OCR F335 The Oceans

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F335 Chemistry by Design: The Oceans
Bonding and Structure
(a) explain the hydrogen bonding in water and explain the usual physical properties of water that arise
from this
Hydrogen bonding in water is particularly strong because there are two lone pairs of electrons and two positively
charged hydrogen atoms per oxygen atom, so intermolecular bonds are maximised.
(i) anomalous boiling point among hydrides of Group 6
The Group 4 data shows how we might expect boiling points to
behave. CH 4 has a low boiling point as it only has instantaneous
dipoleinduced dipole bonds (no hydrogen bonding. The points for
H 2O, HF and NH
3 do not follow the pattern that we expect. H
2O can
form twice as many hydrogen bonds as HF and NH 3, therefore H 2O has
a higher boiling point than the two. HF has a higher boiling point than
NH 3 as fluorine is highly electronegative, making the HF bond high
polarised and so molecules can form strong hydrogen bonds.
(ii) specific heating capacity
Specific Heat Capacity ­ the energy needed to raise 1g of that substance through 1K.
Water has a high specific heat capacity ­ it takes a lot of energy to raise its temperature. A lot of energy is needed
to overcome hydrogen bonding within clusters of water molecules ­ energy is not available for increasing the
kinetic energy (raising the temperature).
(iii) enthalpy change of vaporisation
Enthalpy Change of Vaporisation ­ a measure of the energy
needed to overcome the intermolecular bonds of one mole of
molecules from liquid to vapour.
The enthalpy change of vaporisation follows a similar pattern to the
boiling points, and have the same reasons for the pattern.
(iv) density change on melting
If water at room temperature is cooled to 4o C (277K), the water
contracts and its volume decreases, which is expected from most
substances. However, when the temperature approaches 0o C (273K),
the water expands. Water is unusual as it has a lower density when
frozen, which is due to the effects of hydrogen bonding.
The structure of ice has four groups around each oxygen atom
arranged tetrahedrally in three
dimensions. This arrangement of
water molecules maximises the
hydrogen bonding between them, but leads to a very `open' structure with
large spaces in it. Furthermore, ice has more hydrogen bonds than liquid
water and as these bonds are relatively long, it means the water molecules
are further apart. Therefore the density of ice at 273K is less than the density
of water at the same temperature. When ice melts, the open structure
collapses and water molecules fall into some of the open spaces, thus giving
water a greater density.

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F335 Chemistry by Design: The Oceans
(b) explain the factors (including intermolecular bonds and iondipole forces) determining the relative
solubility of a solute in aqueous and nonaqueous solvents and explain the hydration of ions
Bonds will only dissolve if the bonds between the solute and the solvent are stronger than the bonds within the
solute and solvent.
Most covalent substances only dissolve in nonpolar solvents (e.g. hexane).…read more

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F335 Chemistry by Design: The Oceans
When we are dealing with solvents other than water, we use enthalpy of solvation, Hsolv ,
but the same rules apply.
(ii) describe the solution of an ionic solid in terms of an enthalpy cycle involving these terms
Ions must be separated from the lattice before an ionic solid can
dissolve. This means energy must be supplied to overcome the
electrical attraction between oppositely charged ions.…read more

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F335 Chemistry by Design: The Oceans
S surr = - H
(ii) be able to perform calculations using these expressions
Example: Find the total entropy change when water freezes at 10o C (263K), using Ssys = 22.0 J K
mol1 and H
= 6.01 KJ mol .
Ssys = - 22.0 J K -1 mol-1
-1 -1
Ssurr = -(-6.01 KJ263
mol × 1000)
= -(-6010 J mol )
263 = 22.85171... =+ 22.9 J K -1 mol-1
Stotal = - 22.…read more

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F335 Chemistry by Design: The Oceans
Group 0 elements, with a full outer shell of electrons, have high first ionisation enthalpies ­ they are difficult to
ionise and are very unreactive. Group 1 elements, with only one outer shell electron, have low ionisation
enthalpies ­ they are easy to ionise and are very reactive.
Across a period, the ionisation enthalpy increases. Electrons are being added to the shell but, at the same time,
protons are being added to the nucleus.…read more

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F335 Chemistry by Design: The Oceans
Every acid has a conjugate base and every base has a conjugate acid. They are called conjugate acidbase pair.
(k) explain and use the terms strong acid, strong base, writing equations for their ionisation in water
Strong acids have a strong tendency to donate H+
ions ­ the donation of H+ ions is essentially complete.…read more

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F335 Chemistry by Design: The Oceans
pH = - log[H +(aq)]
(i) strong acids
Since the reaction with water goes to completion effectively, the amount in moles of H+ (aq) ions is equal to the
amount in moles of acid (HA) put into a solution.
Example: For a 0.01 mol dm3 solution of a strong acid HA
pH = - log[0.01]
pH = 2
(ii) strong bases, using Kw
Example: Calculate the pH of 0.1 mol dm3 of NaOH (aq).
1.…read more

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F335 Chemistry by Design: The Oceans
(i) explain the meaning of the term buffer
Buffer ­ a solution that can resist changes in pH despite the addition of acid or alkali. Their pH stays
approximately constant even if small amounts of acid or alkali are added.
(ii) explain how buffers work (including in everyday applications)
Buffer solutions are usually made from:
a weak acid and one of its salts, e.g. ethanoic acid and sodium ethanoate.
a weak base and one of its salts, e.g.…read more

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F335 Chemistry by Design: The Oceans
(o) discuss the global influence of the dissolving of carbon dioxide in water, discuss and explain the benefits
and risks associated with various approaches to reducing atmospheric CO 2 levels including: more
economical use of fuels, the use of alternative fuels (including hydrogen), capture and storage of CO2
and increased photosynthesis
The Global Influence of Dissolving Carbon Dioxide in Water:
Carbon dioxide is soluble in water ­ between 13g of it will dissolve in a litre of water, depending…read more


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