Chemical Kinetics

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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. 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...
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