If the concentration of the acid is increased, the rate of reaction is increased. Increasing the concentration, increases the probability of a collision between reactant particles because there are more of them in the same volume. Examples.
(Source of diagrams internet website for education in science )
Increasing the concentration of acid molecules increases the frequency at which they hit the surface of marble chips to dissolve them (slower to faster).
Increasing the concentration of reactant A or B will increase the chance of collision between them and increase the speed of product formation (slower to faster).
However eventually the rate of the reaction will slow down because the abundance of acid particles will not last for ever as they will be used up in the reaction making it slow down and eventually come to a halt.
Method: -
To carry out the investigation I will be using 5 different concentrations of acid. These will be 0.25M, 0.5M, 1M, 1.5M and 2M. Each solution will have a volume of 20cm3. This is because in my preliminary work I tried using 10cm3 and 20cm3 and 20cm3 gave me better results as the marble chips were all under the acid not partially. I will measure the gas given off every 30 seconds for 4 minutes as this will give me a more accurate graph because I will be able to plot more point s giving it a better line of best fit. I will repeat the experiment 3 times so I can take an average, which will make the results more accurate. To carry out the experiment I set the apparatus up like the diagram below. Then when I put the marble chip into the hydrochloric acid I shook the conical flask at constant rate recording how much carbon dioxide was being produced every 30 seconds as
Calcium Carbonate + Hydrochloric acid = Calcium chloride + Water + Carbon dioxide
Equipment Needed:
-Stop watch for timing the experiment.
-Calcium carbonate 0.7g pieces.
-Measuring cylinder full of water in a bowl of water for measuring the amount of carbon dioxide given off.
-5 different strengths of hydrochloric acid
-Conical flask with rubber stopper to put the acid and marble chip in
-Delivery tube for the carbon dioxide to travel through
-Scales to measure the mass of the marble chip which should be 0.7g
-Safety goggles to protect eyes against acid spillages
Fair Test
To make it a fair test the only variable of the experiment will be the concentration of the acid. Everything else will be kept the same. This includes the mass of the marble chip so you can only have one marble chip otherwise there will be more surface area for the acid to react with making the rate of the reaction faster. The amount of acid as if there is more acid then it will have more energy for the particles to collide and react with each other.
Results
See tables and graphs
Conclusion
According to these results the hypothesis is correct. My evidence shows me that there is a clear relationship between how fast a reaction takes place and the concentration of the acid. As the concentration of acid increases, the rate of reaction increases. This is shown on the 4th graph as a positive correlation and the line of best fit being straight. The straight line through the origin of the graph proves that the two factors are directly proportional. The science that explains this trend is when you have an abundance of acid particles the more collisions there will be and the more likely there will be a reaction between the calcium carbonate and hydrochloric acid. This is clearly shown in the results, as with a concentration of 0.25 molar the rate of reaction is 0.04cm3/sec whereas with an acid with a concentration of 2 molar the rate of reaction is 0.27cm3/sec showing a significant difference. This has occurred because the particles in the stronger acids have more movement energy so there were more collisions and the reaction took place faster. For a chemical reaction to take place, some bonds in the reactants must have broken. The colliding particles must have had enough energy to break these bonds. Only the very fastest moving particles had enough energy to break bonds making it more likely that the stronger the concentration of acid the more likely this would happen. To react, the particles must have collided with enough energy and in the correct orientation for the bonds to be broken. The stronger concentrations of acid will have more energy therefore a faster rate of reaction. The validity of the results was fine as it supported my hypothesis but I would not use the results for any firm conclusion by themselves because they are not this valid you would need something to support them. Graph four shows this because even though the line of best fit is a straight line if I were to draw a line joining all the points together it would b an ‘s’ shape. Eventually the rate of reaction should start to tail off, as there is a maximum rate therefore the conclusion is not strong. There are also gaps in the graph showing that the line of best fit may not be very accurate.
Evaluation
Overall the entire test went well. In the practical, the parts that were most difficult were trying to keep the temperature the same because we did the practical work on more than one day so it was sure to change. This would have affected the experiment because increasing the temperature increases the rate of reaction. At the higher temperature reactant particles move faster and collide more violently. However at the time of the experiment we did not consider taking the experiment so the anomalous result could have occurred because of this. If I were to do this experiment again I would take the temperature using a thermometer.
The anomaly result that I got was on table 3 and you can see this clearly on graph 4, as it does not fit in with the line of best fit. The reason for this result would be that two sets of results were to high making the average higher than it should be. Thus may have occurred because I carried out the experiments on different days when the temperature was different or that the marble chips had a larger surface area making the reaction take place faster because there is more particles of the solid in contact with the acid reactant.
To measure things more accurately I could of used more reliable equipment because the measuring cylinder was not accurate enough. I had to guess at times how much carbon dioxide had been given off but also sometimes it was hard to read the readings as the measuring cylinder was at an angle making it seem more or less than it was. The easiest way to make the results more accurate would be to repeat the experiment more times until you got consistent results.
The reason that not all the repeats are identical is because of accuracy. If everything were a complete fair test then you would expect to find the repeats exactly the same. However the results are similar so you could base quite a firm conclusion but it would be useful to obtain more results making it more accurate as the average will be more precise. The largest range bars are for the 2.0 molar concentrations. This may have occurred because the consistency of shaking the conical flask was not the same making some results have higher rates than others or the accuracy of the results.
To make this practical method more reliable you could again repeat the experiment more times and take more readings for example every 15 seconds opposed to every 30 because this would then enable you to see where it the experiment was going wrong. Also you could extend it for more than 4 minutes because eventually you would expect to find the line curling off because most of the acid particles would be used up making there be less energy for the particles to collide and react.
Extra practical work I could do to back up my conclusion would be to look at the other factors affecting the rate of reactions, which are;
Temperature As the temperature increases in a reaction the rate of the reaction increases because the higher the temperature the faster the particles move and collide increasing the reaction.
Smaller pieces of Marble Breaking up the marble would increase the reaction rate because this increases the total surface area making more particles of the solid have contact with the acid.
Catalyst Using a catalyst increases the rate of a reaction but they are not used up.
