There are a number of signs that a chemical reaction has taken place. These are:
. Bubbles of gas produced
. Temperature change
. Change of colour e.g precipitation
. Light produced
. Sound produced
. Smell produced
. Kinetic energy (movement)
. A reactant dissolving
. A change in pH
A reaction is always fastest at the beginning because this is the time that the numbers of reacting particles is greatest, and so the number of collisions between the reacting particles is greater. When the particles have reacted they can no longer react with other reactants. As the reaction proceeds the reaction slows down as the concentration of reacting particles decreases.
During a reaction molecules collide. When they collide with the correct orientation they slow down, stop and then fly apart. This happens even if they don’t have the required activation energy to react. In a collision that is unsuccessful the molecules separate and remain unchanged, whereas if a collision is effective the activation energy barrier is crossed and the particles that separate are chemically different from the others.
Activation Energy
The activation energy of a reaction can be thought of as an ‘energy barrier’. The reactant molecules must have sufficient energy to get over this barrier if a reaction is to occur, or there will just be a mixture of unreacted chemicals left. Kinetic stability happens when the activation energy of a reaction is so large that there are virtually no molecules in the reaction mixture with sufficient energy to overcome the energy barrier, and so the reaction effectively does not happen.
The difference between the potential energy of the reactants and the potential energy of the products indicates the enthalpy change of reaction. When the products have a lower potential energy than the reactants, the reaction is exothermic, as the kinetic energy of the system and thus its temperature has increased. When the products have a higher potential energy than the reactants, the reaction is endothermic, as a net input of energy is needed to form in the products. The average kinetic energy of the system drops and so does the temperature.
This is an exothermic reaction it provides the energy itself to keep the reaction going. If it was the other way round it would be an endothermic reaction.
Prediction
I predict that the higher the molarity of the acid, the faster the rate of reaction. The reasons that I made this prediction, are that there are more acid particles in the same volume of water, so there is a greater chance of the acid particles colliding and reacting with the particles on the surface of the marble chips. So the rate of reaction will increase.
Plan of Action
I am going to find out how different acid concentrations affect the rate of reaction by setting up the equipment as shown in the diagram below. I will then measure out five lots of 2.5 grams of small marble chips. I will pour one lot into the glass phial, I will then measure out five different concentrations of acid. I will do this by using a two molar acid and will vary the ratio of water, the combinations will be:
Acid Water
25 ml 0 ml = 2 molar
15 ml 5 ml = 1.6 molar
12.5 ml 12.5 ml = 1.2 molar
10 ml 15 ml = 1 molar
5 ml 10 ml = 0.8 molar
Then I will pour the two molar acid into the phial, put the cork on and start the timer as soon as the cork is in place, so we are able to start timing when gas is being collected and not before as this would make it an invalid result of the first ten seconds. I will repeat this process for each concentration. After each experiment I will rinse out the phial, to make sure the marble chips won’t start reacting before I pour the acid in. I will record the measurement from the syringe every ten seconds by folding the tube over to stop the gas from escaping through, and will empty the syringe of gas after each experiment.
Trial Run
Below is a results table showing our first trial run of our plan of action so I can see if anything needs changing.
These results do follow the pattern predicted so I will not change my plan of action for the real experiment.
Apparatus
. 5 measuring cylinders to measure out the different concentrations
. Water
. 2 molar acid
. syringe to measure the gas produced
. 5 lots of small marble chips each weighing 2.5 grams
. Electronic weighing scales to weigh out the marble chips
. 5 beakers to hold the marble chips
. Rubber tube with a cork attached to capture the gas produced
. A phial to hold the marble chips with the acid whilst reacting
. Stop clock to take readings every 10 seconds
Fair Test
I will make sure it is a fair test by rinsing out the phial after each reaction has taken place so the next lot of marble chips won’t start reacting before the experiment begins. I will use the same bottle of 2 molar acid to make sure I don’t use slightly different concentrations and will repeat the experiment three times to get a good average of the results. Also I will use the same measuring cylinders so I will be measuring to the same nearest ml each time.
Method
. Make sure you have all the equipment stated in apparatus list
. Weigh out five lots of 2.5g of marble chips
. Measure out the five different concentrations
. Pour one lot of marble chips into the phial
. Pour acid in the phial with the chips and place on the cork
. As soon as cork is in place start the stopwatch
. Every 10 seconds record the gas measurement from the syringe
. Rinse out the phial and repeat for each concentration
. Record all results on a table
Results
Average Results
Here is a table showing the average of the three sets of results that I have recorded:
Results Table for Rate Of CO2 Production
Evaluation
Overall I think that my results are reliable but not as accurate as they could be. Reasons for this are that the measuring cylinder for measuring the acid and water was only accurate to 0.5ml and the syringe to measure the production of CO2 gas was only accurate to 1ml, or more with the human eye reading it. The balance to weigh out the small marble chips was very accurate to 0.01g but there was no guarantee that the surface area was the same which will have affected the results.
Starting the stopwatch at exactly the same time for each experiment was impossible and will have varied each time affecting the first 10 second readings. In my theory I mentioned about temperature effecting the rate of a reaction, and the room temperature wasn’t the same each day during the experiment so this will also have added inaccuracy’s.
Due to human error the gas may have leaked out through the tube whilst we were trying to read from the syringe resulting in us reading more then what we should have. With all this taken into account I have concluded that although my results follow the patterns and trends predicted they aren’t very accurate.
If I was to do the experiment again then I would have the same amount of marble chips as well as the same weight as this would make the surface area more accurate. I would also do the experiments all in one day so that the temperature would be roughly the same. Also I would probably use a different method of tracking the mass instead of displacement. I would do this because although displacement is very clear and simple, it can also be very inaccurate and is read through the human eye, whereas mass loss would be done with a machine and would be extremely accurate.
I have found no anomalous results as each graph follows the trends and patterns expected. My graphs clearly illustrate that my theory was correct. The collision theory in particular as this was the factoral that we were changing (concentration). The higher the molarity the more particles there were in the same volume of water so the faster the reaction. As you can see from my graphs that the higher the molarity went the faster the gas was produced.
Conclusion
In conclusion the experiment went very well and I have proved that my theory was correct and that my results are reliable. You can see this from the graphs present.
