So:
Building on from this, I will now work out how much more frequently particles will collide with an increase in temperature from 320K to 330K:
So, a temperature change from 320K to 330K causes the average velocity of the molecules increases by a factor of 1.016, about 1.6%. The rate actually increases by much more than this, sometimes by 200% to 300%. Therefore, this demonstrates that temperature not only has an effect on the number of collisions, but also with how much energy they collide.
Maxwell-Boltzmann:
“As the temperature increases, the rate of a chemical reaction also increases. The reason for this is because of the distribution of energies amongst the reacting-particles. This distribution is called the Maxwell-Boltzmann distribution”.
- Extract taken from ‘Revise AS Chemistry for Salters (OCR)’, page 55
In a reaction not every collision will be successful in forming the product. Molecules within a particular substance will be travelling at different speeds. Those moving at slower velocities have lower energies, and hence their collisions may not exceed the activation enthalpy. There will also be molecules moving faster, and so have higher energies. The majority of particles, however, will be moving at a speed very close to the average.
The Maxwell-Boltzmann distribution shows how the speeds (and hence the energies) of a mixture of moving particles varies at a particular temperature. This gives a range of different kinetic energies. Figure 1.2 shows two Maxwell-Boltzmann distributions.
The Maxwell-Boltzmann distribution explains why rate generally doubles by a rise in temperature of about 10°C. As Figure 1.2 shows, with this increase in temperature, the area on the right of the activation enthalpy has effectively doubled, meaning a larger number of reactant particles are able to react and form the product.
The Arrhenius equation
The Arrhenius equation can be used to show the effect of a temperature change on the rate constant, and hence on the rate of the reaction. If the rate constant is doubled, then the rate of the reaction will also double.
By taking the natural log of the Arrhenius equation, a linear equation can be formed.
Therefore this allows a graph of 1/T against. lnk to be drawn. The gradient of this graph (m) will therefore be the negative activation enthalpy (EA) divided by the gas constant, R (8.31 J K-1 mol-1). Substituting the R value will therefore allow me to calculate the activation enthalpy.
General collision theoryFor a reaction to occur the reacting species must come together and collide, causing bonds in the products to stretch and break. The bonds are then reordered and reformed to form the products.
Colliding particles are required to have a minimum kinetic energy, known as the reaction’s activation enthalpy, before it is possible for them to react. Not all of the particles exceed this activation enthalpy and have the right orientation at the moment of impact to cause the reaction to proceed. Therefore, it can be stated that a lower activation enthalpy barrier for a reaction will make it easier and quicker for bonds to be broken in the initial stage of the reaction.
If the colliding particles do not exceed the activation enthalpy barrier, they ‘bounce off’ each other and remain unchanged. Activation enthalpy is shown diagrammatically by Figure 1.3.
Section 2 - Risk assessment
In this investigation, a lot of glassware is being used. The glass can become slippery when wet. If glass smashes, it can cause a major safety hazard, as it has sharp edges and can send shards of glass around. If any glass is broken, it will be brought to the attention of those in the lab immediately. Then, I will clear it up with a dust pan and brush and put it into a glass bin. Then I will check for small pieces of glass which I may have missed.
This investigation involves the use of a water bath, with temperatures up to and including 80°C. At this temperature, it is possible to cause burns to the skin. Therefore extra care will be taken while using water baths. I will make others aware of which water bath I am using, and at what temperature it will be. It is important to note that any apparatus in the water bath will also be warned up. There will be metal items present in the water bath, such as clamp stands and test tube holders. Metal is a very good conductor of heat. These may become too hot to handle and so I will only touch them on areas that have not been submerged in the water.
I will adhere to basic lab safety while I am in the lab. This includes no running, no food or drink allowed into a lab, washing my hands before leaving the lab, wearing appropriate clothing and removing all jewellery, disposing of chemicals in a designated place and not returning unused chemicals to their original containers, to prevents contamination.
In this investigation, many substances are irritant to the skin and eyes. For this reason, safety gloves and eye protection will be worn at all times in the laboratory. To remove the risk of clothes becoming stained or getting in the way of the experiment, a lab coat will be worn.
Whenever liquids are used, there is always a risk of spillage. This can cause the floor to become slippery and unsafe to work on. Extra care will be taken when handling liquids and any spillages will be brought to the attention of those working in close proximity to me.
In the laboratory, there will be other people working at the same time as me. I must always be conscious of how my actions will affect those around me.
Discussion of chemicals used in this investigation
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Potassium Peroxydisulphate(VI), K2S2O8
Harmful if swallowed. Eye, skin and respiratory irritant. May cause allergic respiratory or skin reaction. Wear safety glasses. Keep from contact with organic material.
Stable. Protect from light and moisture. Incompatible with strong reducing agents, strong acids, steel, aluminium, alkali metals, brass, magnesium, zinc, cadmium, copper, tin, nickel and their alloys.
Consumption of large amounts of iodides may harm the growing foetus. Inhalation of dust may irritate respiratory tract. May act as a skin or eye irritant. May cause sensitization or allergic reaction in susceptible people.
Harmful through inhalation and skin contact. Solution is corrosive to skin, and may cause burns if left untreated.
If spilt in lab:
Ventilate room and apply mineral absorbent.
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Sodium thiosulphate Na2S2O3
Stable. Incompatible with strong acids, iodine or mercury. Minimise exposure.
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Starch solution (C6H10O5)n
Combustible. Incompatible with strong oxidizing agents. Minimise exposure.
Section 3 – Materials and methods
Preliminary work
I felt that it was necessary to undertake some preliminary work before starting this investigation. From this, I aimed to:
- Find out a suitable concentration to use for the reactions which involve temperature changes.
- Find a suitable range of temperatures to use
- Get a better understanding of the technique.
Table 3a shows the results of this preliminary.
The rate of reaction roughly doubles by a rise in 10. Therefore, the time taken for the blue/black solution to form will roughly half. Table 3b therefore shows the theoretical times taken for the blue/black solution to appear based on my preliminary results.
From this, I have chosen to use mixture 3 as the set concentration when varying the temperature. I have chosen this as it is a compensation for its time taken at 80 ºC and its overall time duration. Mixture 1 would be have an extremely short time to complete the reaction at 80ºC, and mixture 5 would be too time consuming overall, as there is only a limited duration of time in which to complete this investigation.
Up to 80ºC, the time taken for the blue/black solution to appear would theoretically stay above 1 second for mixture 3. Any further than this and it would be below 1 second. Therefore, I feel that a suitable range in temperature should be no more than 80ºC.
Method - Varying the concentration
- Place a test tube rack inside a waterbath, and set it to 20ºC. Place a white tile onto the bottom of the test tube rack.
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Into a boiling tube, carefully add 2cm3 of potassium peroxydisulphate and place this in the test tube rack. Ensure that the water level of the waterbath is higher than that of the contents of the boiling time so as to better transfer the heat from the water to the contents of the test tube.
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Into a test tube, accurately measure one of the mixtures 1 - 5 listed below in Table 3.3. Consistency is required. Ensure the bottom of the meniscus is used as the reference point to measure volumes for all measurements. This will create more accurate and consistent results. Place this into the water bath. See Figure 3.1 below.
- It is important to allow the contents of both the test tube and boiling tube to reach the desired temperature before proceeding. Measure the temperature of both the boiling tube and test tube contents with two separate thermometers. This is very important so as to prevent contamination of the experiment by the two reactants mixing beforehand.
- Pour the contents of the test tube into the boiling tube, and aim to do this in one quick action. Immediately as the entire contents of the test tube has entered the boiling tube, start the stop clock. Do not stir or swirl the solution, as this will transfer energy to the solution and may be very inconsistent, due to the fact different amounts of energy may be transferred solution.
- As soon as no light can be seen through the blue/black solution, and hence the white can no longer be seen, stop the timer. Record the time taken.
- Repeat each mixture 3 times.
Varying the temperature
- Place a test tube rack inside a waterbath, and place a white tile onto the bottom of the test tube rack. Set the waterbath to the desired temperature (20, 30, 40, 50, 60, 70 or 80ºC).
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Into a boiling tube, carefully add 2cm3 of potassium peroxydisulphate and place this in the test tube rack. Ensure that the water level of the waterbath is higher than that of the contents of the boiling tubes so as to better transfer the heat from the water to the contents of the boiling tube
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Into a test tube, accurately measure out mixture 3 shown in Table 3.4 and place it into the test tube rack. Consistency is required. Ensure the bottom of the meniscus is used as the reference point to measure volumes for all measurements. This will create more accurate and consistent results. Place this into the water bath. See Figure 3.2 below.
- It is important to allow the contents of both the test tube and boiling tube to reach 20°C before proceeding. Measure the temperature of both the boiling tube and test tube contents with two separate thermometers. This is very important so as to prevent contamination
- Pour the contents of the test tube into the boiling tube, and aim to do this in one quick action. Immediately as the entire contents of the test tube has entered the boiling tube, start the stop clock. Do not stir or swirl the solution, as this will transfer energy to the solution and may be very inconsistent due to the fact different amounts of energy may be transferred solution.
- As soon as no light can be seen through the blue/black solution, and hence the white can no longer be seen, stop the timer. Record the time taken.
- Repeat each temperature 3 times.
In order to find the initial rate, I must first calculate the number of moles of thiosulphate ions added to each mixture. This is calculated by:
Following on from this, I now need to work out the amount of iodine used up by these thiosulphate ions. The reaction is:
2S2O32-(aq) + I2 (aq) → S4O62-(aq) + 2I-(aq)
In the reaction, 2 moles thiosulphate ions react with 1 mole of iodine. The reaction is therefore 2:1 and so the number of moles of iodine used up 1 x 10-5 moles
I can now calculate the initial rate of each concentration by dividing 1 x10-5 by the average time taken in each experiment for the blue/black solution to appear. See Table 4.3.
Rate equation
I have established that the rate of reaction is first order with respect iodide ions. The known orders of this reaction are:
Therefore, the rate equation looks like this:
Now I will find the values of ln(k) and 1/T, to plot onto a graph. Rearranging the rate equation gives:
Now I will calculate the gradient of this graph, to give me the value of
The two endpoints of this graph are (0.0028, -6.2804) and (0.0034, -9.3511). These are both extremely close to the trend line, so much so that they will be used to calculate the gradient.
The gradient of two points is given by:
Substituting my values gives:
The value of the gas constant, R, is 8.3145 J K-1 mol-1.
Substituting this into the gradient gives:
Therefore, the activation enthalpy for this investigation was + 43 kJ mol-1
Rate determining step.
The rate equation shows that only the concentration of iodide ions and peroxydisulphate ions affect the rate of reaction. These react together in step 1.
In step 2, the thiosulphate ions have no effect on rate.
Step 1 is slow
Step 2 is very fast
Therefore the rate determining step is step 1
Section 5 - Discussion and evaluation
Precision of equipment
In this investigation, I used the most accurate equipment available to me at that time. However, in reality through reading values by eye there are still small errors associated with recording. I will now calculate the percentage error of all the equipment I used in this investigation, and the discuss their significance on the overall outcome of this investigation
This percentage error is quite large overall. However, as the percentage errors came from individual readings of equipment, and the same number and types of these errors happened in each individual experiment, I do not believe that these errors were significant enough to disprove the findings of this investigation.
Accuracy of method
In this investigation, there was inevitably a time delay between mixing the reactants together and starting the stop clock. When the reaction was longer, small time delays were not as significant. However, for the reactions at high temperatures, time recordings were much shorter, and so this delay will have been much more significant. I do not think this affects the validity of my results, as each time delay would be generally quite similar to each other, as they were each carried out by me.
In this investigation, I was judging the colour change by eye. For the lower concentrations, there was more of a gradual change in colour, and it was difficult to judge exactly when to stop the timer.
The temperature of the lab varied throughout the day. Sometimes, the room would be slightly below 20°C in the morning, and then towards the afternoon would rise slightly above 20°C. Being slightly colder isn’t a problem as such, as waterbath can easily warm the water, however it cannot cool the water, and so if the environment were above 20°C, the waterbath would slowly warm too.
The biggest relative significance comes from the time delay between mixing the reactants and starting the stop clock. As the time delay can vary between each experiment, the significance of the error can increase.
Suggested improvements
If it were possible, a better idea would be to shine a light through the solution and into a detector, which would measure the amount of light passed through the solution. This would ensure that I could stop the timer is at the same point in the reaction much more accurately, without having to judge it by eye each time.
To improve on this method, I would ask for assistance in starting the stop clock as I mix the reactants. In theory, this would eliminate any time delays, however in practice human error still plays a part, and so this would still not be 100% accurate.