Concentration Of The Acid The Reaction Rate example essay topic

Rates of Reaction BACKGROUND INFORMATION What affects the rate of reaction? 1) The surface area of the magnesium. 2) The temperature of the reaction. 3) Concentration of the hydrochloric acid. 4) Presence of a catalyst.

In the experiment we use hydrochloric acid which reacts with the magnesium to form magnesium chloride. The hydrogen ions give hydrochloric acid its acidic properties, so that all solutions of hydrogen chloride and water have a sour taste; corrode active metals, forming metal chlorides and hydrogen; turn litmus red; neutralise alkalis; and react with salts of weak acids, forming chlorides and the weak acids. Magnesium, symbol Mg, silvery white metallic element that is. In group 2 (or IIa) of the periodic table, magnesium is one of the alkaline earth metals. The atomic number of magnesium is 12. Magnesium (s) + Hydrochloric acid (aq) = Magnesium Chloride (aq) + Hydrogen (g) Mg + 2 HCl = Mg Cl 2+ H 2 In the reaction when the magnesium hits the acid when dropped in, it fishes and then disappears giving of hydrogen as it fishes and it leaves behind a solution of hydrogen chloride.

The activation energy of a particle is increased with heat. The particles which have to have the activation energy are those particles which are moving, in the case of magnesium and hydrochloric acid, it is the hydrochloric acid particles which have to have the activation energy because they are the ones that are moving and bombarding the magnesium particles to produce magnesium chloride. The rate at which all reactions happen are different. An example of a fast reaction is an explosion, and an example of a slow reaction is rusting. In any reaction, reactants chemical reactions (R) products. We can measure reactions in two ways: 1) Continuous: - Start the experiment and watch it happen; you can use a computer "logging" system to monitor it. i.e. Watching a colour fade or increase.

2) Discontinuous: - Do the experiments and take readings / samples from the experiment at different times, then analyse the readings / samples to see how many reactants and products are used up / produced. Reaction rate = amount of reactant used up time taken If the amount used up is the same each time then the only thing that changes is the time taken. so, reaction rate u 1 time taken. rate = Time taken. Where K is the constant for the reaction. For particles to react: -a) They have to collide with each other. b) They need a certain amount of energy to break down the bonds of the particles and form new ones.

This energy is called the "Activation Energy" or Ea. When we increase the temperature we give the particles more energy which: 1) Makes them move faster which In turn makes them collide with each other more often. 2) Increases the average amount of energy particles have so more particles have the "activation energy " Both of these changes make the rate of reaction go up so we see a decrease in the amount of time taken for the reaction and an increase in time taken. = 1 Time taken reflects the rate of reaction.

Because temperature has an effect on both the speeds at which the particles react and the activation energy they have a greater effect on the rate of reaction than other changes. A change in concentration is a change in the number of particles in a given volume. If we increase the volume: -a) The particles are more crowded so they collide more often. b) Although the average amount of energy possessed by a particle does not change, there are more particles with each amount of energy; - more particles with the activation energy. a) is a major effect which effects the rate, but b) is a minor effect which effects the rate very slightly. In this experiment we are not concerned with whether the reaction is exothermic or endothermic because we are concerned with the activation energy needed to start and continue the reaction. PREDICTIONS I predict that as we increase the temperature the rate of reaction will increase.

If we increase the temperature by 100 C the rate of reaction will double. I predict that if we increase the concentration of the acid the reaction rate will increase. If the concentration of the acid doubles, the rate of the reaction will also double. LINKING PREDICTION TO THEORY Reaction Rate and Temperature. The collision theory describes how the rate of reaction increases as the temperature increases.

This theory states that as the temperature rises, more energy is given to the particles so their speed increases, this increases the number of collisions per unit of time. This increase in collisions increases the rate of reaction. The collision theory explains how the rate of reaction increases, but it does not explain by how much or by how fast the rate increases. The Kinetic energy of particle is proportional to its absolute (Kelvin) temperature. 1/2 mv 2 u But the mass of the particles remains constant so we can eliminate that part of the equation so; Th V 2 uT Therefore we can fit this into a formula: V 21/V 22 = T 1/T 2 If we substitute the temperature into the formula we can work out the average speed of the formula: V 21/V 22 = 310/300 V 1 = "O 310/300 V 2 = "O 1.033 V 2 = 1.016 V 2 However if we look at this it is only 1.016 times greater than the speed at 300 K, in other words we can see that it has only increased by 1.6%. The frequency of the collisions depends on the speed of the particles, this simple collision theory only accounts for the 1.6% increase in the rate, but in practice the reaction rate roughly doubles in a 10 K rise, so this simple theory cannot account for an 100% increase in the reaction rate.

During a chemical reaction the particles have to collide with enough energy to first break the bonds and then to form the new bonds and the rearranged electrons, so it is "safe" to assume that some of the particles do not have enough energy to react when they collide. The minimum amount of energy that is needed to break down the bonds is called the activation energy (EA). If the activation energy is high only a small amount of particles will have enough energy to react so the reaction rate would be very small, however, if the activation energy is very low the number of particles with that amount of energy will be high so the reaction rate would be higher. An example of a low EA would be in explosives when they need only a small input of energy to start their exceedingly exothermic reactions. In gases the energy of the particles is mainly kinetic, however in a solid of a given mass this amount of energy is determined by their velocities. This graph below shows how the energies of particles are distributed.

This graph is basically a histogram showing the number of particles with that amount of energy. The area underneath the curve is proportional to the total number of particles. The number of particles with EA is proportional to the total area underneath the curve. The fraction of particles with EA is given by the ratio: Crosshatched area under the curve total area under curve Using the probability theory and the kinetic theory of gases, equations were derived for the distribution of kinetic energy amongst particles. From these equations the fractions of particles with an energy EA J mole-1 is represented by the equation: e -Ea / RT where R = the gas constant (8.3 J K-1 mole -1) T = absolute temperature.

This suggests that at a given temperature, T, The reaction rate u e -Ea / RT If we use k as the rate constant, as a measure of the reaction rate we can put this into the equation also. ku e -Ea / Ruth k = A e -Ea / RT The last expression is called the Arrhenius equation because it was developed by Sante Arrhenius in 1889. In this equation A can be determined by the total numbers of collisions per unit time and the orientation of the molecules when the collide, whilst e -Ea / RT is determined by the fraction of molecules with sufficient amounts of energy to react. Putting the probability theory and the kinetic theory together this now gives us a statement which accounts for the 100% increase in the rate of reaction in a 10 K rise. Reaction Rate and Concentration. The reaction rate increases when the concentration of the acid increases because: If you increase the concentration of the acid you are introducing more particles into the reaction which will in turn produce a faster reaction because there will be more collisions between the particles which is what increases the reaction rate. METHOD.

To get the amount of magnesium and the amount of hydrochloric acid to use in the reaction, we have to use an excess of acid so that all of the magnesium disappears. Mg + 2 HCl = Mg Cl 2+ H 21 mole 2 moles 1 mole 1 mole So, we can say that one mole of magnesium reacts with 2 moles of hydrochloric acid. If we use 1 mole of magnesium and 2 moles of hydro chlor ic acid we will get a huge amount of gas, too much for us to measure. We would get 24,000 cm 3 of hydrogen produced where we only want 100 cm 3 of hydrogen produced. So to get the formula for the amount of moles that we have to use the formula: Moles = mass of sample 100 = 0.004 moles. volume with 1 mole 24,000 To get the maximum mass we can use: Mass = moles x RAM. = 0.004 x 24 = 0.0096 g So, this is the maximum amount of magnesium we can use.

To the nearest 0.01 of gram = 0.01. This is the maximum amount of magnesium we can use. Because the reaction reacts one mole of magnesium to two moles of hydrochloric acid we have to make sure that even with the lowest concentration of acid we still have an excess of acid. The acid that we were using was 2 moles per dm 2 which means that it is 0.2 moles per 100 cm 2 of acid.

We need to make the reaction work to have double the amount of magnesium. The maximum number of moles that the magnesium needed was 0.004 moles so the amount of acid that we needed was double that so that equals 0.008 moles. As you can see from the table below we have the acid in excess throughout the experiment. Amount of HCl (cm 3) Amount of H 2 O (cm 3) Moles of acid. 100 00.2 75 25 0.15 50 50 0.1 25 75 0.05 The reason why we used 0.01 g of magnesium was because it was therefore easy to measure because there was not too much, or too little.

Therefore we had no problem with too much gas. Apparatus This is the apparatus we used to measure the amount of H 2 that was produced in the reactions. We measured the amount of gas that was given of every two seconds to get a good set of results. We used this apparatus with the reaction changing the concentration, and then the temperature.

To accurately measure the amount of gas given of we used a pen and marked on the gas syringe at the time intervals. This is the apparatus we used to measure how long it took for the magnesium to totally disappear. We used this apparatus in both of the experiments, changing the temperature and the concentration of the acid to water. Temperature. When we did the experiment changing temperature we used both of the sets of apparatus.

To get a fair reaction we had to keep the amount of magnesium the same and the concentration of the acid. In the experiment we used 0.1 g of magnesium and the concentration of the acid was 50 cm 3 of acid to 50 cm 3 of water. This is because if we used 100 cm 3 of acid the reaction would be too fast. Still we had an excess amount of acid, so one mole of magnesium can react with two moles of HCl. Concentration.

When we did the reaction changing the concentration we changed the concentration until we had just enough for 1 mole of magnesium to react with two of HCl. To get a fair reaction we had to keep the amount of magnesium the same and the temperature. We used 0.1 g of magnesium. RESULTSTemperatureFrom this graph you can see that if we do increase the temperature the rate of reaction also increases, but it does not show that if you increase the temperature the rate of reaction doubles. This graph shows that there is an increase in the rate of reaction as the temperature increases. This shows a curve, mainly because our results were inaccurate in a number of ways.

This is because the concentration is changed during the experiment because at high temperatures the acid around the magnesium is diluted. If this experiment was accurate it would be also a curve but if you made it into 1/time the result would be a straight line showing a clear relationship. Even though I changed it to 1/time it still does not show a clear relationship because of the factors mentioned in the conclusion. Concentration This graph shows an increase in the amount of gas given off and the speed at which it is given off. This graph also does not show the rate increase, it just shows how it increases with a change in concentration. This graph shows that if you increase the concentration of the molar solution of the acid the time in which the Mg takes to disappear becomes a lot slower.

This does not show the rate at which this happens, the graph of rate vs. conc. would show a straight line. This shows a straight line, thus proving that there is a relationship between the time it takes the magnesium to disappear and the concentration of the acid. If we take a gradient of it, it would show the rate at which the reaction was happening. Because this shows a straight line we can say that it is a second order reaction. This graph shows a nearly straight line which shows that there is a relationship between the temperature and the rate of reaction, as the gradient shows the rate of reaction. If you look at this graph it comes out to show that if you increase the temperature by 100 C the gradient of the line is doubled.

This shows that rate u temp. This graph shows that if you increase the molar concentration of the acid, you will increase the rate of reaction. From this you can see from the gradient, that if you double the molar concentration of the acid the rate of reaction will double because the gradient is a way of showing the rate of reaction. If you compare the quantitative observations to see which the faster reaction is you can see that after 10 seconds: Temp. 2 10 20 3040 50 Amount of H 2 produced after 10's 7.5 16 25 5457 83 Even though there is a greater increase in the amount of H 2 given off in each of the different reactions you can see that there is a change in the amount given off, but between the temperatures 30 and 400 C there is not much of a change, this could be because of our human error, there should be a big change in the amount given off. Molar conc.

0.5 1 1.5 2 Amount of H 2 produced after 10's 6 25 60 90 This table shows a nice spread of results throughout the range of concentration. It clearly shows that the reaction is at different stages so is therefore producing different amounts of H 2. This shows also that the reaction is affected by the concentration of the acid. CONCLUSIONS I conclude that if you increase the temperature by 10 oC the rate of reaction would double, this is because of using the kinetic theory and the probability theory. Even though our results did not accurately prove this, the theory that backs it up is sufficient. the kinetic theory explains that if you provide the particles with a greater amount of kinetic energy they will collide more often, therefore there will be a greater amount of collisions per unit time.

The probability theory explains that there is only a number of particles within the reaction with the amount of Ea to react, so if you increase the amount of kinetic energy there will be more particles with that amount of Ea to react, so this will also increase the reaction rate. If you double the concentration of the acid the reaction rate would also double, this is because there are more particles in the solution which would increase the likelihood that they would hit the magnesium so the reaction rate would increase. The graph gives us a good device to prove that if you double the concentration the rate would also double. If you increase the number of particles in the solution it is more likely that they will collide more often.

There should be more H 2 given off if we compare it across the range of temperatures because the reaction is going quicker and so more H 2 is given of fin that amount of time. There is more H 2 given off if you compare it to the range of concentrations that you are using, this shows that the reaction is at different stages and so is therefore producing different amounts of H 2. Also our results were not accurate but this could be because of a number of reasons. There our many reasons why our results did not prove this point accurately. -At high temperatures the acid around the magnesium starts to starts to dilute quickly, so if you do not swirl the reaction the magnesium would be reacting with the acid at a lower concentration which would alter the results.

- Heating the acid might allow H Cl to be given off, therefore also making the acid more dilute which would also affect the results. - When the reaction takes place bubbles of H 2 are given off which might stay around the magnesium which therefore reduces the surface area of the magnesium and so the acid can not react properly with it so this affects the results. To get more accurate results, we could have heated the acid to a lower temperature to stop a large amount of H Cl being given off. The other main thing that could have helped us to get more accurate results is we cold have swirled the reaction throughout it to stop the diluting of the acid and the bubbles of H 2 being given off. If I had time I could have done the reactions a few more times to get a better set of results. This would have helped my graphs to show better readings.