Oceans

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  • Created by: LBCW0502
  • Created on: 30-10-16 11:25
What is a cation?
A positively charged ion
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What is an anion?
A negative charged ion
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Ions build up into what type of structure?
Giant ionic lattice
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In a sodium chloride lattice, how many chloride ions surround one sodium ion?
Six (and six sodium ions surround one chloride ion)
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Why does a lattice hold together?
Overall the attractions are stronger than the repulsions (ionic solids are hard and have a high melting and boiling point)
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What happens when ionic substances dissolve in water?
Ions are surrounded by water molecules. Ions are no longer arranged and are scattered through the water (random). Ions are separated and behave independently of each other
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Describe the content of sea water
A mixture of positive and negative ions dissolved in water
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What must happen before the ionic compound can dissolve in water?
Ions must be separated from the lattice so they can spread out in solution (endothermic process)
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Hydration of ions in solution is an example of which type of reaction?
Exothermic reaction
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What is lattice enthalpy?
The enthalpy change when one mole of a solid is formed by the coming together of the separate ions. When ions are separated they are seen as being in a gaseous state (solid state as a lattice)
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Are all large lattice enthalpy quantities positive or negative?
Large negative quantities (to break down a lattice, you have to put in energy which becomes a positive quantity)
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What are the two reasons for a very negative lattice enthalpy?
The ionic charges increase and the ionic radii decrease
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How can these two factors be summarised?
Lattice enthalpy becomes more negative for ions with greater charge density. Ions with a higher charge attract one another more strongly (electrostatic interactions increase). Attract strongly when closer together. Ions with smaller radii are closer
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How does attraction link to the negativity of the lattice enthalpy?
Stronger attractions mean more negative lattice enthalpies
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Substances with large negative lattice enthalpies are soluble or insoluble?
Usually insoluble
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Why are the covalent bonds in water polar?
There is a difference in electronegativity between oxygen and hydrogen. The water molecule has a bent shape and the whole molecule is polar (behaves as a tiny dipole)
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What are ion-dipole interactions?
Tiny charges on the water molecules are attracted to charges in the ions. This happens on the surface of an ionic solid placed in water so ions become separated from lattice and are in solution
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Describe what happens when ions in a solution are hydrated
In solution, positive ions are surrounded by water molecules with the negative end of the dipole facing towards them. Negative ions surrounded by water molecules with positive end of dipole facing towards them
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What determines whether or not ions are extensively hydrated?
Water molecules bind weakly to some ions (not extensively hydrated). Other ions are extensively hydrated and bind strongly to water molecules
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How does the charge density of an ion affects the size of the hydrated ion?
The higher the charge density of the ion, the more water molecules it attracts and the bigger the hydrated ion (ion has an effect on all the water molecules surrounding it, not just the closest ones)
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What type of bonds are present in water molecules?
Hydrogen bonds
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What is the enthalpy change of hydration?
The enthalpy change for the formation of a solution of ions from one mole of gaseous ions
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What do the enthalpies of hydration depend on?
The concentration of solution produced
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Why are the enthalpies of hydration always negative?
Hydration is exothermic and energy is given out
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Most exothermic values occur for the ions with what features?
The greatest charge and the smallest radii (small, highly charged ions can attract water molecules strongly)
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When is the enthalpy change of solvation used?
When dealing with solvents other than water (molecules of some other solvents e.g. ethanol are also polar and can bind to ions)
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What is the enthalpy change of solution?
The difference between the enthalpy changes of hydration of the ions and the lattice enthalpy. The enthalpy change when one mole of a solute dissolves to form a very dilute solution. This can be measured experimentally
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Which methods can be used to find the enthalpy change of solution?
Enthalpy cycles and enthalpy level diagrams
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Are ionic solids soluble or insoluble in non-polar solvents (e.g. hexane)?
Ionic solids are insoluble in non-polar solvents (non-polar solvents have no regions of slight positive/negative charge so they are unable to interact strongly with ions)
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What is the temperature of the surface of the Sun?
6000 K
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The Sun radiates which types of radiation?
Ultraviolet, visible and infrared regions
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What is the Earth's average surface temperature?
285 K
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What happens to the energy from the Sun travelling towards Earth?
Part of the energy is absorbed by the Earth and its atmosphere and part is reflected back into space. The part that is absorbed helps to heat the Earth and Earth radiates energy back into space
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When is a steady state reached?
Where the Earth is radiating energy as fast as it absorbs it, and the average temperature of the Earth remains constant
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Give two examples of greenhouse gases
Carbon dioxide and methane
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What happens when infrared radiation is absorbed by greenhouse gases
Increases vibrational energy of molecules, bonds vibrate, vibrational energy transferred to other molecules in the air (increases kinetic energy/raises temperature of air). Some IR re-emitted by molecules (towards Earth or space)
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What do scientists believe is the main the cause of global warming?
Increasing levels of carbon dioxide and methane in the atmosphere
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What is the most abundant greenhouse gas in the atmosphere?
Water vapour
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What is the IR window?
Wavelengths of infrared radiation that water vapour does not absorb
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Which combined effects ensures that water vapour helps to keep the temperature balance of the Earth?
Combined effects of absorption and the IR window
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How is the level of water vapour in the atmosphere dependent on temperature?
The higher the temperature, the higher the level of water vapour in the atmosphere
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What is the main cause of increased levels of greenhouse gases?
Human activity e.g. combustion from vehicle engines
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How can we reduce carbon dioxide levels?
Reduce emissions of gases
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What is the general definition of an acid?
A substance which donates H+ in a chemical reaction
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What is a base?
A substance that accepts the H+
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Describe the Brønsted–Lowry theory of acids and bases
An acid is a H+ donor and a base in a H+ acceptor
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What is a conjugate base?
A negative ion in an acid (e.g. ethanoate ion) acts as a base
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What it is a conjugate acid?
Formed when H+ bonds to a conjugate base
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Which compound can act as both an acid and a base?
Water (depends on what the water is reacting with)
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What is produced when water reacts with hydrochloric acid?
An oxonium ion (H3O+) and a chloride ion (Cl-)
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What does pH measure?
The power of H+ (aq)
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How is pH defined?
-log10 [H+ (aq)]
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What happens when [H+ (aq)] changes by a factor of 10?
The pH changes by 1 (logarithmic scale)
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What are strong acids?
Compounds with a strong tendency to donate H+ and dissociate completely in aqueous solution
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What are weak acids?
Compounds with a weak tendency to donate H+ and does not dissociate completely when dissolved in water
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What can be used to calculate [H+]?
The equilibrium constant (also called acidity constant - Ka)
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Which assumptions must be made when calculating [H+]?
[H+] = [A-] and the amount of HA at equilibrium is equal to the amount of HA put into the solution
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What happens to the value of Ka when the acid becomes weaker?
The value for Ka gets smaller
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What happens to the value of pKa when the acid becomes weaker?
The value of pKa becomes greater
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Which special equilibrium constant is used to calculate the pH of a strong alkali?
The ionisation product of water (Kw)
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How is an acidic solution defined?
[H+(aq)] > [OH- (aq)]
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How is an alkaline solution defined?
[H+(aq)] < [OH- (aq)]
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What are buffer solutions?
Solutions that can resist changes in pH, despite the addition of small quantities of acid or alkali
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What are buffer solutions usually made from?
A weak acid and one of its salts (e.g. ethanoic acid and sodium ethanoate) or a weak base and one of its salts (e.g. ammonia solution and ammonium chloride)
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What assumptions are made in order to explain how buffer solutions made from a weak acid and one of its salts would resist changes in pH?
All A- ions come from the salt and almost all the HA molecules put into the buffer remain unchanged
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What happens when a small amount of acid is added to a buffer solution?
Some A- ions from the salt react with extra H+ ions to form HA and water. This removes the H+ ions from the solution and the pH is re-established
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What happens when a small amount of alkali is added to a buffer solution?
H+ ions are removed from the solution. H+ ions can be regenerated from the acid HA and the pH is re-established
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Why is the presence of both a weak act and its salt necessary for a buffer to work?
There must be plenty of HA to act as a source of extra H+ ions when they are needed and plenty of A- ions to act as a sink for any extra H+ ions which have been added
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The pH of the buffer solution depends on which two factors?
The value of Ka and the ratio of [salt] : [acid]
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The choice of a particular weak acid is determined by what?
Which region of the pH range the buffer is in
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What happens when water is added to the buffer solution?
Concentrations of both the salt and the acid are reduced equally so the pH remains unchanged
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What is Ksp?
Solubility product. Represents the conditions for equilibrium between a sparingly soluble solid and its saturated solution
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What is the value of Ksp when solid calcium carbonate is at a equilibrium with its solution at 298K?
5.0 x 10-9 mol^2dm^-6
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How can you use Ksp to predict whether a precipitate will form from a solution
If a dissolved [Ca 2+ (aq)] multiplied by dissolved [CO3 2- (aq)] it gives a value in excess of Ksp (CaCO3 will precipitate). If the value is smaller/equal to Ksp, ions stay in solution
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What is entropy?
A measure of the number of ways of arranging molecules and distributing energy
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What do positive values of ΔS indicate?
Products are more disordered than reactants
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What do negative values of ΔS indicate?
Products are less disordered than reactants
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Which substances have higher entropies?
Gases have higher entropies than liquids (liquids have higher entropies than solids)
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What are the units of entropy?
J K-1 mol-1
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Which two features result in a higher entropy?
A collection of molecules has a higher entropy if: the molecules are spread out more and the energy is shared among more molecules
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What do you need to consider then predicting whether or not a change is feasible?
Entropy changes to the system and its surroundings
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What is the difference between a chemical system and the surroundings?
A chemical system is the products and reactants. The surroundings are everything else
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Is freezing an exothermic or endothermic process?
Exothermic process
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Why can you not work out how much each individual substance in the surroundings has increased in entropy?
It is impossible to say exactly how the energy has been shared out
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How do you convert from degrees Celsius to Kelvin?
Add 273
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How do you convert ΔH in kJ mol-1 to J mol-1?
Multiply by 1000
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