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Chemical Kinetics

Experiment 8 Chemical Kinetics Abstract This experiment was done to determine the effects of the nature of the reactants, concentration, temperature, surface area and catalyst on the rate of chemical reactions. The nature of the reactants implies a difference if the reactants are aqueous or organic, acidic or basic or if they occur in the same phase or not. Acid-base reactions, formation of salts, and exchange of ions are fast reactions while reactions in which large molecules are formed or broken apart are usually slow.

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Generally, the rate of reaction is directly proportional to the concentration of the reactants, as well as to the temperature. Greater surface areas and addition of catalysts also increase the rate of chemical reactions. Introduction Chemical reactions result from effective collisions between particles of the reactants as stated by the Collision Theory. These chemical reactions proceed at different rates. The study of chemical reactions with respect to reaction rates and effects of various variables is known as Chemical Kinetics.

Several factors that affect the rate of chemical reactions are nature of the reactants, concentration, temperature, surface area and presence of catalysts. Methodology Effect of the Nature of Reactants Twenty drops of 0. 1 M potassium permanganate (KMnO4) and 12 drops of 6 M sulfuric acid (H2SO4) were mixed in a 5-mL test tube. Two test tubes were filled with 10 drops each of the resulting solution. Ten drops of 0. 1 M sodium oxalate (Na2C2O4) solution was then added to the first tube and 10 drops of hydrogen peroxide (H2O2) solution to the second. The rates of discoloration of KMnO4 in the two test tubes were compared.

Effect of Concentration Ten drops each of 6 M, 3 M and 1 M HCl were placed into three separate test tubes. A piece of Mg ribbon was added into each test tube and the time when Mg ribbon dissolves completely was noted. Effect of Temperature A 5-mL test tube was filled with ten drops of 0. 15 M sodium thiosulfate (Na2S2O3) and was heated in a water bath until the temperature of the bath was 40o C. Ten drops of 3 M HCl was then added and the time when the solution became cloudy was recorded. The same procedure was done, heating the solution to 60o C and cooling it to 40o C.

Effect of Surface Area A small piece of chalk was placed in a 5-mL test tube. Another piece of chalk with the same size was ground and put in a second test tube. Twenty drops of 1 M HCl was added into each test tube and the relative rates of reaction were compared. Effect of Catalyst Ten drops of freshly prepared H2O2 and a pinch of MnO2 were mixed in a test tube. The evolution of gas was noted. The same procedure was done but this time without adding MnO2. The relative rates of gas evolutions in the two systems were then compared. Results Table 1.

Effect of the Nature of Reactants on the Rate of Disappearance of Pink Color Reducing Agent| Rate of Disappearanceof Pink Color| Na2C2O4| slower| H2O2| faster| Table 2. Effect of the Concentration of HCl on the Dissolution of Mg Ribbon Concentration of HCl| Relative Rate of Reaction| 6 M| fastest| 3 M| fast| 1 M| slow| Table 3. Effect of Temperature on 3 M HCl Temperature (oC)| Relative Rate of Reaction| 20| slow| 40| fast| 60| fastest| Table 4. Effect of Surface Area on the Rate of Evolution of Bubbles State of Solid Substance| Relative Rate of Evolution of Bubbles| Powdered| faster| Granulated| slower|

Table 5. Effect of MnO2 on the Rate of Evolution of O2 Gas Liquid Solution| Relative Rate of Evolution of O2 Gas| Without MnO2| slower| With MnO2| faster| Discussions Nature of Reactants. Acid-base reactions, formation of salts, and exchange of ions are fast chemical reactions. Reactions in which large molecules are formed or broken apart are usually slow. Reactions breaking strong covalent bonds are also slow. In both reactions, exchange of ions occurred. However, the reaction with H2O2 was faster because it is a stronger reducing agent. Moreover, H2O2 easily reacts at room temperature. Concentration.

In the experiment, the 6 M HCl solution had the shortest time in dissolving the Mg ribbon because it has the greatest concentration of HCl. Greater concentration of a reactant means there are more particles within a given space. This increases the number of collisions between the reacting species. Hence, the reaction rate becomes faster. The relationship of the rate of the reaction with the concentration of reactants can be seen by using the Rate Law: r = k[A]m[B]n. A (M)| B (M)| C (M)| Rate (M/s)| 0. 030| 0. 030| 0. 010| 0. 3 x 10-4| 0. 060| 0. 030| 0. 020| 1. 2 x 10-4| 0. 060| 0. 045| 0. 20| 1. 8 x 10-4| 0. 030| 0. 030| 0. 040| 0. 3 x 10-4| Order of reaction * with respect to C [0. 030 M]x [0. 030 M]y [0. 010 M]z k= 0. 3 x 10-4 [0. 030 M]x [0. 030 M]y [0. 040 M]z k= 0. 3 x 10-4 [0. 25]z = 1 z = 0 * with respect to A [0. 030 M]x [0. 030 M]y k = 0. 3 x 10-4 M/s [0. 060 M]x [0. 030 M]y k = 1. 2 x 10-4 M/s [0. 50]x = 0. 25 x = 2 * with respect to B [0. 060 M]2 [0. 030 M]y k = 1. 2 x 10-4 M/s [0. 060 M]2 [0. 045 M]y k = 1. 8 x 10-4 M/s [0. 60]y = 0. 67 y = 1 Order (overall) = 2 + 1 + 0 = 3 Value of k 0. 3 x10-4 M/s = k[0. 030 M]2 [0. 030 M]1 [0. 010 M]0 k = 1. 11 M-2s-1 Temperature.

The HCl solution which was heated to 60o C had the fastest rate of reaction. Increasing the temperature in a solution causes the particles to move faster, and thus, these particles collide with each other more frequently. The greater the number of collisions per second, the greater the reaction rate. However, increasing the temperature will not always increase the rate of reaction. Most enzymatic reactions occur faster in higher temperatures. But, a very high temperature can cause denaturation of these enzymes, thus causing a halt to such reaction. Surface Area. The powdered chalk showed faster rate of reaction.

If a solid reactant is broken down into smaller pieces, the rate of the reaction increases. Smaller pieces of the same mass of solids have a larger surface area and thus a higher chance that reactant particles would collide and react. If the surface area per unit mass is small, collisions can only occur with the outer atoms and is therefore limited. Catalyst. Catalysts are substances which speed up reactions without being chemically altered in the end. These speed up a reaction by providing an alternate route with lower activation energy. Catalysts do not necessarily lower the activation energy.

In certain solutions, reactants can still collide with each other and generate a reaction even without a catalyst. Reactant particles possess kinetic energy but not all particles are strong enough to break bonds and cause a reaction. In the reaction MnO2 + 2H2O2 –> MnO2 + O2 + 2H2O, manganese dioxide is considered as the catalyst (usually not written). Catalysis reactions can either be classified as homogenous or heterogeneous. Homogenous catalysis involves a catalyst present in the same phase as the reacting molecule while heterogeneous catalysis involves a catalyst in a different phase from the reacting molecule.

Conclusion The rate of chemical reaction is affected by several factors. Generally, acid-base reactions, formation of salts, and exchange of ions are fast reactions, while reactions in which large molecules are formed or broken down are usually slow. Greater concentration of reactants results in faster reaction rates because there are more particles which collide with each other. High temperature also speeds up chemical reactions by increasing the kinetic energy within the system.

Smaller pieces of matter have increased surface areas which provide higher chance of collisions, and thus, making reaction rates faster. Catalysts alter the rate of chemical reactions without being used up. These work by changing pathways and causing the reaction mechanism to occur with lower activation energy. Sources Retrieved from http://www. ucc. ie/academic/chem/dolchem/html/elem/elem008. html. 2010, March 18 Retrieved from http://www. docbrown. info/page03/3_31rates. htm. 2010, March 18 Retrieved from http://answers. yahoo. com/question/index? qid=20080301185105AANoOTo. 2010, March 18

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