Write a hypothesis (what you think will happen an why). Look at the 'Factors affecting rates of reaction' page on this Google site for some help with this. For example - As the concentration of acid increases the reaction time will decrease and the reaction rate will increase. The higher concentration of acid contains more acid particles per mL of acid. Therefore, there will be more successful collisions per second between the acid particles and the magnesium particles and the reaction rate will be higher than with lower concentrations of acid.

Background information:

Magnesium ribbon reacts with excess sulfuric acid to make hydrogen gas and colourless magnesium sulfate solution. As this reaction progresses, the bubbles of hydrogen gas can be seen as fizzing and bubbling. Once the magnesium ribbon has been used up and has disappeared, the fizzing and bubbling stops. The reaction time can be measured using a stopwatch. This reaction is exothermic (gives out heat energy) and it is therefore difficult to completely control the temperature of the reaction mixture.

Equipment:

Lab coat, safety glasses, 2 molL^-1 (100%) sulfuric acid, magnesium ribbon, ruler, 10 mL measuring cylinder, boiling tubes, test tube/boiling tube rack, distilled water in a washer bottle, stopwatch, scissors.

Outline method:

Make up a 20% acid solution by measuring 2 mL of 100% (2 molL^-1) sulfuric acid into a 10 mL measuring cylinder. Make up to the 10 mL mark by adding 8 mL of distilled water. Pour this into a boiling tube labelled 20%. Remember to measure the volume by having your eye level with the bottom of the meniscus to reduce parallax error.
Make up a 40% acid solution by measuring 4 mL of 100% (2 molL^-1) sulfuric acid into a 10 mL measuring cylinder. Make up to the 10 mL mark by adding 6 mL of distilled water. Pour this into a boiling tube labelled 40%.
Make up a 60% acid solution by measuring 6 mL of 100% (2 molL^-1) sulfuric acid into a 10 mL measuring cylinder. Make up to the 10 mL mark by adding 4 mL of distilled water. Pour this into a boiling tube labelled 60%.
Make up an 80% acid solution by measuring 8 mL of 100% (2 molL^-1) sulfuric acid into a 10 mL measuring cylinder. Make up to the 10 mL mark by adding 2 mL of distilled water. Pour this into a boiling tube labelled 80%.
Finally, make up the 100% acid solution by measuring 10 mL of 100% (2 molL^-1) sulfuric acid into a 10 mL measuring cylinder. Pour this into a boiling tube labelled 100%. Working from the lowest concentration up to the highest concentration reduces the chances of changing the concentration of the acid by diluting it.
Carefully measure and cut a 1 cm piece of magnesium ribbon. Make sure you place the zero of the scale of the ruler on one end of the magnesium ribbon and carefully measure to the 1cm mark. This increases the accuracy of the measurement by reducing zero error.
Place 1cm of magnesium ribbon into the boiling tube containing 100% sulfuric acid solution. Start the stopwatch as soon as the magnesium touches the acid, and stop the stopwatch as soon as the magnesium disappears and fizzing stops. Record the reaction time in seconds. Remember that there are 60 seconds in a minute.
Repeat steps 6 & 7 for the other concentrations of acid.
Gather the data from two other groups and record this in your results table.
Determine the average reaction time for each concentration.
You can also calculate the reaction rate by using: reaction rate = 1/reaction time. This step isn't essential.
Plot the relevant graph(s) in order to see a trend or pattern in the results and processed.

Independent Variable

Which variable will be changed? The concentration of the sulfuric acid will be changed.

How will the variable be changed? The 100% (2 molL^-1) sulfuric acid will be diluted by adding specific volumes of water to the solution.

Give a suitable range of values for this variable. The concentrations of sulfuric acid to be used are 20%, 40%, 60%, 80% and 100%.

Dependent variable

Which variable will have to be measured or observed in order to get some data or information from the investigation? The reaction time will be measured.

How will the dependent variable be measured or observed? Make your answer here explicit. The reaction time will be measured using a stopwatch. The stopwatch will be started as soon as the magnesium ribbon enters the acid. the stopwatch will be stopped as soon as the magnesium has disappeared and the fizzing/bubbling stops.

What units will it be measured in? Seconds. All times in minutes will be converted to seconds.

Control variables.

Control Variable 1:

What? The temperature of the sulfuric acid and reaction. This is difficult to control in this experiment.

How? The sulfuric acid solution will be kept at room temperature (21^oC) that is controlled by the heatpump.

Why? Temperature affects the rate of reaction. Higher temperatures increase the reaction rate and decrease the reaction time. Lower temperatures decrease the reaction rate and increase the reaction time. Therefore, it is important to carry out all trials at the same temperature, and room temperature is probably the easiest to control. In this particular experiment temperature is difficult to control since the reaction between sulfuric acid and magnesium ribbon is exothermic. An exothermic reaction generates heat which will 'heat' up the reacting particles. This means the particles will have more energy and there may be more frequent successful collisions between reacting particles.

Control Variable 2:

What? Type of acid used

How? Always use sulfuric acid from the same stock bottle.

Why? Some acids are stronger than others and react at different rates so the acid used needs to be kept the same. H₂SO₄ is stronger than HCl as it has two hydrogen atoms per molecule. Whereas, HCl only has one hydrogen atom per molecule. (Please note that strength of acids is not the same as concentration of acids - they are two completely different chemistry concepts).

Control Variable 3:

What? Volume of sulfuric acid used

How? Always use 10 mL of sulfuric acid

Why? The volume of acid should be kept the same to have a fair test. Differing volumes of acid will affect the reaction rate since there may be more or less acid particles present to react with the magnesium.

Control Variable 4:

What? Surface area of magnesium (size of magnesium pieces)

How? Always use magnesium ribbon from the same roll.

Why? The same size magnesium pieces should be used, as smaller pieces (magnesium turnings or magnesium powder) have a larger surface area. Increasing the surface area increases the reaction rate. Therefore, magnesium ribbon from the same roll should be used throughout to keep the surface area the same each trial.

Control Variable 5:

What? Length of magnesium ribbon used

How? Always use 1 cm (10 mm) magnesium ribbon

Why? Using different lengths of magnesium ribbon would affect the reaction rate because you would be changing the amount of magnesium particles available to react. This would make the results invalid. Therefore, the same length needs to be used each trial.

Control Variable 6:

What? Stopwatch used

How? Always use the same stopwatch to measure the reaction times

Why? Stopwatches may have calibration errors. One stopwatch may run faster than another. This would affect the reaction times. Therefore, it is important to use the same stopwatch to ensure valid results. An calibration error from the use of one stopwatch throughout the investigation would be a systematic error.

Control Variable 7:

What? Person operating the stopwatch.

How? Always have the same person using the stopwatch (for a set of results).

Why? People have different reaction times and may be quicker or slower to start and stop a stopwatch. It is important to have the same person operating the stopwatch to ensure valid results.

Control Variable 8:

What? Measuring cylinder used.

How? Always use the same measuring cylinder to measure the volumes

Why? Using the same measuring cylinder to measure volumes will reduce calibration error. The scale on one measuring cylinder to another has a degree of uncertainty.

Method:

This is a more detailed version of the outline method written above.

Results:

Graphs:

Conclusion:

The results and graphs show that as the concentration of the sulfuric acid increased the reaction time decreased and the reaction rate increased. The 20% acid had the longest average reaction time of 619.14 s and the lowest reaction rate of 0.0016 s^-1, whilst the 100% acid had the shortest average reaction time of 37.79 s and the highest reaction rate of 0.0265 s^-1. This conclusion supports the hypothesis that as the concentration of acid increases, the reaction rate increases.

Discussion of science ideas

The magnesium reacts with the sulfuric acid because there are collisions between the reacting particles. For a reaction to occur the collisions between reacting particles must be successful. For a collision to be successful the particles must collide with enough energy and in the correct orientation.

As the concentration of sulfuric acid increases the reaction time decreases and the reaction rate increases. The higher concentration of 100% sulfuric acid has more sulfuric acid particles in a given volume (e.g. more particles per mL). There will be more collisions per second between the reacting sulfuric acid and magnesium particles, Therefore, there are more successful collisions per second (or an increased frequency of successful collisions), and the reaction rate is higher. The lower concentration of 20% sulfuric acid has fewer sulfuric acid particles per mL. There will be fewer collisions per second between the acid and magnesium particles. Therefore, there are fewer successful collisions per second and the reaction rate is lower.

Discussion of reliability

The results appear to be fairly reliable. All 7 sets of data for the 20% acid are reliable as in each case the 20% acid had the longest reaction time, with an average reaction time of 619.14 seconds. The results for Group 1 and Group 5 do not have any outliers and appear to be the most reliable. The results follow the expected trend and as the concentration increases, the reaction time decreases. All other sets of data have one outlier. Possible outliers are highlighted in yellow.

Discussion of validity

The results and therefore the conclusion are valid since this was a fair test investigation. Only one variable (the concentration of the sulfuric acid) was changed. All other variables were controlled and kept the same. The one variable that was difficult to control and that could have affected the reaction rate was the temperature of the reaction mixture. The temperature of the surroundings and the chemicals prior to the reaction was kept the same since increasing the temperature would increase the reaction rate and decreasing the temperature would decrease the reaction rate. This particular reaction is an exothermic reaction and during the progress of the reaction heat energy was generated. It was therefore difficult to keep the reaction mixture during the reaction at room temperature and constant. This may affect the validity of the results to a small degree. All other key variables were able to be controlled and kept the same to ensure validity in the results and conclusion.

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