The light absorbency change - Sample Essay

Section 1: Planning The aim of the investigation is to examine the kinetics involved in the reactions between acids and metals. The investigation focuses on determining if:  The type of acid (strong or weak) and the type of metal have an effect upon the activation energy of a reaction.  The order of the reaction changes as the concentration of the acid is varied. Several methods can be used to follow the reactant, in terms of the rate at which it disappears in the reaction. One approach is to use a colorimeter to measure the light absorbency change.

This would be unsuitable for both experiments because both reacting substances do not produce any colour. An alternate approach could be to use dilation. This also would be an unsuitable method to follow the rate of the reaction because the volumes in all the systems do not change significantly. The third method involves measuring the electrical conductivity using a conductivity meter. Again, this would be of little use because there are no changes in ion concentration in the reaction. It would also therefore be of no use to measure the rate of the reaction using alternating currents.

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A Titration (continuous rate method) method would not be suitable for the investigation. The main reason for this is because the reactant would be hard to measure due to the intense velocity at which it disappears. Another reason is that there would not be a significant change in the concentration of solute. As a result the procedure will be too rapid for the acid to be neutralised via quenching using ice. Measuring the volume of hydrogen gaseous product evolved from the reaction and subsequently collecting it into a measuring cylinder over a water bath or directly into a gas syringe can also follow the rate of the reaction.

In practice this method would create inaccurate results. This is because the hydrogen gas evolved from the reaction can easily leak out of the equipment. The inaccuracies in the quantitative results will reflect on the calculations made in order to find out the activation energy and order of the reactions. This will decrease the validity and reliability of the results produced. Consequently using an initial rate method to find the time for a certain amount of magnesium to be used up will be more appropriate for the reaction as the concentration of molarity in the acid doesn’t need to be continuously varied.

This is the only possibility since the reaction is too fast for a continuous method. This is due to the fact that the amount of acid used in the reaction is in excess of what is needed in making sure the reaction is completed. Therefore, the volume of acid used wouldn’t be a major factor in the investigation since it will be in excess. However, the number of moles of acid must be in excess.  Is the reaction order the same with monobasic (e. g. hydrogen chloride) and dibasic (e. g. sulphuric acid) acids? A continuous rate method such as titration can be used to measure the above.

For example, using a standard solution containing a specific concentration of solute measured in cubic decimetre. As the investigation proceeds, this solution can then be quenched (e. g. using ice) in equal increments. The main advantage of using this method is that a wide variety of concentrations can be produced as each concentration can be diluted to the specific requirements using de-ionised water. This will provide a wider range of results making the experiment more reliable, and more valid. This problem in practice has its limitations primarily because two monobasic acids and two dibasic acids are not available.

Another limitation is that the experiment links order with a monobasic acid, as a result this problem will not be viable.  What effect does agitation have on the activation energy/rate of reaction? Another way of examining the kinetics of acids reacting with metals is to find out what effect agitation has on the activation energy/rate of the reaction. Due to experimental error conducting this experiment practically will produce invalid results. This is because it would be impractical to “agitate” i. e. stirring a suspension manually the same number of times at the same speed for the required time.

This method would lead to inaccuracies in results and therefore any conclusions drawn from it would also be inaccurate. Another reason is that the equipment required for the experiment is not readily available. Background Theory: “In order for a reaction to occur, collisions must occur between two molecules with enough energy to break bonds in either or both of the molecules” Ref: Nuffield Advanced Chemistry Students’ Book, p159. The activation energy may be defined as the minimum energy needed in reacting particles (atoms, ions or molecules) to achieve a transition state where they exist as an activated complex.

“It is the difference in energy between the reactants and the transition state”. “The point where the old bonds are not quite broken and the new bonds are not quite formed is the point of maximum energy and represents the transition state at the top of the energy barrier between reactants and products. Energy is needed to supply reactants to make a reaction take place. ” Ref: Nuffield Advanced Chemistry Students’ Book, p160. The activation energy enables chemical bonds to be stretched or broken as well as allowing atoms, ions and electrons to be rearranged as the reaction proceeds.

Temperature also influences the rate at which the reaction occurs. Ref: Chemistry in Context, G. C. Hill & J. S. Holman, p411. A graph to show the activation energy of a reaction: Ref: Chemistry for Advanced Level, Peter Cane, p158. The reaction profile above shows that the reactants are higher in terms of energy than the products. It also shows that reactants are kinetically stable because their activation energy is high and exothermic. Activation energy is based on collision theory, which states that in order for a reaction to occur, the molecules in the reaction must collide before they react.

Collision theory also states that a reaction will only occur if colliding particles possess more than a certain minimum amount of kinetic energy (1/2 mv2), referred to as the activation energy. Ref: Chemistry in Context, G. C. Hill & J. S. Holman, p411. If molecules possess less kinetic energy than the minimum required energy (E MIN) bond breaking will not occur and therefore there will be no reaction. The rate of the reaction is directly influenced by the size of the activation energy. At low temperatures, the rate of the reaction is also reduced.

This is primarily due to the fact that there is less energy (kinetic energy) resulting in fewer collisions. Maxwell and Boltzman produced a distribution curve, which showed the energy amongst molecules at two different temperatures. A graph to show the distribution of energy: Ref: Nuffield Advanced Chemistry Students Book, Longman, p257. The graph shows the number of particles in each range of kinetic energy. The area beneath the curve is directly proportional to the total number of particles. Using the information about collision theory and activation energy, it is possible to make some predictions regarding the outcome of the experiment.

If the temperature is increased in the reaction, this will in turn increase the percentage of molecules containing activation energy (EA), therefore the particles will be carrying an increased amount of kinetic energy. This suggests there will be an increase in the number of successful collisions per second, which will also increase the rate of reaction. At high concentrations the number of molecules increases per volume. Therefore it can be said that, “The proportion of collisions that can overcome the activation energy increasing the rate of reaction”. Ref: Chemistry in Context, G.

C. Hill & J. S. Holman, p412. A Swedish chemist named Svante Arrhenius derived an equation, to show how a rate constant and temperature is related. Ln k = constant – EA (1/T) R The EA represents the activation energy of the reaction in J mol-1. R represents the gas constant of 8. 31 J K-1 mol-1. T represents the temperature in Kelvin. The Arrhenius equation can be used to determine the activation energy for a given reaction; this can be achieved by using natural logarithms as shown below K=Ae-EA/RT In k= Ln A + Ln e -EA/RT ==>In k=ln A -EA/RT The above can be rearranged as follows:

Y= mx + c This may be compared with the equation below, which is the equation for a straight line. In k = (EA/RT) x (1/T) + Ln A According to this equation the graph produced should be a straight line, as the Ln k against 1/T should be a straight line with a gradient of -EA/R A graph to show how the gradient is worked and therefore the activation energy: Ref: Chemistry in Context, G. C. Hill & J. S. Holman. The gradient will be determined from the graph of results and this will then be used to calculate activation energy of the reaction, using the equation below:

Gradient = (-EA/8. 31) ? (GAS CONSTANT) This can then be rearranged into Ea = -gradient x 8. 31 Temperature is measured in degrees Kelvin so that it is an independent variable for the reaction. Le Chatellier’s principle may then be applied to the system, as the weak acid will dissociate into hydrogen ions to reinstate the concentration of hydrogen ions. Prediction: The background investigation that was performed indicates that the weak acid will have the highest activation energy. This is due to the fact that energy is needed to dissociate the ethanoic acid into ethanoate ions.

A lower degree of energy is needed to split the hydrochloric acid into hydrogen and chlorine ions as the strong acid has already fully dissociated. This theory can be supported by Ka values found in the book of data. Therefore, the assumption is that the activation energy for the weak acid will be higher. Choosing an Appropriate Metal: In order to examine the kinetics in a reaction an appropriate metal must be chosen. There are two choices of metals in this experiment, Magnesium and Zinc. The metals come in different forms, for example Zinc comes in a granulated and foil forms unlike Magnesium, which is available in a ribbon.

The reactants in the reaction are all in different states. This is called a heterogeneous system. Generally, the smaller the size of reacting particles, the greater the total surface area exposed for the reaction. Therefore, the reaction will take place quicker and the availability of the reactants and the surface area exposed is a factor, which must be kept constant throughout the experiments. There is a high variability of the surface area exposed of granulated zinc and zinc foil. As a result is not possible to keep a constant amount of surface area in the experiments.

Each piece has a different thickness and size and therefore the number of moles in the metal will vary. As the amount of surface area of the metal exposed varies, the amount of acid reacting to the surface of the metal will also vary, producing unreliable results and inconsistencies. This suggests that granulated zinc and zinc foil are unviable options. Magnesium ribbon will be more suitable for the investigation as it has a constant width, thickness, mass and surface area exposed. As a result there will be the same number of moles per given length.

Magnesium ribbon is also better than its contenders (granulated Zinc and Zinc foil) on a practical level, as it is easier to measure and cut more accurately, which will produce more reliable results. Another advantage is that magnesium is higher in the reactivity series in the periodic table than Zinc; therefore the reaction will be quicker. The disadvantage of using Magnesium ribbon is that the Magnesium has already reacted with Oxygen from the air forming a thin layer of Magnesium oxide on the exposed surface of the ribbon. This can be overcome by rubbing sandpaper on the metal to remove the thin layer of magnesium oxide.



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