Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 109 - General Chemistry - Spring 2011

Lecture Notes 34: 18 April

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Colligative properties, cont.

Osmotic pressure (pi)

piV = nRT; or, dividing both sides by V, pi = MRT, where M = molarity.

Example: What are the osmotic pressures of 1.00 M sugar and 1 M aluminum chloride solutions at 25°C?

pisugar= MRT = (1 mol/L)(0.0821 L*atm/mol*K)(298 K) = 24.5 atm

piAlCl3= MRT = (1 mol/L)(4 mol ion/mol)(0.0821 L*atm/mol*K)(298 K) = 97.9 atm

Colloids

Colloids are defined by particle size = 1.0 nm< colloid < 100 nm (particles in solution are 0.1 - 1.0 nm in diameter, whereas particles > 100 nm dispersed in a fluid are considered to be in suspension.) Colloids generally do not settle out.

Chemical Kinetics

Study of rates and mechanisms of reactions. Experimentally, look at rates of reactions, use this information to guess mechanisms

Concentration Dependence of Reaction Rates

Concentrations are assumed to be in Molarity unless otherwise specified.

Consider the reaction:

A + 2B+ C right arrow D + E

This expression is referred to as a Rate Lawwith the sum of various exponents referred to as the order of the reaction. The overall order of this reaction is thus 3rd order - it is first order in A, second order in B, and zero order in C.

Looking at the different reaction orders:

So how do we determine the order of a reaction?

The experiment is to increase the concentration of a single reactant, and observe the rate. Sometimes the order will be obvious (i.e. double, double = directly proportional = 1st order). If not, then can take the results of two experiments and divide them and do some algebraic manipulations to find the correct order.

Example: Find the order of the reaction given the data below.

 Experiment [A] rate
1 0.0167 3.61 x 10-3
2 0.0569 4.20 x 10-2

r = k [A]n

r1/r2 = (k [A]1n)/(k [A]2n)

r1/r2 = ([A]1/[A]2)n

But we want to find n, so take logs of both sides:

ln (r1/r2) = ln ([A]1/[A]2)n = n ln ([A]1/[A]2)

n = ln (r1/r2)/ln ([A]1/[A]2)

n = ln(0.0361 / 0.420)/ln(0.0167 / 0.0569)) = 2

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© R A Paselk

Last modified 18 April 2011