## 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, (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.

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.