The Iodine Clock Investigation
Introduction
This is an investigation into the rate of a reaction and the factors
that contribute to how fast a reaction will take place. Through the
recording and analysis of raw data, this investigation also allows us
to apply generally accepted scientific rules and to test them against
results gained from accurate experimental procedures.
Aim
The aim of this experiment is to investigate the rate at which iodine
is formed when the concentration and temperature of the reactants are
varied, and to attempt to find the order and activation energy.
The Chemistry
'THE IODINE CLOCK' - This is the experiment that will be used to
investigate reaction rates, and it is a reaction between acidified
hydrogen peroxide and potassium iodide:
2H+(aq) + 2I¯ (aq) + H[-1] 2O2 (l) ÕI2 (aq) + 2H2O2 (aq)
Iodide ions are firstly oxidised by the hydrogen peroxide, as shown in
the above equation. The iodine that is then produced reacts
immediately reacts with thiosulphate ions as follows:
I2 (aq) + 2Na2S2O3 (aq) Õ 2NaI (aq) + Na2S406 (aq)
As soon as all of the thiosulphate ions have reacted with the iodine,
the excess iodine molecules react with the 2% starch solution that is
present in the reaction. This can be seen as an instant change in
colour, from a colourless solution, to a deep purple coloured
solution. This change in colour denotes the completion of the
reaction.
Factors affecting the rate of reactions:
All chemical reactions occur at a definite rate under particular
conditions. In order to increase the rate at which reactions occur,
the frequency at which reacting molecules collide must be increased.
This may be achieved in a number of ways:
1. By increasing the concentrations of reacting species.
2. By increasing the temperature.
3. By increasing the pressure (only really significant in reactions
involving gases).
4. By the use of a suitable catalyst.
5. In the case of solids, by reducing particle size and thus
Objective: The objective of the experiment is to determine what factors cause a change in speed of a reaction. It is also to decide if the change is correlated with the balanced equation of the reaction and, therefore, predictable. To obtain a reaction, permanganate, MnO_4^(1-), must be reduced by oxalic acid, C_2 O_4 H_2. The balanced equation for the reaction is:
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• An increase in the temperature of the system will increase the rate of reaction. Again, using the Maxwell-Boltzmann distribution diagram, we can see how the temperature affects the reaction rate by seeing that an increase in temperature increases the average amount of energy of the reacting particles, thus giving more particles sufficient energy to react.
There are five factors which affect the rate of a reaction, according to the collision theory of reacting particles: temperature, concentration (of solution), pressure (in gases), surface area (of solid reactants), and catalysts. I have chosen to investigate the effect of concentration on the rate of reaction. This is because it is the most practical way to investigate. Dealing with temperatures is a difficult task, especially when we have to keep constant high temperatures. Secondly, the rate equation and the constant k changes when the temperature of the reaction changes.
Electron affinity is the amount of energy absorbed or released when an electron is added to a neutral atom in its gaseous state. Hence, it is a trend that shows the likelihood of an element to gain an electron in its valence shell. In the periodic table, some elements make bonds only with a group of other elements due to their electron affinities. Iodine and neon are two non-metals that may seem similar, but they react differently in bonds due to their affinity for electrons. To mention, neon is a noble gas and it does not have an affinity for electrons. In comparison, iodine has a greater affinity for electrons than neon due to the following factors: effective nuclear charge, atomic radius, and electronic arrangement.
The ½ concentration however begins to plateau at a faster rate then the full concentration, which takes a much longer period of time to reach its plateau (fig.1). The plateau for both of the different iodine concentration concludes that each system is reaching equilibrium and therefore returning to a homeostatic state. Fick’s Law can be used to better understand the patterns in figure 1. It states that the diffusion rate is equal to the difference in concentration. This means that because the difference in concentration between the two solutions (iodine and soluble starch) used in this experiment were greater when the iodine was at full concentration, the rate of diffusion would also be greater. This occurs as the molecules of the iodine want to go from a place of high concentration to that of low concentration therefore the change in absorbance will be larger as more molecules of are passing through the semi-permeable membrane. We can thus conclude that when the iodine solution is at a higher concentration it will have a higher average absorbance as a greater amount of iodine molecules are needed to pass through the membrane to become saturated and return the system to equilibrium (Fig.
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