring cylinder 50 cm3 of 0. 02M iodine in potassium iodide was added into conical flask 1 and 20 cm3 of 2. 0 M propanone (acetone – CH3COCH3) solution was added to conical flask 2, followed by 25 cm3 of 2. 0 M hydrochloric acid (HCl) by using clean, dry measuring cylinder. After that, using a clean, dry measuring cylinder, 10 cm3 of 0. 5 M sodium hydrogencarbonate (NaHCO3) was added to each of the following conical flasks T3 to T7. Second, a burette was filled with a solution of 0. 010 M sodium thiosulphate (Na2S2O3). When the contents of flask 1 and 2 are mixed, the reaction starts.
Therefore, the contents of flask 2 were poured into flask 1, after which the timing started immediately. The mixture in the flask 1 was swirled for about 1 minute and using a pipette and safety filler 10 cm3 of the reaction mixture was withdrawn. The contents of the pipette were run into flask T3, noting the time and it was swirled until bubbles ceased. The sodium hydrogencarbonate neutralizes the acid catalyst and quenches (stoops) the iodine-propanone reaction. Four more samples were similarly withdrawn at about 5 minute intervals and added to quenching flasks T4, T5, T6 and T7, in turn.
The exact time was noted for each the sample mixes with the quenching NaHCO3. In order to ensure complete quenching the mixtures in the flasks were swirled again, without stopping the clock. Finally, using the burette filled with the solution of Na2S2O3 the contents of each of the flasks T3 to T7 were titrated and a starch was used as an indicator. All the obtained results were recorded in a table. A graph of the titres of sodium thiosulphate versus time from the start of the reaction was plotted from which the order of reaction with respect to iodine was deduced.
Method 2. Determining the rate of a reaction with respect to iodine Using the burettes the four mixtures of hydrochloric acid (HCl), propanone (CH3COCH3) solution and water were made up in the conical flasks according to the table 2. The volumes of iodine needed for each run were measured out from the burette into four test tubes. The tubes were marked as w, x, y and z. By adding the contents of the test tube w to flask W the first run was started. Simultaneously, the timer was started and the time taken for the iodine to disappear was measured in seconds.
