Data and Analysis:
Graph1 ( please refer to the graph paper attached)
The table below shows the volume of sodium thiosulphate solution used against time. ( data for graph 1)
Temperature readings remain constant during the experiment: 22.5℃.
Analysis:
From the table, it was noticed that the amount of sodium thiosulphate solution used is directly proportional to the concentration of the remaining iodine. The slope of graph 1 equals to the negative value of the rate of reaction, it implies that iodine concentration drops at a uniform rate. Therefore, rate of change of the iodine concentration is independent of iodine concentration. The reaction of iodine with acetone is zero order with respect to I2.
Graph2 ( please refer to the graph paper attached)
By combining the results of Group 1, 2 and 3 class results are obtained. The data is used to plot Graph II: rate of reaction against the volume of acetone added.
Class Results
Analysis:
Data marked with ( ) are abandoned because of its great deviation form the estimated value. With the rate of reaction increases with increasing volume of acetone together with a straight line passing through origin, the reaction of iodine with acetone is in first order with respect to acetone.
Volume of mixture left = 47 cm3
Observations:
- Iodine solution was reddish brown in color while the other reactants are colourless.
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Gaseous bubbles were evolved when the reaction mixture is mixed with NaHCO3. due to formation of carbon dioxide gas.
2NaHCO3(aq) + H2SO4(aq) → Na2SO4(aq) + 2CO2(g) + 2H2O(l)
- During the titration process, the colour of the mixture containing iodine changed from reddish brown to straw yellow gradually. Once the mixture reached the straw yellow colour, 2-3 drops of starch solution were added into the mixture. The colour turned to dark blue, the solution became colourless after several more drops of sodium thiosulphate were added.
Precautions:
- For mixing the content of flasks A and B, the solution should be transferred alternately between two flasks for 2 – 3 times. The solution is to swirled gently to allow the content inside to mix throughly.
Conclusion:
For a straight line passing through origin obtained in Graph II and a linear decreasing graph is in Graph I, the reaction of iodine with acetone was to be found in first order with respect to acetone and zero order with respect to I2.
Discussion:
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The experiment cannot be conducted with more than one variable amounts reactions. For reaction more than one reactant, only the concentration of the reactant under investigated (acetone) changes, and the other reactants remain unchanged.
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Quenching:
What is quenching?
The analysis of the changes in concentration of reactants or products with time is direct measure of the rate of a reaction. Yet, since the process of analysis takes time, quenching is necessary to slow down the rate of reaction abruptly and assumed to have stopped.
The methods of quenching of sample mixture include:
- Rapid cooling by ice
- Removing the catalyst
- Removing one of the reactants by adding another reagent which can use up the reaction rapidly
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Dilution with a large volume of water
In this experiment, The addition of NaHCO3 is to neutralize the H2SO4 , with a view to removing the catalyst to lower the rate of reaction for upcoming titration process. Moreover, ice cubes were added to the reaction mixture to lower the temperature and concentration, thus minimize the reaction rate. The volume of NaHCO3 added is unimportant. It should only be added in excess in order to remove all the catalyst H2SO4.
Chemical kinetics:
Chemical kinetics, also known as reaction kinetics, is the study of of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the and , as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. In 1864, and pioneered the development of chemical kinetics by formulating the , which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.
Rate of reaction
Chemical kinetics deals with the experimental determination of from which and are derived. Relatively simple exist for (for which reaction rates are independent of concentration), , and , and can be derived for others. In consecutive reactions the often determines the kinetics. In consecutive first order reactions, a approximation can simplify the . The for a reaction is experimentally determined through the and the . The main factors that influence the include: the of the reactants, the of the reactants, the at which the reaction occurs, and whether or not any are present in the reaction.
Factors affecting reaction rate:
Nature of the Reactants
Depending upon what substances are reacting, the time varies. Acid reactions, the formation of , and are fast reactions. When covalent bond formation takes place between the molecules and when large molecules are formed, the reactions tend to be very slow.
Physical State
The (, , or ) of a reactant is also an important factor of the rate of change. When reactants are in the same , as in , thermal motion brings them into contact. However, when they are in different phases, the reaction is limited to the interface between the reactants. Reaction can only occur at their area of contact, in the case of a liquid and a gas, at the surface of the liquid. Vigorous shaking and stirring may be needed to bring the reaction to completion. This means that the more finely divided a solid or liquid reactant, the greater its per unit , and the more contact it makes with the other reactant, thus the faster the reaction. To make an analogy, for example, when one starts a fire, one uses wood chips and small branches—one doesn't start with large logs right away. In organic chemistry are the exception to the rule that homogeneous reactions take place faster than heterogeneous reactions.
Concentration
plays a very important role in reactions according to the of chemical reactions, because molecules must collide in order to react together. As the concentration of the reactants increases, the of the molecules colliding increases, striking each other more frequently by being in closer contact at any given point in time. Think of two reactants being in a closed container. All the molecules contained within are colliding constantly. By increasing the amount of one or more of the reactants it causes these collisions to happen more often, increasing the reaction rate (Figure 1.1).
Temperature
usually has a major effect on the rate of a chemical reaction. Molecules at a higher temperature have more . Although collision frequency is greater at higher temperatures, this alone contributes only a very small proportion to the increase in rate of reaction. Much more important is the fact that the proportion of reactant molecules with sufficient energy to react (energy greater than : E > Ea) is significantly higher and is explained in detail by the of molecular energies.
The 'rule of thumb' that the rate of chemical reactions double for every 10 °C temperature rise is a common misconception. This may have been generalized from the special case of biological systems, where the is often between 1.5 and 2.5.
A reaction's kinetics can also be studied with a approach. This involves using a sharp rise in temperature and observing the relaxation rate of an equilibrium process.
Catalysts
Generic potential energy diagram showing the effect of a catalyst in an hypothetical exothermic chemical reaction. The presence of the catalyst opens a different reaction pathway (shown in red) with a lower activation energy. The final result and the overall thermodynamics are the same.
A is a substance that accelerates the rate of a chemical reaction but remains unchanged afterwards. The catalyst increases rate reaction by providing a different to occur with a lower . In a reaction product is itself a catalyst for that reaction leading to . Proteins that act as catalysts in biochemical reactions are called . describe the .
In certain organic molecules specific substituents can have an influence on reaction rate in .
Agitating or mixing a solution will also accelerate the rate of a chemical reaction, as this gives the particles greater kinetic energy, increasing the number of collisions between reactants and therefore the possibility of successful collisions.
Increasing the pressure in a gaseous reaction will increase the number of collisions between reactants, increasing the rate of reaction. This is because the of a gas is directly proportional to the partial pressure of the gas. This is similar to the effect of increasing the concentration of a solution. A catalyst does not affect the position of the equilibria, as the catalyst speeds up the backward and forward reactions equally.
References:
- New Way Chemistry for Hong Kong A-Level Book 2 Ch.13-14 by Y.C. Wong & C.T.Wong
- http://en.wikipedia.org/wiki/Chemical_kinetics