Effects of Temperature On the Rate of Reaction When Energy In the Form of Heat Is Given Out of a Reaction It Is an Exothermic Reaction.
Affects of temperature on the rate of reaction
When energy in the form of heat is given out of a reaction it is an exothermic reaction. Exothermic reactions usually need activation energy, the energy to break the bonds in the chemicals and to start the reaction. Once the bonds are broken new stronger bonds are formed. In an exothermic reaction energy in the form so heat is given out to the surrounding when the products are formed. Therefore there is an average rise in temperature; energy is lost which results in a negative ?H.
The outcome variable for this experiment is to conclude what changes can be made to vary the heat energy when magnesium is reacted with sulphuric acid.
Magnesium + Sulphuric Acid ¡ú Magnesium Sulphate + Hydrogen
Mg (s) + H2SO4 (a) ¡ú MgSO4 (a) + H2 (g)
There are many variables which can be changed in some way to effect the reaction, some of which are listed below:-
Volume of Sulphuric Acid
Concentration of Sulphuric Acid
Initial temperature of Sulphuric Acid
Motion of liquid (speed of stirring)
Amount and size of magnesium
Purity of magnesium
The extent of oxidation on the surface of the magnesium
Addition of Catalysts
Size of Container
Temperature of surrounding environment
Pressure at which reaction is conducted
Insulation of the test-tube
Height of thermometer above the base of the test-tube
Explaining variables
In solutions of higher concentration, particles are closer together. They have a greater chance of colliding. Because there are more collisions the reaction rate is greater.
How does temperature affect the rate of a chemical reaction?
When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place. This is collision theory. The two molecules will only react if they have enough energy. By heating the mixture, you will raise the energy levels of the molecules involved in the reaction. Increasing temperature means the molecules move faster. This is kinetic theory.
How do catalysts affect the rate of a reaction?
Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction. This is really because the reaction proceeds by a different pathway when the catalyst is present. Adding extra catalyst will make absolutely no difference.
There are two ways in which catalysts work. When two different molecules bump into each other, they might react to make new chemicals. We usually talk about "collisions" between molecules; it would be much simpler to say that the molecules bumped into each other. How fast a chemical reaction is depends upon how frequently the molecules collide. You have probably been told about the "kinetic theory" which is all about heat and how fast molecules move around. What catalysts are doing when they make a chemical reaction go faster is to increase the chance of molecules colliding. The first method is by "adsorption"; the second method is by the formation of intermediate compounds.
Adsorption This occurs when a molecule sticks onto the surface of a catalyst. It is not the same as absorption. Here is an example: it is possible to use Platinum as a catalyst to make sulphur Trioxide from Sulphur Dioxide and Oxygen. Sulphur Trioxide is very important because it is used to make Sulphuric acid which is needed for car batteries. The molecules of the two gases (Sulphur Dioxide and Oxygen) get adsorbed (stuck onto) the surface of a Platinum catalyst. Because the two molecules are held so close together, it is more likely that they will collide and therefore react with each other. The Sulphur Trioxide easily falls off the catalyst leaving space for more Sulphur Trioxide and Oxygen.
Intermediate Compounds Many catalysts, including all enzymes" work by forming intermediate compounds. What happens is very simple: the chemicals involved in the reaction combine with the catalyst making an intermediate compound, but this new compound is very unstable. When the intermediate compound breaks down it releases the new compounds and the original catalyst.
How does concentration affect the rate of a reaction?
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. Kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
How does surface area affect a chemical reaction?
If one of the reactants is a solid, the surface area of the solid will ...
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How does concentration affect the rate of a reaction?
Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. So this is collision theory again. Kinetic theory is relevant. This is because the molecules in the reaction mixture have a range of energy levels. When collisions occur, they do not always result in a reaction. If the two colliding molecules have sufficient energy they will react.
How does surface area affect a chemical reaction?
If one of the reactants is a solid, the surface area of the solid will affect how fast the reaction goes. This is because the two types of molecule can only bump into each other at the liquid solid interface, i.e. on the surface of the solid. So the larger the surface area of the solid, the faster the reaction will be.
Smaller particles have a bigger surface area than larger particle for the same mass of solid. There is a simple way to visualize this. Take a loaf of bread and cut it into slices. Each time you cut a new slice, you get an extra surface onto which you can spread butter and jam. The thinner you cut the slices, the more slices you get and so the more butter and jam you can put on them. This is "Bread and Butter Theory". By chewing your food you increase the surface area so that digestion can go faster.
What affect does pressure have on the reaction between two gasses?
The atoms or molecules in a gas are very spread out. For the two chemicals to react, there must be collisions between their molecules. By increasing the pressure, we squeeze the molecules together so you will increase the frequency of collisions between them. This is collision theory again.
In a diesel engine, compressing the gaseous mixture of air and diesel also increases the temperature enough to produce combustion. Increasing pressure also results in raising the temperature. It is not enough in a petrol engine to produce combustion, so petrol engines need a spark plug. When the petrol air mixture has been compressed, a spark from the plug ignites the mixture. In both cases the reaction (combustion) is very fast. This is because once the reaction has started, heat is produced and this will make it go even faster.
What effect does insulating the experiment have on the rate of reaction?
Insulating the experiment prevents heat loss therefore it keeps more energy within the experiment having the same effect as raising the temperature of the experiment. At higher temperatures, particles are moving faster, so there are more collisions. Also (and more importantly), the collisions are more energetic. More collisions have an energy greater than the activation energy, so the reaction is faster.
Concentrations
Moles are a way in which concentration can be measured; the higher the concentration the higher the density of atoms in a set volume. The mole is an amount of a substance that contains the same number of particles as there are atoms in 12.00g
of carbon-12. This number of atoms is 6x1023 and it is called the Avogadro constant or number.
Investigation
I am going to investigate the effects of the concentration of the reactants because these will allow me to choose a suitable range of values for the controlled or independent variable. The dependent variable will be the temperature rise. 2cm3 of each concentration of acid, 8 same size boiling tubes, 8 identical thermometers, a test-tube rack, 2.5cm x 1cm strips of magnesium ribbon from the same source, measuring cylinder and a results table will be used.
Prediction
I predict that the higher the concentration of acid will be the faster the reaction will go; therefore a higher temperature will be reached. Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. If we increase the rate of reaction we increase the temperature rise of the reaction; a faster reaction allows more collisions to take place during the same period of time, which means there is a greater chance of more successful reactions when collisions take place. This results in more successful collisions and therefore more energy is released faster, releasing more heat to the surrounding environment.
Increase the concentration of reactants in a solution (Hypothesis)
If you get some really dilute acid and an alkali in solution and imagine all the particles bouncing around at random you can appreciate that for a reaction to occur, the acid particles must hit the alkali particles before anything spectacular can happen. With the acid being very dilute, many of these collisions will be with harmless water molecules, which play no part in the reaction; therefore it may be a while before the appropriate particles collide. However, if you increase the concentration of the acid, there are fewer water molecules to get in the way, so there are more useful collisions per second, so the reaction happens faster.
If gases are used in a reaction, their concentration can be increased by squashing them up (compressing them into a smaller space), so they do not have to go as far in order to meet the particle of a type that will react. Hence, increasing the pressure of gaseous reactants will increase the rate.
Method
) For each concentration of acid 2cm3 should be measured out using a narrow cylinder with accurate gradients, making sure that you look at right-angles to the cylinder so that correct measurements are taken.
2) Pour each concentration of 20cm3 of Sulphuric acid into separate boiling tubes. Each tube must be of the same size and from the same source. Tubes must be washed and dried to prevent impurities from contaminating the experiment.
3) The temperature of the acid must be measured using an accurate thermometer clamped into place, so that its tip would be positioned 10cm3 into the acidic solusion. The readings should be double-checked by different individuals at 900 from the thermometer. The results must be clearly recorded on a results table.
4) Add the measured 2.5 cm strips of standard magnesium ribbon (from the same coil/source) to the different concentrations of acid, having scraped both surfaces with wire wool to remove magnesium oxide and expose the pure magnesium. Each test tube should be stirred to allow reactants to fully come into contact for the same period of time with the same force.
5) Observe the temperature rise while reaction takes place. When highest temperature is reached and temperature begins to descend record the peak temperature that too should be read at 900 and by more than one person.
6) Once the experiment is complete. Repeat all the experiments in the same way for accuracy.
7) From the results averages can be worked out and graphs can be plotted to show trends and patterns or to prove the prediction.
Experiment 1
Concentration of Acid(Moles) Initial Temperature(Degrees) Final Temperature(Degrees) Temperature Difference(Degrees)
0.25 15 26 11
0.5 15 59 44
0.75 17 40.5 23.5
17 43 26
.25 17 48 31
.5 18 51 33
.75 18 52 34
2 16 51.5 35.5
Experiment 2 (Supplementary)
Concentration of Acid(Moles) Initial Temperature(Degrees) Final Temperature(Degrees) Temperature Difference(Degrees)
0.25 23 32 9
0.5 20 38 18
0.75 20 43 23
20 49 29
.25 21 52 31
.5 20 55 35
.75 20 56 36
2 20 56.5 36.5
Averages of both experiments
Concentration of Acid(Moles) Temperature Difference(Degrees)
0.25 10
0.5 18
0.75 23.25
27.5
.25 31
.5 34
.75 35
2 36
Analysis
From the results and graph we can see that an increase in concentration has resulted in an increase in temperature.
Increasing the concentration of the reactants increases the frequency of collisions between the two reactants. If we increase the rate of reaction we increase the temperature rise of the reaction; a faster reaction allows more collisions to take place during the same period of time, which means there is a greater chance of more successful reactions when collisions take place. This results in more successful collisions and therefore more energy is released faster, releasing more heat to the surrounding environment and this is what causes the rise in temperature.
However the temperature increases by smaller quantities as the concentration of acid increases (it seems to be levelling off). Possible reasons for this could be that the hydrogen gases produced at the surface of the Magnesium insulates the magnesium from the acid which slows down the rate of reaction; slowing down the number of successful collisions resulting in less energy being released and therefore less heat is given out to the surroundings.
.¡¤. This factor works against the concentration increase.
An example of this type of reaction could be the addition of Calcium to Concentrated Sulphuric Acid. In which case the reaction proceeds so rapidly that an insulating crust of Calcium Sulphate is formed around the calcium upon contact, preventing the reaction from progressing.
If a dilute acid is used an explosion takes place. But in the case of very concentrated acid, the calcium reacts rapidly with the acid and prevents any further reactions from taking place as the acid can not come in contact with the calcium
Another explanation could be that some of the heat is used back up in the reaction to increase the rate of reaction. The more heat produced the faster the rate of reaction; the faster the rate of reaction the more energy needed to brake the bonds between hydrogen and sulphur so that the next stage of reaction can take place. The energy needed for this could be taken from the energy being released when the bonds brake instead of it being taken from the surroundings, as this would be slower.
It could also be that the heat dissipates at a faster rate to the environment because Heat energy is easily lost as the tendency of systems is to dissipate energy and hot systems tend to cool. Heat can be transferred in three ways.
Conduction of heat occurs mainly in solids. This process is where vibrating particles pass their extra vibration energy to neighbouring particles.
Convection of heat only occurs in liquids and gasses. Convection occurs when the more energetic particles move from the hotter region to the cooler region taking their energy with them.
Radiation of heat can also be called infer-red radiation. Heat radiation can travel through a vacuum. This process is different from the other two it travels in straight lines and at the speed of light. This is the only way heat can reach us from the sun. Heat radiation travels through transparent media like air, glass, and water. No particles are involved in this process it is the transfer of heat energy purely by waves.
I can see that my results support the theory I put forward in my prediction and therefore my prediction supports my findings.
Conclusion
Temperature difference (heat produced by experiment) increases as the concentration of the acid is increased. As the temperature difference begins to reach a threshold very little variation takes place; Even though there is still a temperature increase it is not as rapid.
If the concentration is increased the rate of reaction is increased which also results in more of the product being formed at a faster rate. Therefore releasing more energy in the form of heat.
The increase in the temperature rise may be slowing down as more heat is used up or lost to the environment at a faster rate. It may also be that the surface area of the Mg is limiting the rate of reaction. If only a certain area of the Mg is exposed only a limited amount of acid will come in contact with it in order to react. No matter how much acid or hoe concentrated it is there will be a limit to how many acid molecules can react with ¡°get at the¡± Mg ribbon. Therefore magnesium filings could have been used to give a greater surface area.
Evaluation
My method was accurate and I feel that the experiments were carried out well. Instructions were followed carefully and time was taken to ensure that everything went well. The results are reliable and accurate as they were always double-checked by another observer before they were recorded. Steps in the method such as making sure readings were read at 90¡ã to the thermometer or measuring cylinder prevented miss readings and allowed further dependable results to be obtained.
Each test-tube was washed and dried to prevent contamination from earlier use.
All acidic solutions were acquired from the same source but may have been contaminated before as other classes before have used them. Concentrations may not have been as accurate and may need testing
All the experiments were carried out on the same day at the same time so that the environment would affect each experiment in the same way if it had any affect on the experiment.
There was an anomalous result was recorded during the first experiment. Keen observations have explained the occurrence of this result. In this experiment the clamp used to hold the thermometer had no rubber insulation, and therefore it did not have any grip against the glass. During the experiment the thermometer slipped giving us a false reading that was of the temperature of the solution at a different position in the solution. This fault was emended before the experiment was repeated. The fault was not an error in the experiment but the result of an inadequate clamp. In future experiments, this could be solved by making sure only suitable clamps were used; however this would not be practical as eight clamps are needed at any one time and in a class of 30 and therefore would cost the science department too much money. This anomalous result did not prevent us from gaining the results needed as a second set of experiments were carried out so that averages could be taken and variation could be limited. When drawing the graph the anomalous result was not included
Graphs were draw on the computer, as they were more accurate and were more consistent at giving lines of best fit.
The line of best fit was a smooth curve and was very close to each point many of which it ran through, showing that data was consistent and reliable. The pattern suggests that evidence is exact. This supports my theory further as we know that results were not indefinite but are persistent which is shown through the repeat of the whole experiment.
Further Improvements
@ Use pure Magnesium filings to increase surface area and increase the rate of reaction, which should give grater variation between concentrations.
@ Make sure non-oxidised magnesium strips are used to give more accurate results.
@ Use wooden clamps with better grip to prevent thermometers from slipping.
@ Use more accurate thermometer so that more accurate readings can be read.
@ Make sure that the concentrations of acid were as they stated so that we know that the results were as accurate as we thought.
@ Insulate the test-tube which would prevent heat from being dissipated to the surrounding and allowing the thermometer to get a more accurate reading of the heat produced by the reaction.
@ Control the temperature of the surrounding by encasing each experiment to prevent any changes in temperature at the start of the reaction to the end of the reaction from affecting the experiment.
Extensions to the investigation
Computers could be used to carry out data logs of the temperature during the experiment to give a more reliable and more accurate account of the experiments and to help distinguish patterns between concentration, heat produced and rate of reaction throughout each reaction.
More concentrations of acid could be used to see if they also have similar affects on the temperature rise.
Different substances could be reacted to see if they behave similarly.
More repeats could be carried out to qualify the findings. These could be done under different conditions to see how the reactions react under a colder or hotter environment.
