Inorganic acids containing phosphorus and oxygen and most of the salts of these acids are composed o — Organic Chemistry Chemistry Question
Inorganic phosphates: From solution to crystals
Inorganic acids containing phosphorus and oxygen and most of the salts of these acids are composed of oxygen tetrahedra, each with the phosphorus atom in the center. The tetrahedra can either be isolated or share an oxygen atom so being linked by means of P–O–P bridges.
Draw the structure of the anions present in the neutral salts of the following acids: H3PO4, H3PO3, H3PO2.
Model Answer
[VISUAL]
In neutral salts, the corresponding anions are: phosphate (PO4^3-), phosphite (HPO3^2-), and hypophosphite (H2PO2^-). Phosphite and hypophosphite contain P-H bonds directly, and only O-bonded hydrogen atoms are acidic.
For the series of acids above, reveal the trends in:
1) acidity of the substances (compare the values of pKa1),
2) O–P–O valence angle.
Model Answer
- Strength of the acids decreases from H3PO2 to H3PO4, i.e. pKa1 increases in this sequence. The explanation is based on the fact that one O-terminated side of each POn-tetrahedron with double bond P=O (shifting electron density from protons in P–OH groups due to inductive effect) acts on three P–OH groups in phosphoric acid and only on the sole P–OH group in the case of phosphinic (hypophosphorous) acid.
- According to the Valence Shell Electron Pair Repulsion (VSEPR) theory O–P–O angle decreases in the same sequence. This is due to different polarity of P–O and P–H bonds (it is apparent from the values of Pauling’s electronegativity ΧP for these three atoms ΧP(H) = 2.20, ΧP(P) = 2.19 and ΧP(O) = 3.44). This fact stipulates partial negative charge δ– at oxygen atoms and almost δ = 0 in the case of hydrogen atoms. Thus, the P–O bonds endure higher repulsion from each other than from P–H bonds, and to a first approximation we can ignore the P–H bonds in our consideration. Then the following strong repulsive bonds for the above acids should be taken into account: one P=O and one P–OH for H3PO2, one P=O and two P–OH for H3PO3, and one P=O and three P–OH for H3PO4.
The formula of metaphosphoric acid can be written as (HPO3)n. This acid is composed of the phosphorus-oxygen tetrahedra either. Suggest the structure of this compound assuming the minimal number of phosphorus atoms in its molecule.
Model Answer
Three tetrahedra linked through the common vertices; protons are attached to one oxygen atoms in each tetrahedron so that CN(O)OH = 2.
[VISUAL]
In a species with two phosphorus atoms two tetrahedra should share an edge which contradicts the initial assumption that each two adjacent tetrahedra have one shared oxygen atom. Thus, minimal amount of P-atoms is equal to three. It corresponds to cyclo-trimetaphosphoric acid.
To estimate the relative charge of atoms in PnOk (2k–5n)– anion, let us define a special secondary parameter Ai of an atom i as the oxidation number of this atom, Zi, divided by its coordination number, CNi,:
Ai = Zi / CNi
The sum of the oxidation number (ZN) of an atom N (for instance, phosphorus atom) and Ai values for the atoms forming the coordination environment (for instance, oxygen atoms) of the atom N gives the relative charge Q(N) of the atom N:
Q(N) = ZN + sum_{i=1}^{CN} Ai
Calculate Qm(P) for the PO4 tetrahedron considering m = 1, 2, 3 and 4 of its oxygen atoms being shared with neighboring PO4-tetrahedra.
Model Answer
m = 1: Q1(P) = (–2/1)·3 + (–2/2)·1 + 5 = –2
m = 2: Q2(P) = (–2/1)·2 + (–2/2)·2 + 5 = –1
m = 3: Q3(P) = (–2/1)·1 + (–2/2)·3 + 5 = 0
m = 4: Q4(P) = (–2/1)·0 + (–2/2)·4 + 5 = +1
Perform similar calculations for TO4-tetrahedra linked through the common vertices, where:
1) T = Si,
2) T = S.
Model Answer
1) T = Si:
m = 1: Q1(Si) = (–2/1)×3 + (–2/2)×1 + 4 = –3
m = 2: Q2(Si) = (–2/1)×2 + (–2/2)×2 + 4 = –2
m = 3: Q3(Si) = (–2/1)×1 + (–2/2)×3 + 4 = –1
m = 4: Q4(Si) = (–2/1)×0 + (–2/2)×4 + 4 = 0
2) T = S:
m = 1: Q1(S) = (–2/1)×3 + (–2/2)×1 + 6 = –1
m = 2: Q2(S) = (–2/1)×2 + (–2/2)×2 + 6 = 0
m = 3: Q3(S) = (–2/1)×1 + (–2/2)×3 + 6 = +1
m = 4: Q4(S) = (–2/1)×0 + (–2/2)×4 + 6 = +2
Let us suppose that a tetrahedron with the minimal absolute value of Qm(P) is the most stable towards hydrolysis.
Which value of m corresponds to the phosphorus-oxygen tetrahedron the most stable towards hydrolysis?
Model Answer
m = 3
Which value of m corresponds to the TO4 tetrahedron (T = Si, S) the most stable towards hydrolysis?
Model Answer
m(Si) = 4, m(S) = 2 according to the assumption.
Isolated phosphorus-oxygen tetrahedra (without P–O–P bonding) can be found in crystalline substances. Mixed phosphates (V) MaPOb are known to be composed of PO4 and MO4 tetrahedra with each oxygen atom having the same number of M and P atoms coordinated to it.
Determine the Q(O) value for such compounds.
Model Answer
Since the bonds between M and P are missing, the following equality is to be fulfilled: CNO×b = (a+1)×4, therefore CNO = (a+1)×4/b.
The M to P ratio in an oxygen surrounding, n(M) : n(P), is a : 1, then, the number of atoms of M and P in the coordination sphere of O is:
n(M) = a / (a+1)×CNO = a / (a+1)×(a+1)×4 / b = 4 a / b, n(P) = 4/b,
Q(O) = (5/4)×(4/b) + (Z/4)×(4a/b) + (–2) = (–2 b + 5 + Z a)/b
where Z is the oxidation number of M.
The condition of the charge balance for MaPOb requires that –2×b + 5 + Z×a = 0.
Therefore, Q(O) = 0.
Suggest possible empirical formulas for such compounds.
Model Answer
According to the result above, n(P) = 4/b. Therefore, the number of phosphorus atoms in the oxygen coordination sphere, n(P), can be 1, 2 or 4 since b is an integer. Note that stoichiometry «MaPO» and «MaPO2», b = 1 and 2 respectively, is not possible for a phosphorus atom in the oxidation state +5. Hence, b = 4.
From the condition of the charge balance, –8 + 5 + Z×a = 0. Solving this equation in integers gives Z = +3 (a = 1) or Z = +1 (a = 3). Indeed, the empirical formulas MPO4 and M3PO4 correspond to known compounds such as AlPO4 and Li3PO4. Note that the condition of oxygen atom equivalence is fulfilled here.
Fluorapatite Ca5(PO4)3F is a constituent of human teeth. It can be synthesized using a double-diffusion method with a gelatin membrane separating solutions containing F–, HPO4 2–, and Ca2+ ions. The synthesis leads to a hybrid material – bioorganic polymer/inorganic phosphate, resembling tooth (or bone) tissue.
Give a reasonable composition of two solutions placed on different sides of the gelatin membrane, that allow preparation of fluorapatite as the target substance in this double-diffusion experiment.
[VISUAL]
Model Answer
Since Ca2+ ions when combined with either NaF or Na2HPO4 solutions give precipitates, it is advisable to separate solutions containing calcium cations and phosphate/fluoride anions with the membrane.
Option 1:
Solution 1: Ca(NO3)2
Solution 2: NaF + Na2HPO4
Option 2:
Solution 1: Ca(NO3)2 + NaF
Solution 2: Na2HPO4
(The target concentrations are 5 mM Ca(NO3)2, 1 mM NaF, 3 mM Na2HPO4).
Write down the balanced equation of the reaction described above leading to fluorapatite.
Model Answer
With pH adjustment to alkaline range:
10 Ca(NO3)2 + 2 NaF + 6 Na2HPO4 + 6 NaOH → Ca10(PO4)6F2↓ + 20 NaNO3 + 6 H2O
Without pH adjustment (leads to acidification):
10 Ca(NO3)2 + 2 NaF + 6 Na2HPO4 → Ca10(PO4)6F2↓ + 14 NaNO3 + 6 HNO3
Calculate the osmotic pressure acting on the membrane at the beginning of this experiment (25 °C, activity of all ions is equal to 1).
Model Answer
Dissociation of Ca(NO3)2 and Na2HPO4 gives 3 ions each, and NaF gives 2 ions. The overall concentration of ions at both sides of the membrane is:
c = 5×10–3 × 3 + 1×10–3 × 2 + 3–3 × 3 = 2.6×10–2 mol dm–3 = 26 mol m–3.
p = c R T = 26 mol m–3 × 8.31 J mol–1 K–1 × 298 K = 6.44×10^4 Pa.