Lab manual_4 - UOIT Chem 1800U W10 Exp 4 7.9 4 EFFECT OF...

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UOIT Chem. 1800U, W10; Exp. 4- 7.9 79 4. EFFECT OF CONCENTRATION AND TEMPERATURE ON REACTION RATE Objectives 1) To observe the effect of reactant concentration on reaction rate; 2) to observe the effect of temperature on reaction rate; 3) to determine the order of a simple reaction with respect to one of the reactants; 4) to calculate the activation energy for the same reaction. Introduction Physical chemistry is a huge branch of chemistry and concerns itself with understanding chemical phenomena by applying mathematical and physical models. In the world of chemistry physical chemistry is the branch closest to physics. Indeed, the lines between physical chemistry, chemical physics and physics are very blurry in modern research. In very broad terms “classical” physical chemistry can be divided into two categories: thermodynamics (including equilibria) and kinetics. Crudely speaking, thermodynamics is concerned with whether a reaction will occur and what the relative amounts of reactants and products will be (“where” the reaction is going). Kinetics is concerned with how quickly the reaction gets there. Detailed kinetics studies can also lead to conclusions about the reaction pathway or mechanism. Ultimately, a thorough understanding of a chemical process requires knowledge of both the kinetics and the thermodynamics. For example, thermodynamics predicts that all diamonds will transform into graphite. However, kinetics indicates that under “normal” conditions the transformation will take a phenomenally long time. For this reason chemists refer to substances as either “thermodynamically” or “kinetically” stable.
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UOIT Chem. 1800U, W10; Exp. 4- 7.9 80 Rate Laws and the Effect of Concentration Consider the generic chemical reaction: aA + bB ± cC + dD (4.1) where the lower case letters are used to represent the stoichiometric coefficients. The rate of the reaction can be defined as: (4.2) rate A t a b B t a c C t a d D t = = == Δ Δ Δ Δ Δ Δ Δ Δ [] The negative signs appear for the first two terms because ‘A’ and ‘B’ are disappearing (i.e., ± [A] and ± [B] < 0). A rate law for the process can be written as: (4.3) rate k A B xy = where ‘k’ is the rate constant and ‘x’ and ‘y’ are empirically derived constants. The values of ‘x’ and ‘y’ may be either integral or non-integral and have no necessary relationship to the stoichiometric ratios in the chemical equation (although, by coincidence, they may be the same). The value of ‘x’ is the order of the reaction with respect to ‘A’ and ‘y’ is the order of the reaction with respect to ‘B’. The sum x + y is called the overall order of the reaction. In this experiment the following reaction will be studied: 2H + (aq) + S 2 O 3 2- (aq) ± S(s) + SO 2 (aq) + H 2 O(l) (4.4) The rate of this reaction can be determined by measuring how long it takes for a given amount of sulphur to be produced: (4.5) rate SO t S t = = Δ Δ Δ Δ 23 2 (4.6) rate k S O H S t mn + [ ] 2 Δ Δ
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UOIT Chem. 1800U, W10; Exp. 4- 7.9 81 If [H + ] is constant (as will be the case in this experiment): (4.7) rate S t kSO m ==
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Lab manual_4 - UOIT Chem 1800U W10 Exp 4 7.9 4 EFFECT OF...

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