Why should this coursework be entitled “The Crumbling Taj”? The Taj Mahal is a famous beautiful building found near Delhi in India. It is made of marble, which is a polymorphic form of calcium carbonate. Unfortunately it is slowly being damaged by acidic rain. This occurs in two forms: Natural carbonic acid formed when carbon dioxide dissolves in water. Acid rain made of a mixture of sulphurous acid, sulphuric acid and nitric acid. These acids are formed from pollution from the burning of fossil fuels and from the discharged exhaust from road transport.
Although hydrochloric acid is not one of these mentioned, it is convenient to use in this experiment, as it is available in our laboratory. The active ion, H3O+ in all acids is present in hydrochloric acid just as much as it is present in all the other acids mentioned. AIM The aim of my investigation is to design and carry out a series of experiments to discover how the speed of chemical reaction between calcium carbonate (marble) and acid can be altered. I will be looking at the following equation: CaCO3 + 2HCl –> CaCl2 + H2O + CO2 Calcium Carbonate (marble) + Hydrochloric Acid –> CalciumChloride + Water + Carbon Dioxide.
The aim for my investigation is to also find out the best conditions at which the reaction rate will be at its fastest. These conditions are factors that can normally affect reaction rate. They are the variables. Everyday we are concerned about how fast things happen. We want to know how fast we can get to school, how fast a car is travelling or how fast we can run a hundred metres. Chemists are particularly interested in how fast chemical reactions happen. They want to know how quickly steel rusts, how quickly food cooks, and how quickly stone buildings are weathered by acid rain.
Different chemical reactions happen at different rates. Some reactions, like explosions are so fast they are almost instantaneous. For example, when a burning splint is put into a mixture of hydrogen and chlorine, there is a loud bang and hydrogen chloride is produced. H2 (g) + Cl2 (g) –> 2HCl (g) Other reactions, like rusting of steel and weathering of limestone on buildings, happen slowly that it may be years or even centuries before we notice their effects. Most reactants take place at speeds somewhere between those described above. VARIABLE FACTORS There are at least ten factors that affect reaction rate in some way: 1.
Temperature change: temperature is related to the velocity of particles. As temperature rises so the velocity of the particles increases and so collisions become more frequent and successful reaction becomes more likely. 2. Shape of molecule. E. g. an enzyme only operates a chemical reaction, as a catalyst, if its shape is precise and specific for a given substrate. 3. Catalyst: A substance that helps on or speeds up a chemical reaction without itself being changed by the end of the reaction. 4. Pressure in a gas reaction. If particles are forced together, then they are more likely to collide and therefore more likely to react.
5. Turbulence: if particles are stirred together they are more likely to collide and therefore react. 6. Strength of an Acid or Alkali: e. g. pH. When more H+ ions or OH- ions are present then reactions involving them are likely to be faster. 7. Surface area: Where surface area is increased e. g. by powdering lumps of a solid reactant then particles are more likely to collide and react at a faster rate. 8. Electric current: in an electrolysis reaction. If current is increased then more electrons per second will discharge positive ions and more electrons will be released by negative ions as they discharge.
9. Concentration: in a concentrated solution more particles of one reactant will be available in a given volume, to react with the other reactant 10. Light: some reactions depend on the intensity and frequency of light shining on them. Photography and photosynthesis are two reactions dependent on light. This statement explains why reactions between gases and liquids usually happen faster than reactions involving solids. Particles in gases and liquids can mix and collide much more easily than particles in solids. In a solid only the particles on the surface can react.
During a reaction, reactants are being used up and products are forming. So, the amount and the concentration of the reactants fall as the amount and the concentration of the product rise. The reaction rate tells us how fast or slow the reaction is taking place. We can measure reaction rates by measuring how much of a reactant is used up or how much of a product forms in a given time. Reaction rate = Change in amount (or concentration) of substance Time taken Variable Factors that will affect my experiment The variable factors that can affect my investigation are:
Temperature change Concentration Catalysts Surface Area INDEPENDENT VARIABLE The independent variable or otherwise the variable I have chosen to investigate out of these four is Concentration. DEPENDENT VARIABLE The dependent variable means what I have decided to measure or observe. While carrying out the experiment I would need to observe something that would tell me how much marble is being corroded. One way of telling how much marble has been corroded is by reacting the marble with hydrochloric acid and measuring how much gas (carbon dioxide) is being given off.
This is a scientific way of observing the amount of marble that has been corroded. So my dependent variable will be measuring the amount of gas (carbon dioxide) produced when marble is reacted with acid. CONTROL VARIABLES There are many factors in experiments that need to be kept constant or the same in order to make the experiment fair. These factors are known as control variables. For this experiment the control variables will be: The amount of acid should be kept the same for each experiment. The amount of marble should be kept the same for each of the experiments.
The type of acid should be kept the same. The concentration of acid should be kept the same for each experiment. The Surface area of the marble should be kept the same for each of the experiments. HYDROCHLORIC ACID – THE FACTS This is a solution of hydrogen chloride in water; it contains chloride (Cl-) and hydronium (H3O+) ions. The maximum concentration of the solution is 31% HCl is strong monobasic acid (another good reason for it to be used with marble in the reaction). HCl produces salts called chlorides.HCl is a very strong acid and is 85% ionised.
HCl dissociates in Cl- (aq) and H+ ions combine with covalent H2O molecules to form H3O+ (aq) ions in solution HCl releases carbon dioxide from carbonates and hydrogen carbonates. It can be oxidised to chlorine as seen by the following equation: CaCO3 + 2HCl (aq) –> CaCl2 + H2O + CO2 METHODS OF MEASURING REACTION RATES There are several ways that a reactions rate can be measured. Ten of them are mentioned below. 1. Precipitation: This is when the product of the reaction is a precipitate that clouds the solution. E. g. using the black cross experiment with HCl + Na 2 S2O3 (aq).
2. Disappearance of a solid reactant by dissolving: The time taken for marble (calcium carbonate) to completely react with excess HCl acid, measured using different concentrations of acid for each datum value. 3. Change in mass (usually given off): Any reaction that produces a gas can be carried out on a mass balance, and as the gas is released the mass disappearing is easily measured. E. g. HCL + CaCO3 (marble) or magnesium, etc. 4. The volume of gas given off:
This involves the use of a gas syringe or a graduated gas jar or an eudiometer to measure the volume of gas given off. E.g. HCl + CaCO3 5. Titration of samples: Samples are withdrawn and quenched or frozen. The concentrations of reactants or products are measured using standard solutions. 6. Colorimetry: If one of the reacting substances or products has a colour, the intensity of this colour will change during the reaction. A photoelectric device in a colorimeter could follow the intensity. 7. Dilatometry: In some reactions, the volume of a liquid mixture may change during a reaction. That change could be followed using an enclosed apparatus fitted with a capillary tube. It is called a dilatometer. 8.
Measurements of electrical conductivity: If the total number of ions present in a solution varies during a reaction then there will be changes in the electrical conductivity of the solution. A conductance bridge is applied to the solution using AC. This avoids electrolysis of the solution. 9. Polarimetry: Optical isomers or chiral compounds containing asymmetric carbon atoms will rotate the plane of polarisation of polarised light by a specific angle clockwise or anticlockwise. Some chemical reactions such as the hydrolysis of sucrose (not optically active) produce optically active isomers.
A polarimeter can follow the rate of chemical reaction by measuring angles of rotation per minute. 10. By measurements of changes in other physical properties: e. g. refractive index, magnetic effect, viscosity, pH, etc. Techniques use to measure concentration vary with the reaction and the available apparatus. Titration can be done on reactions in solutions. Changes in colour can be indications of concentration change and can be measured in a spectrometer. Density and electrical conductivity may vary with concentration. For gases a pressure change may tend to indicate concentration change.
The rate of reaction of a reaction is usually measured as a function of the concentration of a reactant or product over time. Industrial chemists want to produce materials as cheaply as possible. In order to do so, they choose conductions which; Increase the reaction rate. Use the most economic method. Methods that either reduce reversible or at least increases the net yield in the shortest possible time. One way to speed up reaction is to increase the concentration of reactants. At the same time, the products must be removed as fast as they form to prevent the reverse reaction happening.
This is so in the Haber process for making ammonia. Catalysts are also important in industrial processes. By using a suitable catalyst it is possible to carry out some processes, which would otherwise be impossible. Catalysts do not increase yield but they decrease the time taken to reach the products required. Other processes can be carried out at lower temperatures when a catalyst is used, which makes them more economical. Temperature and pressure are also selected cautiously in the manufacture of most chemicals. The importance of these factors in the industrial processes is well illustrated by the Haber process.
Rates of Reaction How does a chemical reaction occur? Particles in a substance are moving all the time. In liquids or gasses the particles are free to move their positions and particles are colliding all the time. A chemical reaction occurs when the particles of the reactants collide with enough energy to break the bonds between them. The amount of energy needed to make this happen is called activation energy. The activation energy differs for different reactions. If the activation energy is low there are lots of collisions with enough energy to “react” and the reaction takes place quickly.