Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 6 December Phone: x 5719
Home: 822-1116
e-mail: rap1

Rates of Chemical Reaction

Rate vs. Temperature:

Why do reactions take place faster at higher temperature?

Rate vs. Concentration:

Why do concentrated chemicals react faster? Again, more molecules, more likely a collision will occur. Or, for some chemicals (and radioactivity), there are more particles to break down.

 

Gases and Gas Laws

What is Pressure? Due to collisions of particle with walls of container etc.

Units of Pressure:

Gases are characterized by four properties

Boyle's Law: Boyle's Law describes the relationship between pressure and volume when the temperature and amount of substance are held constant.

PV = c @ constant T & n

Or, "At constant temperature the volume of any quantity of gas is inversely proportional to its pressure." V = k (1/P), or PV = k, & P1V1 = P2V2.

Plotting pressure volume data (keeping n and T constant) gives a graph for a hyperbola (xy = c), as seen below:

Charles' Law: The relationship between volume an temperature was determined much later because accurate thermometers had to be developed first. But once thermometers were available a number of workers determined that volume is directly proportional to temperature. Plotting data for the relation of volume of a gas to temperature between 0° C and 100 ° C gives a plot similar to that below:

Extrapolating this data to V = 0 we can find an absolute minimum value of temperature on the assumption that negative volumes can't exist:

The intercept on the volume axis is then taken as absolute zero = -273.15 °C = 0 K for an ideal or "perfect" gas with particles of zero volume and no interactions other than collisions.

Algebraically we then find that V = k'T, & & V1/T1 = V2/T2.

We can combine these relationships (T was part of the constant for Boyle's Law and P is part of the constant for Charles' Law) to give the "combined Gas law"

(PV)/T = constant.

But of course the constant now includes amount of stuff.

 

22.4 L/mole = molar volume of an ideal gas @ STP (Standard conditions of Temperature and Pressure: P = 1 atm and T = 0° C.


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© R A Paselk

Last modified 6 December 2004