Expt 8-Chemical Kinetics

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Chemistry 14.1 - General Chemistry Laboratory I

Text of Expt 8-Chemical Kinetics

CHEMICAL KINETICSExperiment #8 CAHANDING, Ma. Amadea Teresita C. DIONISIO, Nicole Anna Marie H. Group #8, Chem 14.1, FCD4 Prof. Glennalin Medina

WHAT IS IT?Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction.

WHAT IS REACTION RATE?The reaction rate or rate of reaction for a reactant or product in a particular reaction is intuitively defined as how fast a reaction takes place.

HOW DO YOU MEASURE IT? The rate equation or rate law is a mathematical expression used in chemical kinetics to link the rate of a reaction to the concentration of each reactant. It is of the kind:

k(T) is the reaction rate coefficient or rate constant. A and B are the variables representing the reactants. The exponents n' and m' are called reaction orders and depend on the reaction mechanism.REACTION MECHANISM is the step by step sequence of elementary reactions by which overall chemical change occurs.

SOME OTHER EQUATIONS. For gases the rate law can also be expressed in pressure units using e.g. the ideal gas law. By combining the rate law with a mass balance for the system in which the reaction occurs, an expression for the rate of change in concentration can be derived. For a closed system with constant volume such an expression can look like:

Each reaction rate coefficient k has a temperature dependency, which is usually given by the Arrhenius equation:

The pressure dependence of the rate constant is associated with the activation volume.

WHAT AFFECTS REACTION RATE? There are several factors affecting the rate of the reaction, namely: NATURE OF REACTANTS CONCENTRATION TEMPERATURE SURFACE AREA and CATALYSTS.

LET US RELATE. In our previous experiment, we have experienced first hand how the factors stated earlier affect the results of the reactions of the substances we have encountered. Let us take a trip back to what we did, and this time we will look unto the concepts deeper.

NATURE OF REACTANTS.

EXPLANATION. There are several properties that affect the nature of the reactants. These are state of matter, bond type and bond strength. In states of matter, gas reacts faster than liquids and solid and aqueous solutions react faster than any state of matter. In terms of bond type, reactions involving ionic species tend to proceed faster than reactions involving molecular compounds. Lastly, reactions involving the breaking of weaker bonds proceed faster than reactions Involving the breaking of stronger bonds. In this case, Na2C2O4 is a strong reducing agent because the bond strength of Na2C2O4 is weaker than H2O2 therefore it is easier to break. Thus, the change of color is faster in test tube A.

RESULTSRELATIVE RATE OF DISAPPEARANCE OF PINK COLOR OF THE SOLUTION (faster/slower) Faster Slower

Reducing Agent

Na2C2O4 H2O2

CONCENTRATION.

EXPLANATION. It is evident that the higher the concentration is, the faster it can dissolve a substance. Thus, Concentration of reactant is directly proportional to the time of reaction. The higher the concentration of the reactant, the more molecules are present and the more frequently they collide, and more reaction between them occurs.

RESULTSCONCENTRATION OF HCl 6M 3M 1M TIME (sec) 46 87 773

TEMPERATURE.

EXPLANATION. In this case, it is very similar to the trend of the concentration. The higher the temperature, the faster it dissolves substances. Thus, It was seen that the higher the temperature the faster the reaction. This is because raising the temperature increases the reaction rate by increasing the number and energy of the collision between molecules.

RESULTTEMPERATURE (C) 20 40 60 TIME (sec) 39 21 8.59

SURFACE AREA.

EXPLANATION. In what we have seen, the smaller particles are, the easier they are to be dissolved. This is due to the fact that smaller particles have larger surface area where the reacting agent can penetrate the substance more efficiently. Thus, The greater surface area or particle size per unit volume of the substance, it is allowed to make more contact with the other reactant thus having a faster the rate of the reaction

RESULTSTATE OF SOLID SUBSTANCE Powdered Granulated RELATIVE RATE OF EVOLUTION OF BUBBLES Faster Slower

CATALYST.

EXPLANATION. We have seen that adding a substance can serve as a catalyst that will make the reaction faster. In this case, the MnO2 served as the catalyst that made the reaction faster. Thus, A catalyst is a substance that accelerates the rate of a chemical reaction but remains chemically unchanged afterwards.

RESULTLIQUID SOLUTION RELATIVE RATE OF EVOLUTION OF O2 GAS Without MnO2 Slower With MnO2 Faster

WE THEREFORE CONCLUDE THAT There are different factors that

affect the speed of reaction. Higher temperature, higher concentration, greater surface area, nature of reactant and a catalyst can produce a faster reaction. And all of these are part of the study of CHEMICAL KINETICS.

SNEAK PEEK.

TIME TO WRAP IT UP.Now we know what we did, its concept, and the science behind all of it.Hope you learned MORE about EXPERIMENT NO. 8. Farewell.

THANKYOU!