276 Exam Flash Cards

what are the 8 substances that make up the chemical composition of water?

what are the 8 substances that make up the chemical composition of water?

H20, pH, major cations, major anions, organic matter, trace metals, particles, biological materials.

Give 4 examples of chemical reactions

sorption-desorption, acid-base, precipitation-dissolution, redox

Give 2 examples of biological processes

uptake and release, photosynthesis and respiration

What is the equation for advection

concentration x mean velocity

What are 2 types of diffusion

Eddy turbulence and molecular

What is the equation for diffusion

concentration gradient x diffusion coefficient

3 key facts regarding thermal stratification

Bottom waters horizontally mixed, surface waters mixed by convection, eddy diffusion controls vertical transport

Define eddy diffusion

The diffusion process where substances are mixed due to eddy motion

Example of eddy diffusion

Manganese- released from sediment and diffuses through the stratified bottom layers, well mixed in surface waters.

What is the exponential decay curve equation

C = Coe^-Q/v

What does the exponential decay curve equation show?

How the concentration of a substance decreases with time

What are steady-state conditions, an example and exception

Where the total mass in the compartment equals the rate of input or output. Chemical equilibrium, except slow processes such as weathering

What are non steady-state conditions, an example and exception

Where there is a change in concentration over time, for example in lakes. Chemical disequilibrium, except very fast reacting element under input-output balance

Define residence time

The mean time period that any constituent remain in a specified compartment.

What is the average residence time in the oceans compared to windermere

4000 years to 1 year

What are 3 factors that affect the distribution of substances

Reaction rate, residence time, internal mixing

What does an increase in the residence time suggest

A more complex reaction that is heterogeneous and involves physical mixing

What is the shape of a water molecule

4 tetrahedrally orientated sp3 orbitals

How is polarity produced in the water molecule (2 factors)

A difference in electronegativity between O and H, and the distribution of lone electron pairs

What is a hydrogen bond

electrostatic attraction between H and O

in separate H2O molecules, which links 4-8 molecules so it has a higher effective molecular weight

What substances are soluble

Ionic and polar covalent substances

What substances are insoluble

covalent substances (e.g. PCB)

What 2 factors is solubility linked to

Hydration and the dielectric effect of water (as water has no charge, it electrostatically insulates charged ions in a solution).

What is the size of truly dissolved substances and example

less than 1 nm, viruses

What is the size of colloids and example

1 nm - 1 macrometer, bacteria and algae

What is the size of particulates

More than 1 micrometer

2 factors about transport and location of colloids

Suspension of small particles, dispersed in water with dissolved substances

Define stokes law

where the velocity of gravitational settling is proportional to diameter squared

For particles less than 1 micrometer, why can they not compete with other forces (examples)

because their velocity is too small to compete with Brownian motion and turbulence

4 key properties of colloids

very large specific surface area, reactive, mobile and act as intermediates in solution (particle coagulation)

3 examples of colloids

Sulphide minerals, organic materials, clay minerals

Define chemical speciation

substances with different chemical and physical forms with different chemical and biological properties

When is the Physical form important in speciation

Metal ion in solution vs metal adsorbed to clay particles

When is the Physical form unimportant in speciation and example

Free hydrated metal ions vs metal complexes with inorganic ligands, free hydrated Fe2+ behaves differently to Fe(II)-humic complex (forms insoluble FeS)

What happens with chemical speciation in algae

Only free ions are taken up by the algae (Free Ion Activity Model)

Two examples of chemical speciation and biological uptake

Zn Uptake by Algae, zinc concentration in the water was linear to zn2+ in the blue-green algae. Al Uptake by Fish, Al is bad for fish, in high pH and more humic-rich waters, less Al3+ available so fish are safe

What is chemical speciation important for (biological)?

important for bioavailability of substances

What are the key biological substances that affect the chemical composition of natural waters

micro-organisms such as planktonic algae and bacteria

2 major factors that affect chemical speciation, and example

pH and dissolved organic compounds, Metal ion plus humic acid = metal humic species

What size is the filter for filtration

0.1-1 micrometer

2 problems with filtration

Dissolved substances generally includes the colloidal fraction, and size fractions are also affected by other properties

What 4 factors affect the size fractions in filtration

non-uniform pore sizes, clogging, adsorption onto filter and apparatus, colloidal aggregation

Describe the example of measuring total dissolved substances by colorimetry

Phosphate analysis, acidified molybdate reagent. Produces reduced phospho-molybdate complex that absorbs long wave-length visible light. Intensity of blue colour is proportional to concentration. Includes free orthophosphate and inorganically complexed orthophosphate.

Describe the example of measuring total dissolved substances by atomic absorption spectrometry

Sample is sprayed into the flame and atomised. Light with suitable wavelength is shone through the flame. Light is absorbed by the atoms of the sample. The amount of light absorbed is proportional to the concentration of the element in the sample.

How are free ions analysed?

By Ion Selective Electrode (ISE). A membrane electrode possesses a certain selectivity towards a given ion or ions

Define free hydrated metal ions

No ligands other than the water molecules

Define complexes

Central ion or atom surrounded by ligands

Define ligands

Molecules or ions in metal complexes

How do you measure free ions?

Using a ion selective electrode (ISE)

Define an ion selective electrode

A membrane electrode that possesses a certain selectivity towards a given ion or ions

How do you measure labile species

Anodic stripping voltametry (ASV) or diffusive gradient in thin films (DGT)

Define labile species

Species that can dissociate in response to pertubation

4 advantages of DGT

In situ, time-integrated, measures labile and bio-available species, not affected by environmental conditions

What is the equation used by DGT

C = m delta g / DAt

3 reasons why the concentration of labile species measured using DGT be smaller than using a filter?

Diffusion coefficient could be smaller, complexes may dissociate slowly, DGT doesn't measure colloids

what 5 things does DGT measure?

metals, radionuclides, nutrients, antibiotics, pesticides

What is the main dissolved constituent in natural waters?

inorganic ions

What 3 things does the concentration of inorganic ions depend on?

crustal abundances, aqueous chemistry, biological effects

Give 3 examples of very soluble metal ions

Calcium, sodium, magnesium

Give 2 examples of not very soluble metal ions

Aluminium and iron

Define ionic potential

a charge density term indicating the ionizing power of the ion

What is the equation for ionic potential

ionic potential (z/r) = Ionic charge (z) / ionic radius (r)

What does the interaction strength of elements in dissolved water depend on?

ionic potential

What happens to ions when there is a low ionic potential (under 3) and a low cation electronegativity (under 1.2)

Large, low-charge, electropositive ions surrounded by water molecules with little interaction between ion and water

What happens to ions when there is a medium ionic potential (3-12) and a medium cation electronegativity (1.2-1.9)

Small, highly-charged, electropositive ions, interaction between ion and water, bond to oxygen by abstracting OH liberating hydrogen ion, form uncharged and insoluble hydroxides (Fe, Al)

What happens to ions when there is a high ionic potential (over 12) and a high cation electronegativity (over 1.9)

Smaller, more highly-charged, electronegative ions, interaction between ion and water, bond to oxygen by abstracting O liberating hydrogen ion, form large and stable anions

The elements present in highest concentrations relative to their crustal abundances are...

group 1 metals and the larger group 2 metals, elements with large ionic potentials which form soluble oxyanions, and group 7 anions

Where is dissolved organic carbon (DOC) usually present

In humic substances

What is the master variable in natural waters?

pH

What is the pH of freshwater

5-11

What is the pH of seawater

around 8

define chemical equilibrium and exceptions

The state of dynamic balance where the rates of opposing reactions are equal and the concentrations remain unchanged. exception: very Slow Reaction such as weathering of aluminium-silicate rock, concentration of species unchanging, non-equilibrium

How are equilibrium reactions quantified

using the equilibrium constant (k) derived from the law of mass action

Give the mass action expression for A + B -> C + D

k = [C] [D] / [A] [B]

Define non-ideal behaviour

Where the concentration of a species is not the same as the actual concentration due to electrostatic interactions between ionic species

Define a homogeneous reaction

where the reaction involves only one phase for example gaseous,

What is the constant concentration of most natural and sea water?

55.5 M dm-3

What 3 things can a mass action expression determine?

whether a system is at equilibrium, which way it will go if not at equilibrium, determines the equilibrium concentration of a species

What effects do temperature and pressure have on k?

significant, pressure only significant for gas phase and in deep waters

6 examples of reactions that can be shown by mass action expressions

Dissociation, acid-base, complexation, redox, gas solution, precipitation-dissolution

Define Henry's law

The mass of gas dissolved by a given volume of liquid at a fixed temperature is proportional to the partial pressure of gas

K for a precipitation-dissolution reaction is called a...

Solubility product Ksp

What happens in the mass action equations when k values are obtained from any set of standard tables?

The effective concentrations of an pure solid or liquid are omitted

What happens when the concentration product is larger than Ksp?

Solution is supersaturated, non-equilibrium

What happens when the concentration product is the same as Ksp?

Solution is saturated, equilibrium

What happens when the concentration product is smaller than Ksp?

Solution is undersaturated, non-equilibrium

Define an acid

An acid is a substance that can donate a proton to any other substance in aqueous solution (i.e. a proton donor

Define a base

A base is a substance that can accept a proton from another substance in aqueous solution (i.e. proton acceptor)

Define amphoteric and example

displays both acid and base properties (HCO3-)

Define polyprotic acids and example

can donate more than one proton e.g. H2SO4

Define polyprotic base and example

can accept more than one proton e.g. OH-

What is K for strong acid and bases

infinite

What is K for weak acid and bases

small (e.g. 10^-6.5)

Give the equation for working out pH from hydrogen ions

pH = -log10 [H+]

The net electric charge of neutral waters is...

0, anions = cations

Write out mass action expression for dissociation of water

k = [H+] [OH-]

Effect of pH on reactions - protons and equilibrium

proton transfer usually very rapid in aqueous solution, equilibrium concepts applicable where dissolved species involved.

2 main sources of CO2 for water

Respiration and the atmosphere

K1 for the dissociation of carbonic acid

k1 = [HCO3-] [H+] / [H2CO3]