We can measure rate in lots of ways, though the most common ways are;
- colour change
- pH change
- volume of a gas
- appearance of a precipitate
The rate of the experiment does not stay constant, it is the greatest at the start, and becomes slower throughout the experiment. This is because throughout the experiment the energy gradually decreases.
We can either measure the amount of reactant used up per unit of time or the amount of product produced per unit of time to find the rate.
For this experiment I am going to find the rate of the reaction by measuring the amount of hydrogen gas produced per 180seconds.
The rates of most reactions can be related to the concentrations on individual reactants by an equation:
Rate = k [X]n
This expression, in which X is the reactant under consideration and n is usually 0, 1 or 2, is known as a rate equation or law. The value of n gives the order of the reaction. When n=0, the reaction rate is said to be zero order with respect to X, i.e.
Rate = k [X]0
But, since [X]0 =1,
Reaction rate = k
The reaction rate is the independent of the concentration of X. This means that changing the concentration of X will not affect the rate of the reaction.
When n =1, the reaction rate is proportional to [X]1 and the reaction is said to be first order with respect to X.
When n =2, the reaction rate is proportional to [X]2 and the reaction is said to be second order with respect to X.
Apparatus
Retort stand
Conical flask
Gas syringe
Bung
Burette
Beakers (25ml)
Stop clock
Safety glasses
15x magnesium strips (5cm)
3x each conc. of Sulphuric acid; 0.2, 0.4, 0.6, 0.8, and1mol (25cm³)
Diagram
Method
- Collect the apparatus and set up as shown above
- Cut five, 5cm strips of magnesium
- Measure out 25cm³ of a concentration of sulphuric acid using the burette
- Add the acid into the conical flask
- Then add a strip of magnesium to the sulphuric acid and place bung on conical flask immediately to stop any hydrogen loss
- Start the timer
- Record the amount of gas given off every 20seconds for 3minutes (180sec) by monitoring the gas syringe
- Repeat the above steps three times, for reliable results
- Record the results into the table and find the average volume of hydrogen given off
-
Repeat the above steps for each concentration of sulphuric acid (0.2, 0.4, 0.6, 0.8, or 1mol/dm³)
Strategy for Results
To record my results I collected in my experiment, I will use a table that will look like this:
In total there will be five tables for each concentration. In the table I am going to take the three sets of results for the volume of gas produced, and find the average of each 20second interval.
I will then plot five graphs, one for each concentration. I will plot average volume of H² against time. My graph will be plotted on something like this. I will calculate the gradient of each graph, which will give me the rate of reaction for that concentration.
Gradient = y2-y1
x2-x1
Variables
Independent – concentration of sulphuric acid (H+ ions) will increase by 0.2mols each time (0.2, 0.4, 0.6, 0.8, or 1mol/dm³)
Dependant – the amount of hydrogen gas that is given off (cm³)
Controlled – the starting temperature of sulphuric acid, catalyst, surface area of magnesium (5cm), volume of sulphuric acid (25cm³), the duration of the experiment (180seconds), the number of attempts per concentration (three)
Accuracy and Reliability
My results are recorded very accurately:
- The stopwatch gave a reading +/- second
- The gas syringe recorded to +/- cm³
- The burette recorded to +/- 0.5cm³
To increase the reliability of my results I repeated the experiment for each concentration three times and I also found the average volume of gas produced for each concentration.
Safety
- Make sure that all work surfaces are clear before I begin the experiment
- Wear safety glasses to prevent any acid entering my eyes
- Take care in using the glassware, as it can be sharp when broken
If I follow these points it will help keep the experiment safe.
Prediction
I predict that as the concentration of the sulphuric acid (H+ ions) increases the rate of reaction will also increase. I think this because as the concentration increases the frequency of collisions between the reacting particles will also increase. This is explained through the collision theory. This states that a reaction can only occur when the reactant particles collide with the correct activation energy. If we make one reactant more concentrated, we increase the number of particles at a particular volume. Therefore there will be more collisions. The more successful collisions there are the faster the rate of reaction.
Table of Results
Initial rate
Gradient = y2-y1
x2-x1
Concentration 0.2mols = 2.8- 0 = 0.01 cm³/s
30- 0
Concentration 0.4mols = 15- 0 = 0.05 cm³/s
30- 0
Concentration 0.6mols = 38- 0 = 1.27 cm³/s
30- 0
Concentration 0.8mols = 51- 0 = 1.7 cm³/s
30- 0
Concentration 1.0mols = 74- 0 = 2.47 cm³/s
30- 0
Conclusion
My results proved my prediction. I predicted ‘that as the concentration of the sulphuric acid (H+ ions) increases the rate of reaction will also increase.’ My results and graphs proved this. The higher concentrations of sulphuric acid (0.8mols, 1mol) had a faster rate of reaction, compared to the lower concentrations (0.2mols, 0.4mols).
My graphs also proved this as the steeper the curve, the faster the rate of reaction. From my graphs I can conclude that as the concentration of the sulphuric acid increased, the average rate of reaction also increased.
The rate of reaction of concentration 0.2mols was 0.01 cm³/s, 0.4mols was 0.05 cm³/s, 0.6mols was 1.27 cm³/s, 0.8mols was 1.7 cm³/s and finally the rate for 1mol was 2.47 cm³/s. In conclusion i can say that as the concentration increases, the rate increases due to the acid being more concentrated, meaning that there is more particles available, and therefore there are more collisions. The more successful collisions there are, the faster the rate of reaction. This is why the higher concentrations have a faster rate of reaction.
Evaluation
Overall I am pleased with my results and the way I carried out my experiment. I think my results are fairly accurate and reliable, and they proved my prediction. My experiment was carried out well, except for my first gas syringe was broken, as it wouldn’t move, but I was able to replace it and I started my experiment again, so as my results wouldn’t be inaccurate.
I think this experiment was very suitable to require results, so I could see the effects of concentration on the rate of a reaction. The equipment was suitable and generally quite straightforward and easy to use. I was able to repeat each concentration, to make my results more accurate. In general I had no real anomalous results, as I carried out each experiment three times and worked out the average for each one. I worked with a partner to do this experiment and I think this is why my results are so accurate, as one of us watched the stopwatch (time), while the other recorded the amount of hydrogen gas produced. A gas syringe was used to measure the volume accurately to 0.1 cm³ and the magnesium strips were also measured accurately. I used a measuring cylinder to measure the amount of acid accurately to 0.1 cm³ and the stopwatch also measured the time accurately to 0.01seconds. I think this made the results I obtained reliable.
I think if I was to make my results even more accurate I could increase the duration of the experiment to four or five minutes instead of just three. I could also try and use a gas syringe with a larger scale. Instead of a scale of just 100cm³ I could have a scale of about 150cm³. I could also repeat the experiment more times. I would also use a wider range of concentrations. This could make my results more reliable.
I am very pleased with my experiment and my results reflected my prediction.
Bibliography
‘Chemistry Made Clear’ by Gallagher and Ingram