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Factors affecting the rate of a chemical reaction


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From University of the East

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Factors affecting the rate of a chemical reaction

  2. 2. Aim : To study the factors that affects the rate of a chemical reaction Discussion : Although a balanced chemical equation for a reaction describes the quantitative relationships between the amounts of reactants present and the amounts of products that can be formed, it gives us no information about whether or how fast a given reaction will occur. This information is obtained by studying the chemical kinetics of a reaction, which depend on various factors: reactant concentrations, temperature, physical states and surface areas of reactants, and solvent and catalyst properties if either are present. By studying the kinetics of a reaction, chemists gain insights into how to control reaction conditions to achieve a desired outcome. Two substances cannot possibly react with each other unless their constituent particles (molecules, atoms, or ions) come into contact. If there is no contact, the reaction rate will be zero. Conversely, the more reactant particles that collide per unit time, the more often a reaction between them can occur. Consequently, the reaction rate usually increases as the concentration of the reactants increases. A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate (except for zero order reactions). Similarly, a higher concentation of products tends to be associated with a lower reaction rate. Use the partial pressure of reactants in a gaseous state as a measure of their concentration. Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of thumb for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10°C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop.
  3. 3. Part I : Effect of Concentration on Reaction Rate The reaction of magnesium metal with hydrochloric acid solution of different concentrations will be used to study the effect of concentration on reaction rate. 1. Using a 10-mL graduated cylinder,pour 5.0mL of each of the following hydrochloric acid,HCI,solution into separate clean, dry test tubes: 0.1 M,1 M, 6 M. 2. Sandpaper three 1-cm strips of magnesium ribbon to remove the tarnish. 3. Drop a strip of magnesium ribbon into each of the test tubes.Note the time of the reaction ends when bubbling ceases or no magnesium is left.Test for the gas formed by holding a burning sprint near the mouth of each of the test tubes.A popping sound indicates the presence of hydrogen gas. 4. Record your observations in the data table. Reaction Condition Time Started Time Ended Duration of Reaction,min Observations 0.1 M hydrochloric acid 9:59a.m On going On going The test tube with magnesium ribbon produce bubbles and the dissolving is on going. 1 M hydrochloric acid 9:59a.m On going On going The test tube with magnesium ribbon also produce bubbles and the dissolving is on going. 6 M hydrochloric acid 10:00a.m On going 30sec ½ mins The Temperature of the test tube become warm and the magnesium ribbon Is dissolves fast and it produce also bubbles.
  4. 4. Part II: Effect of Temperature on Reaction Rate The reaction between magnesium and hydrochloric acid will be used to study the effect of temperature on reaction rate. 1. Pour 5.0 mL of 6M hydrochloric acid,HCI,measured with a 10-mL graduated cylinder,into each of three clean, dry test tubes. (a) Place one of the test tubes in a 250-mL beaker with an ice-water mixture. (b) Place another test tube in a hot water bath kept at 50 oC. © Place the third test tube in a test tube rack at room temperature. 2. Drop a shiny 1-cm strip of magnesium ribbon to each tube. Record the time of the addition and the time each reaction ceases. 3. Record your observations in the data table. Reaction Condition Time Started Time Ended Duration of Reaction, min Observations Ice water 0 oC 9:53a.m 9:54a.m 50secs It produce bubbles and smoke , the magnesium ribbon dissolves fast and it becomes hot. Room Temperature 9:48a.m 9:48a.m 22secs It produce white smoke going up of the test tube and the magnesium ribbon dissolves faster.
  5. 5. Hot Water 50 oC 9:46a.m 9:46a.m 18secs The middle part of the test tube is become moist ,it produce bubbles,and the magnesium ribbon dissolves more fast. Part III: Effect of Catalyst on Reaction Rate The decomposition of hydrogen peroxide into water and oxygen will be used to study the effect of catalyst on the reaction rate. 1. Place 3.0 mL of 3% hydrogen peroxide, H2O2,measured with a 10-mL graduated cylinder,into each of two clean,dry test tubes. 2. Put the two tubes in a test tube rack. 3. Add a pinch of manganese dioxide,MnO2,into one of the test tubes.Observe and compare the rates of evolution of oxygen gas in the two tubes. 4. Record your observation in the data table. Reaction Condition Observations Hydrogen peroxide Hydrogen peroxide is colorless and no reaction Hydrogen peroxide and manganese dioxide It produce bubbles and when we start , it becomes color black in the top and white in the lower part and the recipitation is on going.The coloris become black (charcoal) Part IV : Effect of Particle Size and Surface Area on Reaction Rate The reaction of iron with copper(II) nitrate,Cu(NO3)2 is used to investigate the effect of particle size or surface area on reaction rate. 1.Weigh an iron nail,and put it in a clean,dry test tube.
  6. 6. 2.Weigh an equal amount of powdered iron and put it in another test tube. 3.Put the two test tubes in the test tube rack. Add 3.0 mL of 0.1 M copper (II) nitrate, Cu(NO3)2, to each tube.Observe and compare the rates at which the color of the solution changes in the two tubes. 4.Record your observation in the data table. Substance Tested Observations Iron nail The color of the iron nail is black and it becomes bronze and the edge of the iron nail is become color pink. Powdered Iron The color of the powdered iron is black and when we put the CU(NO3)2 it is become bronze and it produce tiny bubbles. Questions: 1.Write a balanced equation for the reaction of magnesium metal with hydrochloric acid. 2.Write a balanced equation for the decomposition of hydrogen peroxide. 3. Write a balanced equation for the reaction between iron and copper(II) nitrate. 4.Describe how reactant concentration, reaction temperature, catalyst, and the surface area of liquid or solid reactant affect the rate of a chemical reaction. 5.Explain the difference in the rates of reaction of iron nail and powdered iron with copper(II)nitrate. 6. Explain why a miner is never permitted to smoke inside a coal mine? 7.Why does milk to remain unspoiled for two weeks when refrigerated but turns sour when left at room temperature after a couple of days? 8.Why does a cold automobile battery may not generate enough energy to crank the engine on a cold morning?Explain briefly. Answer: 1. Mg (s) +2 HCl (aq) yields MgCl2 + H2 (g)
  7. 7. Mg was oxidized sice it went from zero to +2. H was reduced as it went from +1 to zero. 2. 2H2O2 = 2H20 + O2 3. 4. The concentration of reactants plays an important role in chemical kinetics. It is usually true that by increasing the amount of reactants, the rate of reaction is increased. As we know that an increase in concentration of reactants increases the number of molecules. According to collision theory, the greater the number of molecules the higher is the collision ratio, consequently faster is the rate of reaction. In heterogeneous reactions, the rate of reaction depends upon the surface area of solid reactant. Greater the surface area, higher is the rate of reaction. For example finely divided calcium carbonate (marble) reacts more quickly with hydrochloric acid than calcium carbonate chips. It is due to the fact that powered calcium carbonate offers larger surface area to the reacting acid. In other words, by increasing the surface area of reactant, rate of reaction increases due to greater contact between individual particles and also due to the fact that the surface molecules reacts more quickly. The rate of reaction increases with the rise of temperature. It can be explain by the fact that at higher temperature, a greater fraction of colliding molecules posse the necessary energy of activation Generally an increase of every 10oK in temperature doubles the rate. As the temperature increases the velocity of molecules also increases which results in the increase in the frequency of collision. The rise in temperature rises the kinetic energy of each molecule. It has been found that by raising the temperature by 10k,the fraction of molecule possessing threshold or activation energy becomes double. As a result the no of effective collision is also double,hence rate is doubled 5. Powdered Iron will react in a faster rate than a solid mass of iron. That's because the higher surface area of powders. Means that's the total surface of Iron exposed to Copper(II) Nitrate solution is much greater in case of the powdered form. 6. Miners break up coal from seams and that releases trapped combustible gas (methane). Fires in coal mines then trap miners who smoked. 7. Milk has a large amount of bacteria in it. These bacteria can do various things, but basically they eat and transform parts of the milk.
  8. 8. The bacteria go dormant (or die) at low temperatures, as well as high ones. This is why boiling water is good if you don't have a proper water supply, because it will kill the majority of the bacteria. Room temperature is usually slightly below the ideal conditions for bacteria, but still good. This will lead to the milk quickly spoiling. The cold refrigerator forces them to remain dormant. 8. the chemical fluid that makes the electricity to power the mechanical crank to start the car is responsible.during the winter the cold slows down the chemical reaction so it cant produce enough electricity to power the car.the opposite happens during the summer , the heat makes the fluid accidentally react when it shouldn't and it then leaks the electrical potential so when you actually go to your car it doesn't have enough juice.