IB Docs (2) Team Language of Chemistry
Introduction
Consider the following question which you could give your Higher Level students after they have finished studying Topics 7 & 17 : Equilibrium.
Hexaaquacopper(II) ions, [Cu(H2O)6]2+, undergo a spontaneous ligand displacement reaction with aqueous ammonia. During the process four of the water ligands are replaced by ammonia ligands.
(i) Write the equation for the reaction.
(ii) State the expression for the equilibrium constant, Kc.
(iii) At 298 K the value for the equilibrium constant is 1.98 x 1013.
Suggest with an explanation whether this value will be higher or lower at 323 K.
Here are the answers.
(i) [Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(H2O)2(NH3)4]2+(aq) + 4H2O(l)
(ii) [ [Cu(H2O)2(NH3)4]2+(aq) ]
Kc = ――――――――――
[ [Cu(H2O)6]2+(aq) ] x [NH3(aq)]4
(iii) The reaction is spontaneous and the entropy change will be small so the enthalpy change for the reaction is very likely exothermic. Hence, as the temperature increases the position of equilibrium will shift to the reactant side. The value for Kc will, therefore, be lower than 1.98 x 1013 at the higher temperature of 323 K.
The question is reasonably hard but does not contain any chemistry that is not on the syllabus. For those that have some difficulties it is worth considering whether it is their actual understanding of chemistry that is the problem, or whether the real reason why they are having difficulties is that they simply do not understand the language the question is phrased in.
Look at the question again and consider the parts highlighted in yellow. Each one requires specific knowledge about how chemists communicate with each other.
Hexaaquacopper(II) ions, [Cu(H2O)6]2+, undergo a spontaneous ligand displacement reaction with aqueous ammonia. During the process four of the water ligands are replaced by ammonia ligands.
(i) Write the equation for the reaction.
[Cu(H2O)6]2+(aq) + 4NH3(aq) ⇌ [Cu(H2O)2(NH3)4]2+(aq) + 4H2O(l)
(ii) State the expression for the equilibrium constant , Kc.
[ [Cu(H2O)2(NH3)4]2+(aq) ] [H2O(l)]
Kc = ――――――――――
[ [Cu(H2O)6]2+(aq) ] x [NH3(aq)]4
(iii) At 298 K the value for the equilibrium constant is 1.98 x 1013. Suggest with an explanation whether this value will be higher or lower at 323 K.
The reaction is spontaneous and the entropy change will be small so the enthalpy change for the reaction is very likely exothermic. Hence, as the temperature increases the position of equilibrium will shift to the reactant side. The value for Kc will therefore be lower than 1.98 x 1013 at the higher temperature of 323 K.
I only really realised when I started writing chemistry books how difficult and precise the language of chemistry is. We do specifically teach our students some aspects of chemistry language. For example, we teach them how to name organic (e.g. but-2-ene) and inorganic (e.g. iron(II) sulfate) compounds using the IUPAC Stock-naming system. (See separate page on IUPAC). However I believe there are many other aspects of the language of chemistry where we just assume they will ‘pick it up’ as they go along. Students who do not acquire this knowledge of the language of chemistry are at a huge disadvantage and it may well explain some of their difficulties with scoring high marks in the subject. They simply do not understand the question in the first place. We need to place stronger emphasis on our teaching of chemistry language so that all our students can both receive and give information accurately, i.e. they can communicate in chemistry
Some specific points about the language of chemistry
You might like to think of specific language of chemistry points which we expect our students to be able to know and use by the time they take their final examination. The following are some of the ones I have come up with. Ask yourself how many of these you actually teach and how many you expect students to pick up by 'osmosis'.
1. Formulas of substances
Elements containing two atoms in a molecule (diatomic) are written as such but if there are more than two atoms in a molecule of the element (polyatomic) the element is often written as if only one atom is present, e.g.
S + O2 → SO2
In fact S + 2O → SO2 is more consistent and S8 + 8O2→ 8SO2 is more correct.
2. Order of elements in a compound
Usually the more electropositive element comes first, e.g. NaCl and H2O. Logically ammonia and methane should therefore be H3N and H4C respectively.
3. Physical constants
Should be written in italics. e.g. R – the gas constant; K – the equilibrium constant, k – the rate constant.
4. Shorthand notation
[X] is shorthand for ‘the concentration of X’ – although actually it is the activity of X and has no units.
This causes a problem when calculating equilibrium constants for the IB since if X is in mol dm-3 then the equilibrium constant may have units whereas in reality they do not have units.
5. Different meanings of words in English and Chemistry[1]
Words such as strong, spontaneous and reduce have very specific and different meanings in Chemistry compared to everyday English.
6. Words with different meanings within Chemistry
Stable can mean thermodynamically stable or it can mean kinetically stable and should not be used on its own.
7. The Greek alphabet
We assume students know the Greek alphabet as we frequently use Greek letters which have chemical meanings, e.g. Δ, π and σ.
8. The use of commas and full stops
6,290, 6.290 and 6.290 can all mean the same thing and can all mean completely different things.
9. Reciprocal units
Concentration can be expressed as mol dm−3, mol L−1, mol/dm3, mol per litre, mol/l and M. Since miles per hour is often shortened to mph presumably the units of concentration could also be molpl? When putting data in a table reciprocal units can also be given at the top of the column so that the data can just be given as numbers. For example 0 s, 5 s, 10, s, 15 s can be written as 0, 5, 10, 15 if "time / s" is written at the top of the column.
10. Oxidation states/molecular mass
When determining oxidation states chemists assume that covalent compounds are ionic, e.g. CH4 (C= -4; H= +1). However when referring to ‘molecular mass’ ionic compounds cannot be assumed to be molecular so the term cannot be used for ionic compounds and we have to use ‘formula mass’ instead.
11. Organic structures
It is normal to use angles of 90o when drawing the structures of alkanes even though they are tetrahedral in shape with bond angles of 109.5o.
12. Representation of radicals
A chlorine atom is represented by Cl but a chlorine radical (which is also a chlorine atom) is represented by Cl or Cl.. A methyl radical which is neutral is represented by CH3, CH3. or CH3– even though "–" normally means a pair of electrons.
13. Using tautology
The initiation step in free radical substitution reactions involves 'homolytic fission' – this translates literally as 'same breaking breaking'.
14. The difference between oxidation state and oxidation number
For example, in FeO the oxidation state of Fe = +2 and the oxidation number of Fe = (II)
Note that on the old syllabus these were the other way round!
15. Reading from right to left or left to right when numbering organic compounds
HOCH2CH2CH3 and CH3CH2CH2OH are both propan-1-ol
16. Use of upper and lower case for constants
K is the equilibrium constant; k is the rate constant.
17. Symbols with different meanings within chemistry
The use of square brackets, [ ], can have completely different meanings depending upon the context in which they are used. For example, [CH3COO−(aq)] means the concentration or activity of ethanoate ions, [Ar] means the electron configuration of argon, and [Mn(H2O)6]2+ represents the hexahydrated manganese(II) complex ion.
Footnotes
- ^ Some other words that could be included in this list are: degenerate, weak, metathesis, mole, inductive (effect), element, oil, orbital, phase, salt, Contact (process), saturated, shell, and volatile.
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