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Integrated Rate Laws and k Units

The atmosphere surrounds the Earth and holds the air we breathe; it protects us from outer space; and holds moisture (clouds), gases, and tiny particles. In short, the atmosphere is the protective bubble in which we live. This protective bubble consists of several gases (listed in the table below), with the top four making up 99998% of all gases. Of the dry composition of the atmosphere, nitrogen by far is the most common. Nitrogen dilutes oxygen and prevents rapid burning at the Earth's surface. Living things need it to make proteins.

Figures (2)

Concentration of A (A0 = 0.25 mol/L) and B versus time reaching equilibrium k1 = 2 min−1 and k−1 = 1 min−1
Time course of two first order, competitive reactions with differing rate constants.

Key Points

  • The half-life of a first-order reaction is often expressed as t1/2 = 0.693/k (as ln(2)≈0.693).
  • The mole fraction yi is constant when the amount of each substance is constant, so we have Œ@ ln.yip=pı/=@pT;fnig D 1=p.
  • The scientific unit of pressure is the Pascal (Pa), named after Blaise Pascal (1623-1662).
  • Time zone offsets are identified as -12 UTC through 0 to +12 UTC, with the minus values signifying time "before" or ahead of prime meridian (which is the Western Hemisphere).
  • Concentration is sometimes called "amount concentration" or "molar concentration" to avoid confusion with number concentration (the number of particles per unit volume).
  • To use this method, we plot experimental values of the quantity V=n (where n is nA C nB) versus the mole fraction xB.

Terms

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