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Chem 116 Lecture 4

# Chem 116 Lecture 4 - Quantitative Descriptions Be careful...

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7/1/10 Be careful to state what species is being used to express the rate: S2O32- + 2H+ ° SO2 + H2O + S We could say: rate = - u [_______]/ƒ t or rate = - z [____]/b t Quantitative Descriptions but ° [_____]/° t = 0.5 x û[_____]/° t OR ° [___]/° t = 2 x ° [_______]/° t

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7/1/10 Expressing Rate in Terms of Changes in Concentration with Time 2H2( g ) + O2( g ) 2H2O( g ) (a) Express the rate in terms of changes in [H2], [O2], and [H2O] with time. (b) When [O2] is decreasing at 0.23 mol/L*s, at what rate is [H2O] increasing? Choose [O2] as a point of reference since its coefficient is 1. For every molecule of O2 that disappears, 2 molecules of H2 disappear and 2 molecules of H2O appear, so [O2] is disappearing at half the rate of change of H2 and H2O.
7/1/10 In general, for the reaction: aA + bB cC + dD The numerical value of the rate depends upon the substance that serves as the reference. The rest is relative to the balanced chemical equation. Caution: We will use rate but specify with respect to what. Quantitative Description

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7/1/10 The Rate Law expresses the relationship between reactant concentration and reaction rate aA + bB ± cC + dD rate = - ± [A]/D t = k [A] m [B] n m, n = 0, 1, 2 … integers k = rate constant (a proportionality constant) 1.
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Chem 116 Lecture 4 - Quantitative Descriptions Be careful...

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