module 10

Question 1

Explain why different stable isotopes of the same element can react in the same chemical or physical reactions at different rates

Answer 1

Heavy and light isotopes of the same elements react in the same reactions at
different rates

mass difference affects bond strength which results in different reaction rates

The rarer stable isotopes often are heavier (i.e., more neutrons)
-True for most smaller elements (C, H,
N, O, S)

Heavier isotopes
-stronger bonds ->more energy to break
 form bonds and stay bonded
 stay in solid phase
 don’t react as fast during bond breaking

Lighter isotopes
-weaker bonds -> less energy to break
 Don’t form bonds quickly and leave bonds more readily
 More likely to go into liquid or gas phase
 react quicker during bond breaking

product has a different heavy/light isotope ratio than the reactants
“Mass-dependent isotope fractionation”

Question 2

Explain how stable isotopes of water can help infer temperatures of ancient water sources (e.g. ice cores)

Answer 2

Water evaporation
-breaking H bonds in H2O
-lighter isotopes tend to evaporate faster (less energy to break bonds)

 Over time, residual reactant pool (ocean/lake/pond) has more heavy isotopes, more light ones have left in
vapour

Question 3

Stable isotope ratios

Answer 3

use ratios (R) of the amounts of two isotopes of the same element when discussing fractionation

R = rare/abundant isotope (or heavy/light isotope)

For the common stable elements:
less abundant/more abundant isotope
E.g., 13C/12C, 2H/1H, 15N/14N

compare to a standard with a known ratio and express as a per mil (‰) difference

Question 4

hot vs cold

Answer 4

At colder temps, fractionation is
greater, and vapor becomes even
lighter, despite the heavy ocean

Seasonality – a is higher (more extreme)
at low temperature (also higher for
sublimation from vapour to ice)

water: bond strength controls isotope fractionation.
hydrogen bond between the heaviest isotopes will be the strongest, and more energy if required to break the intermolecular forces in water.

An increase in temperature,
increases the kinetic energy (i.e. KE α T), and more intermolecular bonds will break as temperature increases.

The amount of fractionation decreases as temperature increases

Question 5

equilibrium/kinetic

Answer 5

-chemical equilibrium between reservoirs
-reservoirs must be well mixed,
- closed systems.

-Kinetic
-disequilibrium fractionation
-open systems
-reservoirs can be removed or replenished,
-accelerated rate in one reaction direction while the other is diminished. -Raleigh distillation.