Rates of reaction.

NIKESH PARMAR 11E

CHEMISTRY COURSEWORK

RATES OF REACTION

Many people are interested to know how to alter the rates of chemical reactions. Fertiliser manufacturers are interested in speeding up the formation of ammonia from nitrogen and hydrogen. Car manufacturers are interested in slowing down the rate at which iron rusts. This shows that rates of reaction is a very important factor within industries today, and in order to demonstrate how a particular factor can alter the speed of a reaction the outcome variable of my investigation to follow is: How a chosen factor affects the rate of reaction of Magnesium and Hydrochloric Acid.

Reaching this outcome will consist of choosing a relevant factor with the assist of background knowledge, applying the factor whilst experimenting, measuring the rate of reaction and concluding the outcome. In order to choose a relevant factor I will initially need to observe the different variables:

1. The concentration of a reactant.

The higher the concentration, the more molecules per unit volume are available for the chemical reaction, thus more successful collisions occur and reaction rate increases. In regard to my investigation: increasing the concentration of hydrochloric acid molecules would increase the frequency or chance at which they would hit the surface of magnesium in order to dissolve them, which would increase the speed of product formation.

2. The temperature of the reactants.

Higher temperature results in particles moving faster, successfully colliding more often and an increase in the reaction rate. For example by increasing the temperature, the magnesium and hydrochloric acid molecules will gain kinetic energy which will cause them to move faster. The increased speed will also increase the chance of collision, hence the rate will increase.

3. The surface area of a reactant.

The larger the surface area, the more collisions and the greater the reaction rate. For example smaller pieces of the same mass of magnesium would have a greater surface area compared to larger pieces. Therefore there is more chance that a magnesium particle will hit the solid surface and react.

4. Adding a catalyst to the reaction.

Catalysts increase the rate of a reaction by helping to break chemical bonds in reactant molecules by lowering the activation energy. Particles need less energy to react and the process proceeds more quickly. Alternatively a negative catalyst slows down the rate of a chemical reaction.

5. Stirring the reactants.

Stirring brings substances together so they can react and/or mix quickly therefore stirring is an important rate factor.

Each of the above mentioned factors do have a form of influence to the rate of reaction, however not all are applicable in accordance to my experiment. Therefore I will rule out each of the factors that are not appropriate.

The first exclusion is stirring the reactants. In terms of measuring the results and fairness, if the reacting mixture is not stirred evenly, then the reactant concentration in solution will become much less near the solid, which will tend to settle out. Therefore this will diminish the reliability and fairness of my results. The second exclusion is adding a catalyst. This is primarily due to the restricted apparatus I am provided with to conduct the experiment where it would be too difficult to setup and sustain. These are the only relevant exclusions, so this leaves me with concentration, temperature and surface area all of which can be altered to demonstrate an influence.

These 3 factors will all have a form of involvement, however I will primarily only concentrate on one, thus my aim:

AIM: How temperature affects the rate of reaction of Magnesium and Hydrochloric Acid.

Therefore temperature is the independent variable. As an independent variable, it simply means I will need to change the temperature throughout the experiment in order to demonstrate a possible influence.

I have chosen temperature because I believe this will have a powerful relevance to the rate of reaction of magnesium and hydrochloric acid. Also as this is set as an independent variable, I will have independent control over the values I pre-set it to prior to experimentation.

The dependant variable is the variable dependant on others to produce a result and also what I am trying to find out about. Therefore the dependent variable is the time of reaction.

And the concentration and surface area are set as the control variables. These will need to be kept constant throughout the experiment in order to ensure a fair test as any unexpected alterations during the experiment may affect the accuracy of my results.

BACKGROUND KNOWLEDGE:

‘Rate of reaction’ simply means ‘how fast is the reaction’. Primarily this depends on two factors:

The number of collisions per unit time between the reacting species,
and the fraction of these collisions that are successful in producing a new molecule.

According to the collision theory particles can only react when they collide. If you heat a substance, the particles gain kinetic energy, therefore move faster and so collide more frequently. However if two or more molecules collide but are not orientated correctly or collide without sufficient energy then no reaction will take place.

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In order for particles to successfully collide and actually break existing bonds to make new a new product, instead of bouncing off each collision, the particles will need to possess enough kinetic energy to cause or initiate a reaction. For this to occur the particles will need to have enough activation energy to break the energy barrier. Activation energy is the minimum kinetic energy needed for a successful collision and the energy barrier is the level of activation energy needed to start a reaction. Therefore when heated molecules have a greater kinetic energy, a greater fraction of them have the required activation energy to exceed the barrier to react.

For example temperate helps to do this as increasing the temperature increases the range of kinetic energies, which in return increases the average kinetic energy and therefore increases the population of particles with more than the activation energy to exceed the energy barrier.

Furthermore an influence of temperature can be seen in the graph below:

The graph shows that an increase or decrease in temperature will change the shape of the curve which means fewer or more particles will have the activation energy.

PREDICTION:

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Furthermore an influence of temperature can be seen in the graph below:

The graph shows that an increase or decrease in temperature will change the shape of the curve which means fewer or more particles will have the activation energy.

PREDICTION:

From the background knowledge I have concluded that as you increase or decrease the temperate of a reaction, this will change the movement of the particles and in return influence a change in reaction. Therefore I predict that as I increase the temperate of the magnesium and hydrochloric acid substance, the reaction will also increase.

Below is a graph of my predicted result:

The graph shows that the rate changes meaning there isn’t a constant rate throughout the reaction. Also as can be seen from the curve, the reaction is fastest at the start and gradually becomes slower as the reaction proceeds. This shows that the rate of reaction increases at a decreasing rate. And also the curve of the graph goes flat towards the end, this is the point where all reactants have been used up, hence meaning the reaction is complete and the product is formed.

In addition as can be seen from my graph, I am aiming to determine the varying rates of reactions, where-as the dependant variable is the time of reaction. Therefore the rate of reaction can be calculated using its formula:

Reaction rate = 1

PRELIMINARY WORK:

In order to ensure a fair test is carried out and the best possible variables are used in order to conduct an accurate experiment, I will need to carry out preliminary work. This will consist of: experimentation of concentration, surface area and temperature.

To produce reliable results I will need to ensure that only one of the variables is changed at a time and each recording is repeated twice. From this I should be able to calculate an average which will help me in choosing a reasonable criterion for each variable.

Concentration and surface area need to be experimented in order to determine the most advantageous amount of hydrochloric acid and the optimal length of magnesium to use. Primarily this is to ensure that the variables compliment and help produce accurate results when varying temperature, and also helps to determine amounts that are straightforwardly measurable and ones that will not take too long to record. For example 3cm of magnesium and 5ml of hydrochloric acid at whichever temperature may take more than twenty minutes to react, and keeping in mind the restricted time I have for the whole experiment, this would be far too long.

The independent variable, temperature, also needs to be covered in my preliminary work to determine the best range of values I will set it to for the actual experiment. Also this will make it easier for me and save time later as I would already be familiar with the varying times of reaction.

Preliminary work will also allow me to identity any arising problems at an early stage and determine the required apparatus needed to complete the experiment.

Preliminary results:

Varying temperature

The results for temperature show that that there is a big difference in time of reaction when increasing the temperature by 20ºC each time. However from this I have decided to pre-set the intervals by 10ºC for the actual experiment because I believe this will allow me to notify the decreasing trend in more detail.

Varying concentration of 2HCL

Room temperature (22ºc)

I wasn't able to complete this table due to the limited time; however I was able to get one recording for each varying concentration.

I have found that diluting the concentration has a dramatic effect on the time of reaction and due to the time elapsing out of proportion when adding water; I have decided to not add any water to the hydrochloric acid when conducting the actual experiment.

Varying length of magnesium

Room temperature (22ºc)

Again I wasn't able to complete this results table due to the elapse of time during that particular lesson. However I was able to find that as the length of magnesium increased so did the time of reaction.

I wasn't able to take recordings for 3cm, so it would be a risk keeping this consistent throughout the experiment. So I have decided to use 2cm of magnesium, primarily due to the reason that if I use 1cm constantly, when the temperature gets to a high end, the reaction may be too rapid. This will reduce the accuracy of my results. Therefore I will use a constant length of 2cm.

Possible errors during the preliminary work may have been starting and stopping the stop watch at precise timings. It was often difficult to observe when the product had actually been formed, however knowing this early now I am able to pay extra attention during the experiment.

Other profound methods of fault may have been the amount of hydrochloric acid, as a small portion of liquid may have been left in the measuring cylinder after the contents were poured to the required beaker. Also the measurements of the magnesium strips may have not been 100% accurate.

The only striking element of impediment whilst conducting the preliminary experiment was time. Some of the reactants took far too long to react, which held up the whole experiment and didn’t allow me to complete the preliminary work as intended. However the results taken should be enough for me to choose relevant criteria for each variable, of which should be manageable within the time frame.

As these difficulties have been identified early on, I am able to apply this and avoid the faults during the actual experiment.

METHOD:

The following apparatus will be needed to carry out the experiment:

2cm magnesium strips (Approx. 15)
Bottle of hydrochloric acid and pipette
100cm beaker
100ml measuring cylinder
-5ºC -110ºC thermometer
Bunsen burner
Heatproof mat
Tripod
Wire Gauze
Splint
Digital Stop Clock
Scissors
Ruler

Below is a diagram showing the required set-up of apparatus:

The following steps will need to be carried out in order to conduct an accurate and more importantly successful experiment.

Begin by setting out the apparatus as shown in the diagram above.

i: Ensure the heatproof mat is placed on a clean and flat surface on the desk.

ii: Measure 15ml of hydrochloric acid using the measuring cylinder and pour the contents into the beaker ready to be heated.

iii: The 2cm strips of magnesium should be measured using the provided ruler and then cut out accordingly using the scissors.

Using a splint, safely, ignite the Bunsen burner and adjust the air hole so it is half way open and the flame is blue.

Knowing the first range of temperature is 30ºC, pay close attention to the ºC shown on the thermometer and carefully remove the beaker from the gauze once the required temperature is reached.

Put a 2cm strip of magnesium into the beaker and instantly start the stop watch timer.

As soon as the magnesium disappears, stop the stop watch, and record the time into your results table.

Repeat stages 3-5 again, but stage 3 should be altered so the temperature of the substance is 10ºC higher than the one before.

i: This procedure should continue until you have recorded all results up to º90 C.

Repeat the whole experiment again in order to increase the accuracy of results which will make an average calculation possible for each recording.

Safety is a major factor that needs to be kept in mind at all times:

Before the apparatus are setup, it is required safety goggles are worn, the desk is clear of all stationary and books and bags are placed away from the table.

Great care is taken whilst the Bunsen burner is ignited, and whilst recording results where a substance is not being heated, the flame is bright and visible.

When removing the beaker from the setup apparatus area, it is necessary great care is taken as it may be hot. Therefore a towel may be useful.

Contact of hydrochloric acid to the skin should be rinsed through ally under a cold tap.

All apparatus should be handled with care and primarily with the glass equipment, placed away from the edge of the table.

The outcome of the usefulness and findings of my preliminary results, in accordance to the actual experiment, can be found in the variable table below:

RESULTS:

Concentration and carefulness helped me in producing more reliable results than those of the preliminary work.

The lengths of the magnesium were all matching in length, and I assured the concentration of hydrochloric used was as precise as possible to the constant rate.

As can be seen, each recording was repeated twice. This was to ensure the most accurate results were produced and with two sets of recordings this would allow me to calculate an average.

ANALYSIS:

Now that I have completed the practical experiment and results have been recorded, I am able to analyse the results and come to a conclusion in contrast with my set prediction

I will begin by producing a line graph showing the relationship between temperature and the average time of reaction. The recordings are also tabulated below.

Refer to: GRAPH 1

The basic trend of the graph shows that time of reaction (y) decreases at a decreasing rate. More specifically the curve of best fit shows that the average temperature (y) is inversely proportional to the average time of reaction (x). This forms an equation of y=m× (1/x)

Time of reaction = constant x (1/temperature).

As the constant is set as 1, this can be interpreted as y=1/x.

In order to validate this I will produce a line graph showing the relationship between the two. The calculations of 1/temperature are also tabulated below.

Refer to: GRAPH 2

As can be seen from the graph the time of reaction increases at a constant rate which proves that the time of reaction is proportional to 1 over temperature. Although the points do not form a perfect straight line, using the line of best fit it is possible to determine the time taken by any given temperature.

For example a 1/temperature of 0.015 ºC would cause the reaction to take approx. 60 seconds. And by dividing 0.015 by 1, it is possible determine the actual temperature of the reaction.

In this case:

1/temperature = 0.015
1/0.025 = 66.66
Therefore temperature = 66ºC

Hence it would take approx. 60 seconds for a product to be formed when the temperature is 66ºC.

I have analysed the time of reaction as this was the dependant variable throughout my experiment, but the aim of my experiment is to determine the relationship between temperature and rate of reaction. Therefore I will begin by producing a scatter diagram showing all the recordings. This will allow me to observe the accuracy of results and also, through a visual display, identify any possible recordings that don’t follow the usual trend.

Using the recordings of time of reaction, it is possible to convert this into a rate using the following formula:

1 / time taken

Below are the rates of reaction calculations tabulated and therefore a scatter diagram demonstrating this:

Refer to: GRAPH 3

The scatter diagram shows positive correlation meaning as the temperature increases, the rate of reaction also tends to increase.

In order to distribute the recordings further and more importantly notify a relationship I will tabulate and produce a line graph showing the average time of reaction recordings against increasing temperature.

Refer to: GRAPH 4

The basic trend of the graph shows that as the temperature increases so does the rate of reaction, which proves my prediction is correct. However as can be seen, the relationship is not linear. The curve of best fit shows that the average rate of reaction (y) is directly proportional to the square root of temperature (x).

I have calculated this below and in order to test a relationship I have produced a line graph:

Refer to: GRAPH 5

The graph shows that the average rate of reaction increases at a constant rate, excluding the second point, which proves that rate of reaction, is directly proportional to the square root of temperature.

Using the line of best fit it is possible to find the rate of reaction by choosing any temperature on the graph.

For example a √temperature of 3ºC would cause the reaction to take approx. 0.00565 seconds. And by squaring the √temperature of 3ºC, it is possible to determine the actual temperature of the reaction.

In this case:

√temperature = 3ºC
√temperature of 3 = 3 x 3
Therefore temperature = 9ºC

Rate of reaction = 0.00565
1 / 0.00565 = 177
Therefore time taken = 177 seconds

Hence a reaction which is 9ºC would take approx. 177 seconds to complete.

Overall from the analysis I have proved:

As the temperature increases the time of reaction decreases.
As the temperature increases the rate of reaction also increases which validates my prediction is correct.

And from demonstrating these findings on line graphs, I was able to determine and prove that:

Time of reaction is proportional to 1/temperature and
rate of reaction is proportional to the square root of temperature.

EVALUATION:

Accuracy:

As can be seen from the circled point on the majority of graphs produced from the results taken, this is identified as an anomaly as it disperses away from the usual trend of the graph. There are many reasons to indicate why an anomaly is present; this consists of human errors and equipment restrictions and possible unruly scientific theories that affect the whole experiment.

The method of using a stop watch to measure the time of reaction was not as reliable as to using more precise recording apparatus such as a time measuring and indicating device for example. This is 1) due to the unit of time where all recordings were automatically rounded to 2d.p. and 2) the mature condition of the stop watch used at times caused a lockage in the start and stop buttons, in a case where the time distinction of getting the stop watch to start and stop may have affected the accuracy of a recording.

Furthermore it was impossible to observe and acknowledge exactly when a reaction had been completed; hence product formation. Extra attention was paid to the reaction, however the stop watch timer was only stopped when I believed the reaction was complete. This is unreliable and has no form of justification as this was determined solely through manual procedure.

Again another profound error may have been the amount of hydrochloric acid. I ensured as much solution as possible was poured into the required beaker after being measured in the measuring cylinder, however there were always small drips of liquid that were inevitable. It is illegible whether this has an influential affect; however this regardless opposes the accuracy of the recordings.

The temperature of the solution may have not been as intended at the start of the reaction (when the magnesium was put in). This is primarily due to that the beaker was removed from the heat once the thermometer hit the intended temperature, however the distinction in time of removing the beaker from the heating setup area to the flat desk and entering the magnesium may have caused the temperature to fall (significant/insignificantly is unknown). This is due to the reaction being exothermic, transfer of heat to surroundings, which would have caused dramatic changes in temperature throughout the experiment.

There are also reasons to compliment the accuracy of my results:

Each recording was repeated twice. This enabled me to calculate an average and gave me the advantage of being able to produce a better analysis overall.

I was able to identify early on during the preliminary work that the magnesium strips may have not been the same consistent size throughout, but during the actual experiment I took extra care in measuring, cutting and comparing the strips to the required identical length.

Reliability:

In addition to the scopes of errors mentioned above, there are many other incontrollable factors that may have had a form of influence or obstruction. However no significant problems or difficulties were encountered whilst conducting the experiment.

My results and conclusion were accurate and reliable enough to verify a relationship that as the temperature increases, the rate of reaction also increases, hence agreeing with my set prediction. I was also able to determine that an increase in temperature, decreases the time of reaction, furthermore the time of reaction is proportional to 1/temperature and the rate of reaction is proportional to the square root of temperature.

Improvements:

No matter how accurate produced results are, due to the restriction in apparatus provided and the time to complete the investigation, improvements will always be applicable. Possible improvements:

In order to exclude the scopes of errors mentioned above regarding changing temperature, it would be convenient to conduct reactions in a thermos or similar container which would trap the heat. This would keep the temperature constant and in return increase the accuracy of results.

Even though two repeats for each recording was accurate enough to produce a reliable average, increasing this to three or four recordings would produce a range of results which could then be analysed in more detail. From this possible impediments of restrictions may also arise, which whilst analsying may bring about new theories affecting the reactions.

As mentioned earlier, a more precise time measuring device would be useful instead of a stop watch. This would obviously be to produce better, précised results and one that would possibly exclude the need of manual operation.

Extending the investigation:

It would be useful to experiment temperatures below 30ºC. This would enable me to observe the reactions and behavior of lower temperatures. And primarily with the last three temperature recordings it was noticeable that the points were closer which explains the flat curve produced at the end of graphs that produced a curve. This would enable me to observe the point at which reactions are unable to exceed; therefore it would be of use to observe temperatures exceeding 90ºC.

Another method of measurement would be counting the bubbles formed in a reaction. This would require the need of better equipment, but I believe this would form interesting results.

It would also be interesting to experiment the influences of the other variables, as mentioned at the beginning of my coursework. I was able to briefly experiment concentration in my preliminary work, but it would be interesting to do a continuous specialised experiment and also a new variable that comes to mind: light.

SECONDARY SOURCES OF INFORMATION:

Internet sources used to construct my background knowledge: