Many people are interested to know how to alter the rates of chemical reactions. Fertiliser manufacturers are interested in speeding up the formation of ammonia from nitrogen and hydrogen. Car manufacturers are interested in slowing down the rate at which iron rusts. This shows that rates of reaction is a very important factor within industries today, and in order to demonstrate how a particular factor can alter the speed of a reaction the outcome variable of my investigation to follow is: How a chosen factor affects the rate of reaction of Magnesium and Hydrochloric Acid.
Reaching this outcome will consist of choosing a relevant factor with the assist of background knowledge, applying the factor whilst experimenting, measuring the rate of reaction and concluding the outcome. In order to choose a relevant factor I will initially need to observe the different variables: 1. The concentration of a reactant. The higher the concentration, the more molecules per unit volume are available for the chemical reaction, thus more successful collisions occur and reaction rate increases.
In regard to my investigation: increasing the concentration of hydrochloric acid molecules would increase the frequency or chance at which they would hit the surface of magnesium in order to dissolve them, which would increase the speed of product formation. 2. The temperature of the reactants. Higher temperature results in particles moving faster, successfully colliding more often and an increase in the reaction rate. For example by increasing the temperature, the magnesium and hydrochloric acid molecules will gain kinetic energy which will cause them to move faster.
The increased speed will also increase the chance of collision, hence the rate will increase. 3. The surface area of a reactant. The larger the surface area, the more collisions and the greater the reaction rate. For example smaller pieces of the same mass of magnesium would have a greater surface area compared to larger pieces. Therefore there is more chance that a magnesium particle will hit the solid surface and react. 4. Adding a catalyst to the reaction. Catalysts increase the rate of a reaction by helping to break chemical bonds in reactant molecules by lowering the activation energy.
Particles need less energy to react and the process proceeds more quickly. Alternatively a negative catalyst slows down the rate of a chemical reaction. 5. Stirring the reactants. Stirring brings substances together so they can react and/or mix quickly therefore stirring is an important rate factor. Each of the above mentioned factors do have a form of influence to the rate of reaction, however not all are applicable in accordance to my experiment. Therefore I will rule out each of the factors that are not appropriate.
The first exclusion is stirring the reactants. In terms of measuring the results and fairness, if the reacting mixture is not stirred evenly, then the reactant concentration in solution will become much less near the solid, which will tend to settle out. Therefore this will diminish the reliability and fairness of my results. The second exclusion is adding a catalyst. This is primarily due to the restricted apparatus I am provided with to conduct the experiment where it would be too difficult to setup and sustain.
These are the only relevant exclusions, so this leaves me with concentration, temperature and surface area all of which can be altered to demonstrate an influence. These 3 factors will all have a form of involvement, however I will primarily only concentrate on one, thus my aim: AIM: How temperature affects the rate of reaction of Magnesium and Hydrochloric Acid. Therefore temperature is the independent variable. As an independent variable, it simply means I will need to change the temperature throughout the experiment in order to demonstrate a possible influence.
I have chosen temperature because I believe this will have a powerful relevance to the rate of reaction of magnesium and hydrochloric acid. Also as this is set as an independent variable, I will have independent control over the values I pre-set it to prior to experimentation. The dependant variable is the variable dependant on others to produce a result and also what I am trying to find out about. Therefore the dependent variable is the time of reaction. And the concentration and surface area are set as the control variables.
These will need to be kept constant throughout the experiment in order to ensure a fair test as any unexpected alterations during the experiment may affect the accuracy of my results. BACKGROUND KNOWLEDGE: ‘Rate of reaction’ simply means ‘how fast is the reaction’. Primarily this depends on two factors: 1. The number of collisions per unit time between the reacting species, 2. and the fraction of these collisions that are successful in producing a new molecule. According to the collision theory particles can only react when they collide.
If you heat a substance, the particles gain kinetic energy, therefore move faster and so collide more frequently. However if two or more molecules collide but are not orientated correctly or collide without sufficient energy then no reaction will take place. –> In order for particles to successfully collide and actually break existing bonds to make new a new product, instead of bouncing off each collision, the particles will need to possess enough kinetic energy to cause or initiate a reaction. For this to occur the particles will need to have enough activation energy to break the energy barrier.
Activation energy is the minimum kinetic energy needed for a successful collision and the energy barrier is the level of activation energy needed to start a reaction. Therefore when heated molecules have a greater kinetic energy, a greater fraction of them have the required activation energy to exceed the barrier to react. For example temperate helps to do this as increasing the temperature increases the range of kinetic energies, which in return increases the average kinetic energy and therefore increases the population of particles with more than the activation energy to exceed the energy barrier.
Furthermore an influence of temperature can be seen in the graph below: The graph shows that an increase or decrease in temperature will change the shape of the curve which means fewer or more particles will have the activation energy. PREDICTION: From the background knowledge I have concluded that as you increase or decrease the temperate of a reaction, this will change the movement of the particles and in return influence a change in reaction. Therefore I predict that as I increase the temperate of the magnesium and hydrochloric acid substance, the reaction will also increase. Below is a graph of my predicted result:
The graph shows that the rate changes meaning there isn’t a constant rate throughout the reaction. Also as can be seen from the curve, the reaction is fastest at the start and gradually becomes slower as the reaction proceeds. This shows that the rate of reaction increases at a decreasing rate. And also the curve of the graph goes flat towards the end, this is the point where all reactants have been used up, hence meaning the reaction is complete and the product is formed. In addition as can be seen from my graph, I am aiming to determine the varying rates of reactions, where-as the dependant variable is the time of reaction.
Therefore the rate of reaction can be calculated using its formula: Reaction rate = 1 PRELIMINARY WORK: In order to ensure a fair test is carried out and the best possible variables are used in order to conduct an accurate experiment, I will need to carry out preliminary work. This will consist of: experimentation of concentration, surface area and temperature. To produce reliable results I will need to ensure that only one of the variables is changed at a time and each recording is repeated twice. From this I should be able to calculate an average which will help me in choosing a reasonable criterion for each variable.