Dependent Variable (Variable that I am measuring): The time taken for all the magnesium ribbon to react with the hydrochloric acid is to be found with a stopwatch and rounded off to the nearest second. I will then find the rate of the reaction by dividing the time taken for the magnesium to react by 100 and rounding off the answer to 2 decimal points.
Control Variables (Variables that I am keeping constant to make it a fair test):
1. Length of Magnesium Ribbon
An increase in the length of magnesium can alter the rate of reaction because more magnesium is exposed to the acid particles. Thus, it is important to keep the length of magnesium constant. This will be decided after conducting my trials.
2. Volume of Hydrochloric Acid
The volume of hydrochloric acid can affect the rate of reaction. A large volume of acid would make the rate faster because there are more acid particles. I will plan to use 25cm³ of hydrochloric acid.
3. Concentration of Hydrochloric Acid
The concentration of hydrochloric acid has already been decided for us. It will be 0.6 Molars.
4. Surface Area of Magnesium Ribbon
The surface area of magnesium ribbon can affect the rate of reaction e.g. if the magnesium was split into two pieces, more surface area will be exposed to the acid increasing the rate of reaction.
Apparatus List:
250ml Beaker (Water Bath)
100ml Beaker
50ml Measuring Cylinder
Thermometer
Magnesium Ribbon (length is to be trialled)
Pair of Scissors
15cm Ruler
400ml Hydrochloric Acid (Concentration of 0.6 Molars and 25cm³ of acid for each experiment)
Kettle
Hot Water/Cold Water (Volume varies for each experiment)
Ice Cubes (Varies for each experiment)
Stopwatch
Safety Glasses
Diagram of Apparatus Needed: (diagram)
Method:
- Set up apparatus as shown in the diagram above.
- Using a ruler and a pair of scissors, measure and cut 10 strips of magnesium ribbon required for the trial experiments (2cm x 5 and 1cm x 5). *
- Pour some hot water or cold water in a 250ml beaker (water bath).
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Then measure 25cm³ of hydrochloric acid (concentration of 0.6 Molars) using a 50ml measuring cylinder and add into a 100ml beaker. Place the beaker gently inside the water bath.
- Put a thermometer into the 100ml beaker to check the temperature of the acid. If the temperature has not reached the proposed temperature, add more hot water or ice cubes into the water bath until it does so. Make sure the level of the water is above the level of the acid so that the acid will fully be cooled or heated.
- When the temperature of the acid reaches the proposed temperature, put the magnesium ribbon into the acid. Start timing using a stopwatch and record the starting temperature.
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When the magnesium ribbon fully reacts with the hydrochloric acid, stop the stopwatch. Record both the temperature after the reaction and the time taken for the magnesium to completely react.
- Do procedures 1 to 7 for temperatures 10ºC, 20ºC, 30ºC, 40ºC, and 50ºC.
Note: In my trials I can only use a maximum of 20cm magnesium ribbon for experimentation but I will be using 15cm of magnesium altogether (2cm x 5 = 10cm and 1cm x 5=5cm, therefore 10cm + 5cm = 15cm). When it comes to doing my actual method I will use a constant length of magnesium which will be decided from the preliminary trial. The maximum length of magnesium I can use in all the actual experiments is 40cm.
Safety: The reaction between magnesium ribbon and hydrochloric acid can be quite violent when at high temperatures and can give off acid spray so safety goggles should be worn throughout.
Preliminary Trial: Initially, to determine an appropriate range of temperatures and decide some suitable quantities, I conducted a preliminary investigation in which I recorded the time taken for the magnesium ribbon to react with the hydrochloric acid at various temperatures. To decrease the temperature of acid to 10ºC and 20ºC, I will add ice cubes to a 500ml beaker (water bath) filled with water. While for temperatures 30ºC, 40ºC, and 50ºC I will increase the temperature of the acid by adding hot water from a kettle to the beaker. I received the following results:
Constant Variables: 1. Volume of Hydrochloric Acid= 25cm³
2. Concentration of Hydrochloric Acid = 0.6 Molars
Modification of Method: After this trial, I decided that the length of magnesium to use in my actual experiments is 1cm because changing the length to 2cm will not have much effect on the rate of reaction and that it may be wasteful if I use a larger piece. Therefore, I will change step 2 of my method to:
2. Using a ruler and a pair of scissors, measure and cut 21 strips* of 1cm magnesium ribbon.
Each experiment will also be repeated 3 times to find an average. However, I will not plan to test how long it takes for the magnesium ribbon to react with the hydrochloric acid for 10ºC because it takes too long. Instead, I will plan to test temperatures: 20ºC, 25ºC, 30ºC, 35ºC. 40ºC, 45ºC and 50ºC. This means that I will change step 8 to:
8. Do procedures 1-7 for temperatures 20ºC, 25ºC, 30ºC, 35ºC. 40ºC, 45ºC, and 50ºC and repeat the experiment 3 times to get an average reading to improve the reliability of my results.
I will not change anything else in my original plan because my results seem to be accurate and reliable. It can be noticed from the table that as the temperature increases, the time for the magnesium ribbon to react with hydrochloric acid increases.
Analysis:
Results: Effect of temperature on the rate of reaction between magnesium ribbon and hydrochloric acid
Constant Variables: 1. Volume of Hydrochloric Acid = 25cm³
2. Concentration of Hydrochloric Acid = 0.6 Molars
3. Length of Magnesium = 1cm
4. Surface Area of Magnesium = Constant as length is also constant
(Time Graph)
From the graph above I can see that an increase in temperature affects how quickly the time takes for the magnesium ribbon to disappear. As I increase the temperature of hydrochloric acid, the time taken for the reaction to occur increases. This means that hydrochloric acid with a higher temperature would make the magnesium react faster than hydrochloric acid with a lower temperature.
As the temperature increases, the time taken for the magnesium ribbon to react with the hydrochloric acid increases. Temperature affects the speed of chemical reactions when the particles collide more often in a certain time. When the hydrochloric acid particles are heated up, they have more kinetic energy causing the particles to move around and collide more quickly with the magnesium. Therefore, the reaction time increases as the temperature increases. However, there is another reason why temperature affects the speed of the reaction. This is by making it more likely that collisions result in a reaction. This is where the temperature is increased to give the acid particles more kinetic energy and bounce off each other hard enough to start a reaction. This energy needed to start the reaction is known as the activation energy. This means that more collisions have an energy greater than the activation energy increasing the time taken for the reaction to occur.
Rates of Reaction Table: Effect of temperature on the rate of reaction between magnesium ribbon and hydrochloric acid
Note: To calculate the rate of reaction I used the formula:
Rate of Reaction = 100
(per s)
Time taken for Mg to react (s)
(Rate Graph)
The graph above shows that my results disagree with my prediction: that as the temperature increases by 10ºC, the rate of reaction doubles meaning that temperature is directly proportional to the rate of reaction. Thus, using the graph, the actual relationship between the temperature and the rate of reaction is that as ‘x’ (the temperature) increases so does ‘y’ (the rate of reaction) but is not directly proportional since it is a curve. This means that when the quantity on the x-axis doubles, the quantity on the y-axis more than doubles. I can show this by taking some readings from the graph:
The total factor increase in rate is … (1 decimal point) as … + … + … + … ÷ 4 = …. This shows that 10ºC is not sufficient to double the rate of reaction because the factor increase is less than 2 i.e. … Therefore, from these results I am able to say that an increase in temperature does certainly increase the rate of reaction between magnesium ribbon and hydrochloric acid. This is because at a higher temperature the hydrochloric acid particles gain more kinetic energy making them move more faster. This rapid movement causes the hydrochloric acid particles to have more collisions with the magnesium ribbon. As the chance of collisions increases, the rate of reaction increases. However, it is not enough for hydrochloric acid particles and magnesium particles to collide. Bonds between the particles of hydrochloric acid must be broken before new particles can be made. Each acid particle needs a minimum amount of energy to break the bonds before a reaction can occur. This minimum energy is called the activation energy of the reaction. This means that when the acid particles are heated, they have more kinetic energy causing a greater proportion of them to have the required activation energy to react with the magnesium. Here is a diagram to summarise my point:
(diagram)
Since most of the points on the two graphs are close to the best fit curve, the results are accurate and reliable, so all my evidence supports a firm conclusion.
Evaluation: Although I think that my experiment was sound overall, there were few points at which the accuracy was not perfect. Referring back to the graph, all my results were reasonably accurate, however there were two anomalies. The first anomalous result (state temp, exp no, and time) was probably due to the fact that it was not easy to see when the magnesium ribbon had fully reacted with the hydrochloric acid. This could have caused big errors in the reliability of my results. For example, the time of the chemical reaction was measured to an accuracy of 1 second. Thus, the percentage error on measuring the reaction lasting … seconds for ..ºC is …%. This could even account for the points not exactly being on the line of best fit. This is a major source of error when doing faster reactions at higher temperatures as the time could increase. The variability of all my results was reasonably similar for each temperature e.g. …, …., and …. For …ºC except the temperatures that contained anomalies.
There are several reasons for any anomalous results in this experiment. The second anomalous result (state temp, exp no, and time) that did not fit the pattern was probably due to not measuring the volume of acid accurately. If the volume of acid is more, this will cause the time to increase and if the volume of acid is less, this will cause the time to decrease. As I measured 25cm³ of acid in a 25cm³ measuring cylinder, the percentage error is equal to 4% (1/25 x 100 = 4%). However, this is a minor source of error.
This investigation can be improved in many ways. Inorder to improve the reliability of the method, more smaller and accurate apparatus (burettes and pipettes) can be used to measure the volume of acid. I can also try to monitor and keep my control variables as constant as possible to increase the reliability of my experiment. For example, I could measure the length of magnesium as accurately as possible and avoid wetting the apparatus (50ml beaker with 25cm³ of acid) with water for each experiment to avoid minor errors in my results. The results could also be recorded more frequently (5 repeats) to increase the reliability and a bigger range of temperatures can be included with a 5ºC interval e.g. 10ºC, 15ºC, 20ºC, 25ºC, 30ºC, 35ºC, 40ºC, 45ºC and 50ºC. However, due to practical time constraints in taking the readings for my investigation, and some consequential problems relating to time extension, I could not in fact make these adjustments.
A definite trend can be drawn from the results for this investigation but to ensure that this is a definite conclusion the ideal method for measuring accurately the rate of reaction between magnesium ribbon and hydrochloric acid with temperature is too collect the volume of hydrogen gas given off using a syringe. The volume of gas can be measured up to a certain time period and the initial rate of reaction can be calculated. This experiment could also be carried out by using data logging equipment. This is where computer based logging and monitoring equipment is set up to measure and record the actual temperature used, the volume of hydrogen gas produced, the time taken for the magnesium ribbon to react, and the rate of the reaction. However, the ideal method was not suitable for this investigation because this equipment was not available in the laboratory and it’s too expensive. More time is also needed to time the reaction in order to calculate the rate. To extend my investigation, I could investigate the effect of temperature on the rate of reaction between magnesium and sulphuric acid. Other reactive metals and acids can also be used.
