Scientific theory to explain prediction
EXO and ENDOTHERMIC reactions
Chemical reactions involve rearranging the atoms in some molecules to make new molecules. This means breaking the bonds between the atoms in the original molecule and then making some new bonds. The type of reaction I would be looking for to make my prediction correct would be an exothermic reaction. These reactions are accompanied by a temperature rise. They are known as EXOTHERMIC reactions as heat energy is transferred into the surroundings. This happens because new bonds are made. When new bonds are made energy (heat) is released to the surroundings, so bond making is exothermic.
However in an endothermic reaction, energy is taken in from the surroundings. The surrounds then have less energy than they started with so the temperature falls. This is because making and breaking of bonds involves energy. You need to supply energy (heat) to break bonds, so bond breaking is endothermic.
Exothermic
The products are at a lower energy level than the reactants. The drop in level between them is the amount of energy that has been released – which we see as a rise in temperature. The activation energy (Ea) is the energy that must be put in to get the reactant molecules colliding powerfully enough to react. This is the step up in energy level from the reactants.
The energy level then drops down past the original energy of the reactants, down to the level of the products. The amount of activation energy put in is balanced by the amount of energy given out, down to the original level of the reactants. The further drop from the original level of the reactants, down to the level of the products, is the energy released as an exothermic reaction.
Endothermic
The products are at a higher energy level than the reactants. The rise in level between them is the amount of energy that had to be taken in from the surroundings to get them to react. We see this as a drop in temperature. The activation energy here is actually the same as the energy taken in.
Collision theory and activation energy
The particles of the reacting substances must collide with each other for a chemical reaction to take place, and must have the sufficient amount of energy for the to react. The minimum amount of energy needed to cause this reaction is called the activation energy or Ea.
I have chosen to keep the amount of acid constant and change the amount of magnesium turnings during my experiment. Explaining this in terms of collision theory it should become apparent throughout the experiment that there will be a greater amount of energy dissipated into the surroundings when the mess of magnesium is increased. In theory this is because there will be more magnesium particles to collide with and so when there are more collisions a better reaction will take place. This should result in a temperature rise being dissipated as energy into the surroundings when the amount of magnesium is increased.
Trial experiment
In order to keep this investigation fair I am going to do a trial experiment and a method in order to do so. This will help me to make clear of any mistakes I might make in my method and enable me to make changes.
I am going to test both magnesium ribbon and turnings in this trial just to prove the collision theory that the surface area will affect the temperature increase of this reaction.
Apparatus
- Polystyrene cup
- HCL 2 mole
- Magnesium turnings
- Thermometer
- Piece of card
- Weighing scales (A level scales, very accurate)
- Piece of paper (to carry magnesium turnings on from scales)
- Goggles
- Measuring cylinder
Trial method
First begin with taking the starting temperature of the HCL in the Polystyrene cup and draw a table to record results.
- Measure out 25cm3 using the measuring cylinder.
- Measure out 0.1g of magnesium turnings. Do the same for each result until 0.6g
- Pierce a whole in the card and slide thermometer in, this will act as a rest and will make sure heat doesn’t escape into the surroundings.
- Add the 25cm3 of HCL into the polystyrene cup
- Add the magnesium to the HCL.
- Quickly bring down the thermometer in the card to ensure minimal heat loss (most heat given off at the start of reaction)
- Record the temperature increase
**Do the same for magnesium ribbon but measure 0.5cm of it until 3cm and record results**
How to keep this experiment fair
In order to keep this experiment fair it should be taken on the same day, as the room temperature is never completely constant and a slight temperature increase or decrease on a different day could affect my results. I also have to keep the amount of acid constant as the factor I have chosen to change is the mass of magnesium. Another point I have to be sure of is that I don’t dip the thermometer in the HCL when it is reacting with the magnesium, as heat rises and I would not get fair results
Safety
In order to keep this experiment fair I have to follow these safety points
- Take care while using acid in the classroom as the acid can irritate skin.
- Wear goggles to protect eyes from acid.
- The reaction can get very hot and so safety must be taken in order not to get burned.
- Do the experiment in a mature way and take care with the reactants and products I will eventually achieve.
Conclusion from trial
I believe that my two graphs and the tables themselves prove that using magnesium turnings would be better as it has a big surface area and a much bigger temperature increase. I believe my method worked well and I will be using it again for my real experiment. However I will record more results to give me a more spread graph, 0.1 to 1g. It is also quite clear from this when using the turnings that my prediction was correct and that the temperature increased as the amount of magnesium went up.
Analysis
As you can see from the graph and the table my results do prove my prediction to be correct. The table shows us that when the amount of magnesium is increased the temperature goes up, then when I worked out E=mc T it was apparent that when there was an increase in temperature the amount of energy was increasing. We can see this from taking the first and last results
It is also apparent that the delta H goes down as the amount of energy is increased. This is because the delta H shows us that amount of energy in KJmol-1; it takes for the reaction to come to completion. As shown in my results as the amount of energy the products make increases the delta H decreases meaning less energy is needed fro the reaction to come to completion.
Conclusion
My results show that my prediction and scientific theory were correct. I predicted that as the mass of magnesium is increased the temperature would rise. This will show us that the amount of energy being dissipated into the surroundings is increased. This reaction then would have been an exothermic reaction. They are known as EXOTHERMIC reactions as heat energy is transferred into the surroundings. This happens because new bonds are made. When new bonds are made energy (heat) is released to the surroundings, so bond making is exothermic. I also stated that the particles of the reacting substances must collide with each other for a chemical reaction to take place, and must have the sufficient amount of energy for the to react. The minimum amount of energy needed to cause this reaction is called the activation energy or Ea.
Evaluation
As seen in my results the amount of energy went up as the amount of magnesium was increased. I feel that I carried out the experiment in a mature manor and that the experiment as a whole went according to my method. However some errors did occur.
I noticed that when measuring the magnesium that often not all of the magnesium would fall in the cup but would fall on the floor or around the desk. Also some of the magnesium was obviously hitting the acid before others, this would have affected my results as some of the energy would have been lost. I fell a way both of these points could be avoided is by putting the magnesium in the cup first and then adding the acid. Acid, being a liquid, would be a lot easier too pour in the cup and I feel it would make this test fairer.
Another point I found was that the position of the thermometer was not always stationary. This meant that the temperature shown on the thermometer would have been affected. Sometimes it would slip down into the acid. A way in witch this could be avoided is that we could hold the thermometer in place with some sort of grip or vice, as we tried to hold the thermometer during the experiment but it got too hot.
There are also many other ways we could have made this experiment fair.
- We could use more insulation around the cup in order to prevent heat loss into the surroundings. This could be done with something as basic as rapping paper towels around the polystyrene cup.
- The beaker used in our experiment could be bigger; if this were the case then there would be an even bigger surface area and a better spread of results.
- If more acid was used then we would see an even better reaction as there would in terms of collision theory be a better reaction. However this would require more acid and maybe even a waste of acid if there was too much for the reaction.
- Using a burette we could measure the acid more accurately, which would give us a better and more accurate set of results.
- To record the temperature more accurately we could use a digital thermometer. This would give us more precise results. However apparatus like this are very expensive and would be quite difficult to use in an hours lesson.
Further work
There are few different ways in which I could change this experiment in order to get more accurate or a different range of results. I believe that within the experiment itself that if we put the magnesium in the polystyrene cup before adding the acid that it would be more precise. And also less energy in the form of heat would be lost. I believe this would be a good idea without changing the experiments properties.
Another way would be to change the factor I would be looking at from the mass of magnesium to the concentration of acid. In this experiment my prediction would be that as there is a higher concentration of acid there will be more energy dissipated into the surroundings in the form of heat. This is a very similar way of doing the experiment but I believe that it would give different results maybe more accurate as the mass of magnesium would be constant.
I can also find one problem with this and that is there will be a waste of acid. I would need about ten different concentrations, and to make this there would have to be some left over. Also the concentration would be getting too high to deal with in a school laboratory and it wouldn’t be safe.
Again we could look at trying more masses of magnesium, but as seen in the experiment so enough we would have the maximum temperature rise if we went too high and we would not be able to record them. This along with doing more in between masses would be a waste of acid.
