Investigation of energy changes in a displacement reaction Introduction The theory that I am looking at is that: ‘A more reactive metal will displace a less reactive metal from a compound’ The reaction that I will be looking at is zinc replacing copper. This happens, as zinc is higher in the reactivity series than copper. The ionic equation is shown below: Zn(s) + Cu2+(aq) i?? Cu(s) + Zn2+(aq) With any reaction with a more reactive metal, displacement occurs. Plan To make a prediction for this investigation, certain aspects of energy changes need to be looked into first: Displacement reactions
Displacement reactions are type of redox reaction. These are reactions where a more reactive metal will replace a less reactive metal in a salt. E. g. Zinc + Lead Nitrate ? Zinc Nitrate + Lead As zinc is more reactive, it replaces lead in the salt Oxidation-reduction reactions, redox reactions, are an important part of this experiment. In an oxidation reaction, a product will gain oxygen ions and will loose electrons. In a reduction reaction, a product will loose oxygen ions and gain electrons. In the following example, copper reacts with oxygen ions to form copper oxide. 2Cu + O2 ? 2CuO.
The copper atoms are converted into copper ions Cu2+ in the reaction and the oxygen is converted into oxygen ions, O2-. The copper has lost electrons: Cu ? Cu2+ + 2 electrons The oxygen ions have gained electrons: O2 + 4 electrons ? 2O2- This method occurs with all oxidation-reduction reactions. Another important aspect of these experiments is that they are exothermic. An exothermic reaction gives out heat as a source of energy. A reaction that is exothermic uses less energy to break the old bonds than is emitted to create the new bonds, therefore heat is given off. The higher the energy given off, the higher the heat will be.
The recording of heat can show the increase in energy. The energy change can be shown as: The main aim of this experiment is to find the energy changes in a displacement reaction. To find the energy changes, the following equation can be used: Heat = mass of solution x 4. 2 x temperature rise This will give the amount of energy released in the experiment. The equation is repeated for each mass of zinc tested. As the amount of zinc increases, it will reach a point where it will no longer be able to react with the copper sulphate and displace the copper. Therefore, the graph below shows expected results:
For my preliminary work, I can work out what the maximum amount of magnesium is that I can use before no more dissolves in the lead nitrate. My calculations are below: Magnesium + Lead Nitrate ? Magnesium Nitrate Preliminary work Method First of all, 50cm3 of lead nitrate is measured out into a 100cm3 measuring cylinder. This is then poured into a polystyrene cup.
Chosen masses of 0. 1, 0. 2 and 0. 3 of magnesium are measured out on some electronic scales. The starting temperature is measured using a mercury thermometer. One of the masses is added and the solution is stirred. It is continually stirred until the temperature stops going up. This final temperature is then noted. This method is repeated for the other masses and then they are all repeated again to have 2 sets of results. These results are shown below: Mass (g) Initial Temperature (? C) Final Temperature (? C) Temperature Change (? C).
Conclusion During this preliminary work, I have found that the temperature rose when the mass of magnesium rose. However, the results are inaccurate for many reasons. I can improve on the main experiment by: Placing the cup in a beaker sot hat it is insulated. When the preliminary experiment was done, the cup was held in the hand so heat would have escaped easier. Use the same person for stirring. In the preliminary work, different people stirred different masses. Different vigour used causes different reaction speed and results. A 50cm3 cylinder is used instead of a 100cm3.
This makes the volume of lead nitrate less accurate than using a 50cm3 cylinder, as there is a smaller error. The calculations for the main experiment is shown below: Prediction Now that I know the maximum mass, I can plan my experiment and make a prediction. I predict that the reaction will be exothermic and that there will be more heat when more mass of magnesium is put in.
I also think that the amount of heat energy produced will be directly proportional to the amount of solid added. I can predict this as the results in my preliminary follow this pattern. I also predict that the maximum amount of zinc added will be 1. 6g and that any readings after this will not have increased temperature. I can predict this as the calculation proves that the value is 1. 6g. Method Apparatus Polystyrene Cup Beaker Mercury Thermometer Zinc (Chosen Masses) Copper Sulphate (0. 5M, 50cm3 at a time) Measuring cylinder Electronic weighing scales Spatula Goggles.
Lab coat Diagram Method Lab coat and goggles are put on Polystyrene Cup is placed in the glass beaker 50cm3 of copper sulphate is measured out and poured into the cup. The temperature of the copper sulphate is then taken and noted. Zinc is measured out on electronic weighing scales and is added to copper sulphate The copper sulphate and zinc are then stirred. When the reacting has stopped, the temperature is noted. This process is repeated for the values every 0. 25g from 0. 25g to 3. 0g. My values have gone past the maximum value to show the graph straightening out. Variables.