Basic principles and thermodynamics

Question 1

What are some differences between the real system and the thermodynamic model system?

Answer 1

Real:
- Changes with time
- Contains many different species
- Reactions controlled by kinetics
- Unknown factors

Thermodynamic:
- Incomplete and inaccurate to some degree
- Complete equilibrium assumed
- Data quality might be poor

Question 2

What is meant by local equilibrium?

Answer 2

Small parts of a system can be at equilibirum, with no changing properties over time. Thermodynamics can be applied to these areas

Question 3

Describe the charge balance

Answer 3

Electroneutrality requires that the ionic species in a water sample
maintain a charge balance. Therefore, the quality of a water analysis can be judged based on the charge balance.

Question 4

What does a large charge imbalance indicate?

Answer 4

 that a major ion has not been included in the analysis
 problems with sampling or analytical errors
 the presence of OH complexes of Al and other elements (for example, the charge of Al(OH)2+ is different from that of
Al3+

Question 5

Which ions can be used to balance a charge balance?

Answer 5

Inert ions (chloride for example) or an element which is already found in a high amount.
Na+ and to a lesser extent Ca2+ can be used.

Question 6

Describe molar concentrations

Answer 6

Molarity = mole/litre
Molality = mole/kg
Used in freshwater solutions since the activity coefficient is approximately 1. In more concentrated solutions (saltwater) the activity is less than the molar concentration.

Question 7

What is the ion activity product?

Answer 7

IAP is the multiplication of the concentration of the products formed in a dissolution.

Question 8

What is the saturation index?

Answer 8

Calculated from the solubility product Ksp and the ion activity product as SI = log(IAP/Ksp). SI determines if a mineral dissolves, precipitates or is at equilibrium in a solution.
<1 dissolving
>1 precipitation
= 1 equilibrium

Question 9

Which are the five main redox couples in natural waters?

Answer 9

O2 / H2O
NO3- / N2
Mn4+ / Mn2+
Fe3+ / Fe2+
SO42- / S2-

Question 10

What is the electron activity?

Answer 10

Indicates the tendency to donate or accept a proton.
Low pe - solution donates electrons (the solution is reducing)
High pe - solution accepts electrons (the solution is oxidizing)

Question 11

What is Eh?

Answer 11

the potential of a solution relative to the SHE.
SHE (standard hydrogen electrode) have constant conditions when no reactions occur

=16.9 * pe

Question 12

Describe the solubility product

Answer 12

It is an equilibrium constant for a dissolution reaction. It states the ratio of product and reactant activities if the reaction reaches equilibrium