A rate of reaction is measuring how fast two substances can react to form two new substances in a set period of time. The measurement of the reaction can take place by finding out how much of a new substance is formed. In most cases, and as in my experiment, the volume of gas produced can be recorded. In other experiments the mass of a solid or the volume of a liquid can be found. A reaction takes place because bonds are broken and new ones are formed. In my investigation I am reacting hydrochloric acid with magnesium. This will form magnesium chloride and hydrogen.
The formula is: HYDROCHLORIC ACID + MAGNESIUM MAGNESIUM CHLORIDE + HYDROGEN The balanced symbol equation is: 2HCl (aq) + Mg (s) MgCl2 (aq) + H2 (g) This means that the hydrogen bond with the chlorine is displaced by magnesium, therefore forming magnesium chloride. This proves that magnesium is higher in the reactivity series than hydrogen. Knowing that the reaction takes place through the displacement of bonds between magnesium and hydrogen one can prove another vital point. That is, that the hydrochloric and magnesium atoms must come in contact for the reaction to take place.
Therefore, the rate of reaction can speed up or slow down depending on how many times hydrochloric acid particles and magnesium atoms collide. So, if one can control the amount of hydrochloric acid particles colliding with magnesium atoms you can, in effect control the rate at which the two substances form. As a brief note, I would like to explain 3 terms used during this investigation. Activation energy: this is the amount of energy needed to break bonds. In our case it is the amount of energy required to break the bond between hydrogen and chlorine in the hydrochloric acid particle.
Without this energy the bonds will stay together and will not break, meaning that no reaction will take place. The collision theory: this is a theory that states when there are more chances of more particles colliding with each other the rate of reaction increases. The kinetic theory: this states that when the particles in a solution receive more energy they transfer it into more kinetic energy. This means that the particles travel faster and collide with each other with a greater force. Therefore, one must research into the variables that can affect the number of times these two atoms meet.
I have found 6 variables: Temperature of the acid ‘When two chemicals react, their molecules have to collide with each other with sufficient energy for the reaction to take place’ 1. This is activation energy, the energy needed to break bonds between atoms. Therefore, if you can give them more energy they will move faster and therefore collide more often thus increasing the chance that there will be sufficient energy to break these bonds. So therefore, there are more successful collisions.
A way in which you can increase the amount of energy is by heating the solution. If more kinetic energy is given to the atoms they will move faster and will collide more often. And to give the atoms more kinetic energy you can firstly supply it with more heat energy. This heat energy will then be converted into kinetic energy and the atoms will collide more often. So if you increase the temperature you increase the amount of kinetic energy and therefore, increase the rate of reaction. The collision theory comes into play concerning the speed.
When the atoms gain more kinetic energy they move around faster, and therefore more atoms collide more often. Concentration of the acid If you increase the concentration of the acid you increase the number of hydrochloric acid particles in the solution. Therefore, the magnesium atoms have more hydrochloric acid particles to collide with and the rate of reaction increases. This is again related to the collision theory. Also, in affect you are increasing the surface area of all the hydrochloric acid particles, and therefore there are more collisions. Surface area of the magnesium
Increasing the surface area is related to the concentration as you are exposing more magnesium atoms to hydrochloric acid particles therefore increasing the chance of collision. Catalyst A catalyst is a chemical intended to increase the rate of a reaction. A catalyst works by decreasing the activation energy. This means that particles have to collide with less force for there to be a successful reaction, therefore increasing the chances of successful reactions, and increasing the rate of reaction. It is unchanged after the end of a reaction and can be used many times during the period of the reaction
Volume of acid Increasing the volume of acid is again related to the collision theory as increasing it means that there are more hydrochloric acid particles to collide with magnesium atoms and therefore increasing the rate of reaction. Pressure of the container By increasing the pressure, you squeeze the molecules together so you will increase the frequency of collisions between them. This is collision theory again. However, this will not be relevant in my experiment due to the fact that I am using a liquid and the pressure of a liquid cannot be increased.
TimeThe longer the magnesium is left in the Hydrochloric acid the longer it has for the reaction to finish, so therefore there will be a greater yield of hydrogen. Out of these 7 variables I have decided to investigate the temperature. I feel that this will give me the most interesting set of results and there are many different theories and explanations for what will happen during the reaction. Another less significant fact is that the reaction between hydrochloric acid and magnesium is exothermic (i. e. it gives out heat) which means the temperature of the solution will increase.
This is because when bonds are broken (in our case hydrogen and chlorine) they use up energy, and when new ones are formed they they give out energy. In the experiment there has to be more energy required to make new bonds than to break old ones as energy is given out, in the form of heat. If we study the energy level graph below you can see that more energy is given out than is used up. We can see this as the energy levels in the solution, and I must stress that it is in the solutions and not outside, decrease. This means that energy is given out. In the graph we see four main points, A, B, C, and D.
The energy level is steady as the magnesium has not been inserted yet. Then we see the energy level dramatically increase (B). This is the activation energy, and the energy needed to break the bonds of the hydrochloric acid particles. After that there is a decrease (D). This is the energy being given out as new bonds are formed between magnesium and chlorine. And then there is a settled energy level at the end. We can see from this that the energy level at the end is less than at the beginning of the reaction. The decrease is shown by A. This means that more energy is lost than is gained.
Therefore, proving the experiment to be exothermic. C is the energy left over after the reaction is complete. It is obvious to see that it is less than the amount of energy to begin with, so energy must have been lost. To show the effect of a catalyst on activation energy a dotted line is drawn to show how much energy is required if a catalyst was added. As is obvious by the graph the activation energy decreases by a considerable amount. PREDICTION AND HYPOTHESIS I predict (using information earlier in this section) that as you increase the temperature the rate of reaction will increase.
This is because the heat energy, which is supplied to the atoms, is converted into kinetic energy and therefore the atoms move and collide with each other causing reactions. If you add more heat energy (i. e. increasing the temperature) then the atoms will have more kinetic energy and therefore move faster. If they are moving faster the rate of reaction increases because the atoms will collide more often and more and the reaction will speed up. This also means that there will be more successful collisions, as the atoms are colliding with a greater force.
Another prediction that I have thought of is that even though an increase in temperature will affect the rate at which the reaction takes place, but at the end of it the same amount of hydrogen will be formed. This is because no matter which temperature the solution is at the same amount of hydrochloric acid and magnesium is being used. The only difference is that at a higher temperature this maximum amount of hydrogen that can be formed, from the amount of hydrochloric acid and magnesium added, will be obtained quicker. The graph below shows this.
As you can see line x is the maximum amount of hydrogen that is formed as there is no more magnesium to react with the hydrochloric acid. This is the end point of the reaction and at a higher temperature this point is achieved faster as the rate of reaction is quicker. This graph illustrates my prediction and hypothesis number 2. My 3rd prediction, which is mentioned in the corresponding hypothesis, is that the temperature of the solution will increase, as it is an exothermic reaction. This means that will give out heat during the reaction. I will therefore measure the temperature of the solution to prove this.