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:

• Cooking at sea level vs. at altitude, vs. pressure cooker.
• Oxidation of paper (browning) vs. burning.
• Alka Seltzer in book.
  • Graphs vs. tables; line plots vs. bar graphs; trends and kids.
• Many reactions approximately double in rate with a 10°C increase in temperature. Though not always true, its still a nice way to guess how fast things might occur.

Why do reactions take place faster at higher temperature?

• Particles moving around faster, so run into each other more frequently.
• Run into each other harder, so easier to overcome repulsion of outer electrron 'envelope'.

Rate vs. Concentration:

• Bleach.
• examples?

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:

• mmHg

• atm
• others include psi (pounds/square inch), pascals, Torr, etc.

Gases are characterized by four properties

• Amount of substance, n  (in moles)
• Volume, V (in Liters)
• Pressure, P (in atm, though often measured in mmHg)
• Temperature (in K)

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

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"

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