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Purpose: The lab seeks to investigate rates of reactions, practice measuring them using the microscale technique, and examine how reaction rates change depending on various conditions.
Procedure: (See citation for materials list and experimental procedures).
Why does the reaction rate change as concentrations of the reactants change?
Reaction rate depends on the rate of collisions between the particles of reactants. If a higher concentration of reactant is present, the reaction rate will increase because of simple probability: there is a higher likelihood that the reactants will collide in the correct orientation and chemically react than if a greater concentration of the reactant was present. Some of the reactants affect the reaction rate more than others and therefore, the change in rate differs depending on which reactant concentration varies.
Explain the general procedure used to find the rate law.
To determine the rate law of the reaction, essentially the only pieces of information needed are the degree to which each reactant affects the overall rate. Since the rate constant and rate always vary with temperature, the reactants and rate orders are the only part of the rate law that is the same in every case. To determine these, data must be collected at a constant temperature and analyzed to calculate the change in rate compared to each reactant concentration changing. Several trials must be run with some reactants held constant as others change and times recorded to calculate rate for each set of concentrations. Each reactant’s rate order can then be individually calculated as the others are held constant. The constant is later calculated by plugging in concentrations and rate for a specific temperature.
Why does reaction rate change as temperature changes?
Since reaction rate depends on how rapidly the particles of reaction collide in correct orientation and with the proper activation energy, the amount of thermal energy involved in the system affects the overall rate. If the temperature is higher, the average kinetic energy and average speed of the particles is higher and therefore the probability of them colliding under the right conditions is also more likely.
Explain the general procedure used to determine activation energy.
Activation energy is the energy needed as input per mole to make the reaction occur spontaneously. To calculate the activation energy, the reaction rate must be measured under different external temperature conditions but with other conditions held constant. The rate constants at a specific temperatures are then related mathematically to form an overall trend (seen graphically) where each point is the constant at a temperature (inverse). The average change of the trend (the slope) is then used in the Arrhenius equation to calculate the activation energy of the reaction (an unchanging value).
Differentiate between reaction rate and specific rate constant.
The specific rate constant changes only as temperature changes. There is only one rate constant for a specific temperature. However, the reaction rate itself, while dependent on temperature, also depends on the concentrations of the reactants present. The rate constant is a constant at a specific temperature and does not change with concentration. The calculation of rate constant is also how the temperature effect is mathematically calculated into the rate itself.
Comment on the effect of a catalyst. Predict how the activation energy changes when a catalyst is added to the reaction.
When a catalyst is added, it causes the reaction rate to increase, as demonstrated by the data. The catalyst reduces the activation energy of the reaction, making the reaction proceed faster because it takes less time for the reactants to collide if they don’t need to reach as high an energy state.
Make a general statement about the consistency of the data as shown by calculating the orders of reactants and by graphical analysis which leads to activation energy. Were the calculated orders close to integers? Did the check of the order give the same value for the order? Were the points on the graph close to a straight line?
The data was very consistent (the data used for calculations). The largest difference in check calculation the rate orders was 0.1. They were all close to whole numbers and confirmed each other sufficiently. The rate law made sense based on rate changes with concentration change. Both the point connections and trend line on the graph are shown and they are very close. This demonstrates that the rates calculated from the data were very accurate.
Write out the “two-point” form of the Arrhenius equation which relates rate constants, temperatures and activation energy.
The “two-point” form is…
ln(k2) = – Ea x ((1/T1) – (1/T2))
*k is the rate constant (point 1 and 2)
*T is the temperature in Kelvin (point 1 and 2)
*The equation basically follows the same format as the one used to find activation energy from slope. This equation accounts for just two points while the one used in the calculations used the trend of three points (more accurate).
How could you improve the data?
The data could be improved by using an automated machine to add drops of reactants so the concentrations are closer to the numbers calculated. The microscale technique requires exact measurements, and any variation in the tilt of the dropper can throw off calculation of rate laws and constants. The temperature also should be exactly regulated because the constant differs by temperature, but it is possible that it varied slightly during the course of the experiment. The data used for calculation is near ideal; see the conclusion for other experimental errors involved in the data collected during experimentation. In addition, the data used to calculate Ea becomes increasingly accurate the more temperatures it is tested at. The trend becomes clearer and clearer on the graph as the number of points increases. Here only three temperatures were tested, but it would have been more accurate if there had been more to make sense of.
In the lab, the reaction between iodide ion, bromate ion, and hydrogen ion was conducted and timed in various concentrations. The microscale technique was employed to increase measuring and accuracy skills but also led to some inaccuracy in results (see next paragraph for experimental errors). Another set of data (attached) was used to make calculations and determine the rate law for the reaction. The factors tested were concentrations of reactants, temperature, and presence of a catalyst. From the results, all three have an impact on reaction rate. Increasing the concentration of reactants caused the rate to increase because more well-oriented collisions between particles occur, making the reaction proceed faster. Increasing the temperature increases the average speed and kinetic energy of the particles, also increasing the likelihood of collision and rate at which the particles meet the required activation energy for the reaction to proceed. Finally, the data shows conclusively that the the presence of a catalyst (in this case Cu(NO3)2) makes the reaction proceed at a faster rate. This is because the activation energy needed as an input for the reaction is lowered by the catalyst. The collected lab data was accurate enough to confirm these general trends.
The other objective of the lab was to calculate the rate law and activation energy of the reaction. This is the segment where the erroneous data presented most of the issue. While the data was sufficient enough to draw general trends about how factors affected the rate, the precise calculation necessary to make rate law calculations was not possible from the collected data. A combination of several errors may have caused this. Most significantly, the microscale technique itself lends to errors in that drops must be exactly the same size to be accurate. Any slight variation in tilt of the dropper would cause the concentration measurements used in calculation to be inaccurate from what they were determined to be (using measured volume of a drop). Secondly, any temperature variation would have caused an incorrect rate constant and thus reaction rate. The experiment was conducted over several days so it is certainly plausible that the laboratory temperature fluctuated to some degree between testing periods. As seen in data table two, the temperature recorded for experiments one through four is 19.3ºC while that recorded for experiments five through seven is 20.1ºC. Finally, any residue from previous experiments left over in the testing wells may have introduced contaminants that acted as reactants not accounted for by the experiment. Especially when a reaction occurs on such a small scale, any added substance can make a big difference in an individual trial.
The calculations of rate law and activation energy rely on a few different reasoning processes and equations. The rate law is the general expression of a reaction that relates the rate of reaction (in M/s) to the concentrations of the reactants involved. These reactants each carry “rate orders” which mathematically account for the relative affect each ion’s concentration has on the rate. For example, in this experiment the rate order of the hydrogen ion was 2. As the concentration of hydrogen doubled, the rate increased four-fold. The other ions, with rate orders of 1, had a more directly variable relationship with the overall rate. Rate order could be calculated because of the way the experiment was designed – sets of trials were set up so only one reactant changed while the others were held constant. The constants could then be cancelled and the rate orders solved using logarithms. The last part of the rate law is the rate constant (k). This constant holds a specific value for each reaction depending only upon temperature, not on concentration. At a given temperature, this constant is multiplied by the concentrations to get the reaction rate. Since temperature also affects the collision rate of particles and thus the rate, the constant also mathematically represents the affect of temperature on the reaction rate. The reaction rate is specific to a set of temperature and reactant concentrations, making it very precise and also easy to manipulate in a practical sense. By investigating the different factors that affect this rate, the lab demonstrated how reaction rate could potentially be manipulated in a practical sense to yield a product more efficiently.
Activation energy (Ea) was calculated using a variation of the Arrhenius equation, which relates the rate constant (as a natural logarithm) to the temperature (inverse) and sets up a trend among the measured temperatures. The slope of the trend line is used to calculate activation energy in the equation. Activation energy is constant for the reaction, not even changing with temperature. A certain amount of energy is always required to begin the reaction (unless a catalyst is present to lower it); higher temperatures simply increase the average kinetic energy of the particles so more of them reach the activation energy more rapidly. When enough of the particles achieve the required activation energy, the reaction may then proceed to completion.
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