Study guide questions for Midterm 2 NRES201 Ch5 Know the gravimetric and volumetric water content calculation see practice questions Practice calculation questions to estimate Ksat using Darcy s Law see practice questions Ch8 What are general physicochemical properties of soil colloids 8 1 Size o Colloids are generally defined as particle size of less than 1 um They belong to the clay fraction 2 um Surface Area o 10 800 m2 g the smaller the size of the particle in a given mass the greater the surface area Expose a large external and internal surface within a clay crystal between layers Adsorption of Cation and Anions o Attraction of cations Al Ca Mg K H Na to the surfaces of negatively charged soil colloids Under natural conditions the proportions of specific cations present are largely influenced by the soil parent material and the degree to which the climate has promoted the loss of cations by leaching o Anions Cl NO SO are attracted to colloids that have positive charges Not as extensive as that for exchangeable cations Important mechanisms for holding negatively charged constituents Adsorption of Water o hold a large number of water molecules The charges on the internal and external surfaces attract the oppositely charged end of the polar water molecule Water adsorbed causes the layers to move apart making clay more plastic and when the water is removed layers move closer together Highly reactive materials Describe the structural difference between 1 1 and 2 1 silicate minerals The ratio of 1 1 and 2 1 is the ratio of Si tetrahedral and Al Mg Fe octahedral sheets 1 1 means one Si tetrahedral and one Al octahedral sheet 2 1 means two Si tetrahedral sheets and one Al octahedral sheet Define isomorphous substitution IS replacement of ions by another of similar size ions in a mineral without fundamentally changing the structure Occurs during mineral formation Responsible for the Cation Exchange Capacity in the clay fraction of soils Name all of 1 1 and 2 1 silicate minerals that are covered in class 1 1 type Silicate Clay Kaolinite 2 1 Non expanding Clay Minerals Pyrophyllite 2 1 Expanding Clay Montmorillonite Smectite 2 1 Non expanding silicate clay Mica Which soil minerals have IS and or permanent charge Smectite Mica Montmorillonite NET CHARGE silicate tet al oct tet Soil Colloid Charge highlighted red Kaolinite 0 Pyrophyllite 0 Montmorillionite 0 4 Fine mica 0 8 Which layered silicates are expansive and non expansive Non expanding 2 1 Silicate Clay Mica min Swelling Kaolinite no swelling Smectite max swelling Montmorillonite max swelling When you wet kaolinitic and montomorillitic soils what will happen to the volume of soils Can you explain why Kaolinite 1 1 adjacent layers are bound by hydrogen bonding which prevents expansion between the layers when the clay is wetted No swelling Montmorillionite The soils shrink and swell markedly with changes in moisture content Adsorption of water in these ultramicropores wavy stacks leads to severe swelling The expansion upon wetting contributes to the high degree of plasticity stickiness and cohesion Why is mica illite a non expansive clay The negatively charged interlayer 0 8 attracts cation like K By their mutual attraction for the K ions in between adjacent layers in fine grained micas are strongly bound together making the micas non expansive clay Two major sources of charges on soil colloids 1 Hydroxyls OH and other functional groups on the surfaces of the colloidal particles that by releasing or accepting H ions can provide either negative or positive charges 2 the charge imbalance brought by the isomorphous substitution in some clay crystal structures of one cation by another of similar size but differing charge permanent charges What is pH dependent charge Where does it come from Note Table 8 5 pH dependent charge that portion of the total charge of the soil particles that is affected by and varies with changes in pH All colloids organic and inorganic exhibit charges associated with OH groups and are largely pH dependent pH of solutions that surround colloids affects the charge properties of colloids They can be negatively or positively charged How pH dependent charges develop at the broken edge a one charge from octahedral oxygen lost its H ion by dissocation b One half 1 2 charge from each octahedral oxygen that would normally be sharing its electrons with a second aluminum Effect of acidification increases H ions in the soil soltuion c One 1 charge from a tetrahedral O2 atom that would normally be balanced by bonding to another silicon if it weren t at the broken edge Effect of acidification increases H ions in the soil soltuion Understand the relationship between soil pH and or charge development at pH dependent edge sites Deprotonation dissociation of hydrogen atom as pH increases d increases and protonation association of hydrogen atom at the broken edge site of clay minerals Negative charges Under moderately acidic alkaline pH hydrogen atom dissociates from colloids resulting in net negative charge Positive charges Under moderately extremely acidic pH the surface OH group gains a hydrogen atom resulting in net positive charge Edge sites contain two different hydroxyl OH groups Which charge i e pH dependent or permanent charge is more important in ion exchange processes Permanent charge What are six principles governing cation exchange reactions Can illustrate specific reaction steps and explain each step 1 Reversibility a Displacement of Na ion from the colloid surface with a hydrogen ion A double arrow indicates that the reaction is reversible 2 Charge equivalence a Exchange is chemically equivalent Two singly charged H can replace a divalent Ca ion 3 Ratio law a Since the different ratio of each species are present the reaction continues until it reaches equilibrium At equilibrium the ratio of cations on the colloid is the same as the ratio in solution 4 Anion effects on mass action a When cations are originated from precipitates or strongly associated with anions the cation exchange reaction tends to proceed to right Not reversible 5 Cation selectivity a Some cations are much more tightly held on the colloid surfaces Higher charge is better Smaller hydrated ionic radii is better applies to Ca and Mg 6 Complementary cations Colloids are always surrounded by many cationic species Cation exchange reaction is not only two ion reactions is rather complex due to the presence of neighboring ions How do cations adsorb on