I will take 5 readings and do each reading three times to make sure that they are accurate. I will also work out the average of my results and they will be organised into a table. A graph with line of best fit will be drawn to illustrate my results and to identify the faults within my experiment.
Risk Assessment
I will follow the following rules if I will do all my work safely. The important things are:
Concentrate on my own experiment; do not tamper with equipment that is not part of my experiment.
Wear safety glasses and apron.
Follow directions carefully. Make sure I am using the correct chemicals in correct quantities.
Do not do experiments of my own devising without checking with my teacher.
Write down my observations as soon as I have made them.
Use clean apparatus. Wash up and tidy up after a practical lesson. Put solid waste into bins, not into sinks.
In case of any accidents, a cut or splash of some chemicals, e.g. acid, wash with plenty of cold water and inform my teacher immediately.
All glassware should be handled carefully to ensure no breakages. If any breakage occurs, it should be cleaned and cleared away to avoid any risk of injury.
Wash my hands after the practical work.
Fair Test
My controlled variables in this experiment are the concentration of the acid and the amount of limestone powder that will be added, so I have to measure it carefully in order to obtain the best results and keeping the test fair at the same time. It is important to keep the reactants separate whilst setting up the apparatus so that the starting time of the reaction can be measured accurately. The amount of limestone powder will be carefully measured on the electronic balance before the experiment, for if it is in irregular weight, anomalous results would be obtained; less weight would increase the speed, whereas more weight would decrease the speed.
Volume of the solution will remain constant throughout the experiment – 30mls, though the concentration may vary. Same amount of water must be added in order to keep the acidity constant and I will keep the measuring cylinder on the bench to see whether the level of liquid are accurate or not. I will also stir the solution with a glass rod to make sure that no lumps form to influence the results.
Prediction
There are six factors that will affect the rate of a chemical reaction:
Concentration
Particle size
Pressure
Temperature
Light
Presence of a catalyst
For this experiment, the pressure and light factors will not be considered. Pressure only affects reactions that involve gases and the exposure to light does not affect limestone.
In my experiment I am going to change the concentration of the acid.
Yet, before looking at the factors that can alter the rate of reaction, we must first consider what happens when a reaction takes place.
First of all, the particles of the reacting substances must collide with each other and, secondly, a fixed amount of energy called the activation energy must be reached if the reaction is to take place. If a collision between particles can produce sufficient energy (i.e. if they collide fast enough and in the right direction) a reaction will take place. Not all collisions will result in a reaction.
A reaction is speeded up if the number of suitable collisions is increased.
The higher the concentration of the acid, I predict, the quicker the speed at which the limestone powder is reacted. I think the time of the reaction would increase quite regularly as the amount of water increases in the solution. This is because:
In a dilute solution there are more water particles which get between the acid and powder and so the average distance increase as acid has to travel further to reach the powder. The water particles will slow down the acid particles. When the acid collides with the powder, it reacts if it has enough energy. If it does not have enough energy it will bounce off and not react.
In a more concentrated solution, there are more of acid particles. They can attack the powder more quickly and more of them can react at the same time, as the water is not in the way as much. There will be more of them with enough energy to react when they collide. More collisions mean a faster reaction.
This also explains why the greatest rate of reaction is usually as soon as the reactants are mixed, i.e. they are both at their highest concentrations. As the reaction proceeds the concentrations of the reacting substances decrease and the rate of reactions decreases.
But how will other factors influence my experiment if I am going to do more tests on rates of reaction?
I predict that the powder would take more time to disappear when there is more water. Because:
Using powders instead of a lump means that the surface area is greater, which means a greater area of reactant is exposed and so available for a collision. More collisions mean a faster reaction.
However, if the reactant is a lump, acid particles can collide only with atoms on the outside of the substance as smaller surface area is exposed, thus less collision and slower the reaction. This explains why it takes hundreds of years for the acid rain to corrode the statue as a whole body.
An increase in temperature produces an increase in the rate of reaction. When the mixture is heated, the particles move faster. This has two effects. Since the particles are moving faster, they will travel a greater distance in a given time and so will be involved in more collisions. Also because the particles are moving faster a larger proportion of the collisions will exceed the activation energy and so the rate of reaction increases.
A low temperature slows down the reaction rate, as in a cold solution, the particles collide less often and with less energy.
A catalyst is a substance which can alter the rate of a reaction but remains chemically unchanged at the end of the reaction. Catalyst usually speed up reactions by proving an alternative pathway for the reaction, i.e. one that has a much lower activation energy. More collisions will, therefore, have enough energy for this new pathway.
Also, catalyst provides a surface for the molecules to attach to, thereby increasing their chances of bumping into each other.
Observing
Recording
In my table of results I will put the amount of acid I used, amount of water added; first time, second time and third time for the reaction and the average time.
Units
The units of my results will be:
Amount of acid – mls
Amount of water – mls
Reaction time – seconds
Amount of calcium carbonate - gm
Results table
Analysis
My results show that from 30mls of acid to 25mls of acid, the time for the reaction went up by 3.63 seconds. The other average changes were 4.34 seconds between 25 and 20mls of acid, 5.59 seconds between 20 and 15mls of acid and 8.6 seconds between 15 and 10mls of acid.
Graph
I have plotted my results on a graph. It shows a steady decrease, telling me that the average time for the limestone powder to react decreases as the amount of acid increases. The time went up in a reasonably regular pattern. From this I can conclude that the limestone powders react quicker in a more concentrated acid.
Evidence
I predicted that the higher the concentration of the acid, the quicker the speed at which the limestone powder is reacted. As shown in my graph, the results I have obtained fit the general pattern of my prediction. The experiment shows that as the acid solution becomes more dilute the time for the reaction is longer. From the table of results we can see that as the amount of water went up by 5mls, the time did not go up regularly as I thought it would have. Thereby, my prediction that the reaction would increase quite regularly as the amount of water increases in the solution was inaccurate.
My graph has negative correlation and got a negative gradient which means that the amount of acid is inversely proportional to the average time taken – as the average time increases as the water concentration increases.
Evaluation
The three results at each concentration were reasonably accurate given the limits of the experiment. The set of results that were nearest to each other were with 20mls of acid where the range in time for the reaction was 1.31 seconds. The largest range of results was 4.06 seconds with10mls of acid.
As shown in my graph, there are readings which seem to be out of the line of best fit; these can undermine my prediction. There is an anomalous result which I have circled around. It is probable that the acid is less concentrated than it ought to be as I might have added more water in. The amount of powder may also have been weighted wrongly, more could have been added and thereby there were more reactions and took much longer to react.
Inaccuracy was due to human error, it was inevitable that some of the limestone powder was left out or spilt when I was trying to put them into the beaker, so less limestone powder is added and altered my results.
There were no results that were odd or left out of my calculation and the apparatus and process I used were fine.
Looking at the overall experiment, I have thought of a number of improvements to give me more and better accurate results. For this experiment, the reliable results will only be obtained if I use the equipment with precision and I could improve my results further if I had time to repeat the experiment I could have done each test more times e.g. 5 times for each concentration, but this would have taken too long. I could also have used a gas syringe to measure the amount of gas produced, so to give better results for rates of reaction. I could extend this experiment by investigating the rate of reaction with a different factor as opposed to average time taken. For example, I could investigate the effect of surface area and temperature on rate of reaction - I could use a lump of limestone and then the same weight of powder to see if this changed the rate. I could also heat the acid to see if the reaction went faster when there was an increase in temperature. Finally, to make better investigation with better quality results in the future, I would watch the experiment more carefully and making sure that correct volumes of water would be added.