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Química Geral · Atoms, moles and stoichiometry

Nomenclatura de compostos inorgânicos

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O que você vai aprender

  • Write the formula of an ionic compound from its name by balancing ion charges, and name an ionic compound from its formula.
  • Use Roman numerals for metals that form more than one cation, working out the metal's charge from the anions.
  • Recognise the common polyatomic ions and the -ate/-ite and per-/hypo- pattern of oxoanions.
  • Name binary molecular compounds with Greek prefixes and write their formulas from names.
  • Name binary acids and oxoacids and connect each oxoacid to its anion.
  • Name hydrates, find the number of waters of hydration from heating data, and classify compounds as ionic or molecular.

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Ionic compounds: name the ions, balance the charges

An ionic compound is named cation first, anion second. A monatomic cation keeps the element name (sodium, calcium, aluminium); a monatomic anion takes the stem plus -ide (chloride, oxide, nitride, phosphide). Subscripts are not named, because the charges fix them: the formula is the smallest whole-number ratio of ions that makes the total charge zero. Calcium phosphate combines Ca²⁺ with phosphate, PO₄³⁻ (drawn): three Ca²⁺ give +6 and two PO₄³⁻ give −6, so the formula is Ca₃(PO₄)₂. Parentheses go around a polyatomic ion whenever more than one of it is needed; a single polyatomic ion takes none (CaCO₃, NH₄Cl). Do not confuse a polyatomic anion with the monatomic one: calcium phosphide is Ca₃P₂.

Metals with more than one charge

Many transition metals and some heavier main-group metals form more than one cation, so their names carry the charge as a Roman numeral in parentheses (the Stock system): FeCl₂ is iron(II) chloride and FeCl₃ is iron(III) chloride. To name a formula, work out the metal's charge from the anions. In Cr₂(SO₄)₃ the three sulfates carry 3 × (−2) = −6, shared by two chromium ions, so each is +3: chromium(III) sulfate. The subscript on the metal is not its charge. Common pairs: Cu⁺/Cu²⁺, Fe²⁺/Fe³⁺, Sn²⁺/Sn⁴⁺, Pb²⁺/Pb⁴⁺, Co²⁺/Co³⁺. Mercury(I) is the two-atom ion Hg₂²⁺, so mercury(I) chloride is Hg₂Cl₂. Metals with only one common charge take no numeral: group 1 (+1), group 2 (+2), aluminium (+3), zinc (+2) and silver (+1).

Polyatomic ions and the oxoanion pattern

Learn these: ammonium NH₄⁺, hydroxide OH⁻, cyanide CN⁻, acetate CH₃COO⁻, nitrate NO₃⁻, nitrite NO₂⁻, sulfate SO₄²⁻, sulfite SO₃²⁻, hydrogen carbonate HCO₃⁻, carbonate CO₃²⁻, phosphate PO₄³⁻, permanganate MnO₄⁻, chromate CrO₄²⁻ and dichromate Cr₂O₇²⁻. In an oxoanion pair the -ate ion has one more oxygen than the -ite ion, with the same charge. Chlorine shows the full series: perchlorate ClO₄⁻, chlorate ClO₃⁻, chlorite ClO₂⁻ and hypochlorite ClO⁻, all with charge −1; per- adds one oxygen to the -ate ion and hypo- removes one from the -ite ion. Bromine and iodine follow the same pattern (bromate BrO₃⁻, hypoiodite IO⁻). Sulfate and sulfite are drawn side by side: same charge, one oxygen apart.

Binary molecular compounds: Greek prefixes

Two nonmetals share electrons and form molecules, and nothing about charge fixes the ratio, so the name states it with prefixes: mono- 1, di- 2, tri- 3, tetra- 4, penta- 5, hexa- 6, hepta- 7, octa- 8, nona- 9, deca- 10. The first element gets a prefix only when there is more than one of it; the second element always gets one and ends in -ide. N₂O₅ is dinitrogen pentoxide, CO is carbon monoxide, SF₆ is sulfur hexafluoride and P₄O₁₀ is tetraphosphorus decoxide. The a or o of a prefix is usually dropped before oxide (pentoxide, monoxide); pentaoxide and decaoxide are also accepted. The element written first is the less electronegative one, so fluorine always comes last (OF₂, oxygen difluoride) and oxygen comes after chlorine (Cl₂O, dichlorine monoxide). Hydrogen compounds are the usual exceptions: HCl is hydrogen chloride without prefixes, and NH₃ (ammonia), CH₄ (methane) and H₂O (water) keep common names and traditional formula order. Never use prefixes for ionic compounds such as CaCl₂ (calcium chloride).

Acids and oxoacids

An acid is named from its anion. A binary acid, H plus an -ide anion dissolved in water, takes hydro-…-ic acid: HCl(aq) is hydrochloric acid and H₂S(aq) hydrosulfuric acid. As a pure gas, HCl is hydrogen chloride. Cyanide is polyatomic but ends in -ide, so HCN(aq) is hydrocyanic acid. An oxoacid contains H, O and another element and never takes hydro-: an -ate anion gives an -ic acid and an -ite anion an -ous acid, while per- and hypo- stay. Nitrate gives nitric acid, HNO₃ (drawn); nitrite gives nitrous acid, HNO₂; perchlorate gives perchloric acid, HClO₄; hypochlorite gives hypochlorous acid, HClO. The number of H atoms equals the size of the anion's charge: sulfate SO₄²⁻ gives sulfuric acid H₂SO₄, phosphate PO₄³⁻ gives phosphoric acid H₃PO₄.

Hydrates

A hydrate holds a fixed number of water molecules in its crystal per formula unit, written after a raised dot and named with a Greek prefix plus -hydrate: CuSO₄·5H₂O (drawn as its ions and waters) is copper(II) sulfate pentahydrate, and MgSO₄·7H₂O is magnesium sulfate heptahydrate. The dot means the waters belong to every formula unit, so CuSO₄·5H₂O contains 10 H atoms and 9 O atoms. Heating drives off the water and leaves the anhydrous salt. With Cu 63.55, S 32.07, O 16.00 and H 1.008 g/mol, CuSO₄·5H₂O has M = 159.62 + 5 × 18.016 = 249.70 g/mol and is 90.08 ÷ 249.70 = 36.08% water by mass. To find x in a hydrate, convert the mass of water lost and the mass of anhydrous salt left into moles and take their ratio.

Ionic or molecular?

Decide first, because the two classes are named by different rules. A compound of a metal and a nonmetal, or one that contains a polyatomic ion, is ionic: NaCl, Fe₂O₃, KNO₃. A compound made only of nonmetals is molecular (CO₂, PCl₃, N₂O), with one important exception: ammonium salts such as NH₄Cl contain only nonmetals but are ionic, built from NH₄⁺ and Cl⁻ ions. A salt of a polyatomic ion has both kinds of bonding: ionic attraction between the ions and covalent bonds inside the polyatomic ion, such as the N–O bonds of nitrate in KNO₃.

What to carry forward

Classify first. Ionic: cation then anion, no prefixes, subscripts from charge balance, and a Roman numeral for any metal with more than one common charge, worked out from the anions. Molecular: Greek prefixes, no mono- on the first element, less electronegative element first. Acids: hydro-…-ic for -ide anions, -ate → -ic and -ite → -ous for oxoacids, with per- and hypo- kept. Hydrates: a prefix plus -hydrate, and every water counts in each formula unit.

Problemas de prática (40)

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Preveja o produto 9

Ionic compounds and metal charges

  • Formula of ammonium sulfateWrite the formula of a salt made of two polyatomic ions, using parentheses where needed.
  • Formula of calcium phosphateWrite the formula of an ionic compound containing a polyatomic anion by balancing the ion charges.
  • Formula of lead(IV) sulfateTurn a Roman numeral into a cation charge and combine it with a polyatomic anion.
  • Formula of mercury(I) nitrateWrite the formula of a mercury(I) salt, recognising that mercury(I) is the diatomic ion Hg₂²⁺.

Acids and oxoanions

  • Acid of the chromate ionApply the anion-to-acid rules to a transition-metal oxoanion.
  • Acid of the iodate ionName and write the oxoacid that corresponds to a given -ate anion of a halogen.
  • Formula of sulfuric acidWrite the formula of an -ic oxoacid from the charge and formula of its -ate anion.

Binary molecular compounds

  • Formula of dinitrogen trioxideWrite the formula of a binary molecular compound from its prefix name.

Hydrates and ionic vs molecular

  • Formula of sodium carbonate decahydrateWrite the formula of a hydrate from its name, balancing the salt and adding the waters.

Classificar 4

Acids and oxoanions

  • Which acid takes hydro-?Classify acids by whether their anion is an -ide ion, including the polyatomic cyanide ion, to decide on the hydro- prefix.

Hydrates and ionic vs molecular

  • Ionic and covalent bonding togetherRecognise that a salt of a polyatomic ion contains both ionic and covalent bonding.
  • Pick the molecular compoundClassify binary compounds as ionic or molecular from the kinds of element they contain.

Binary molecular compounds

  • Which compound gets prefixes?Classify compounds as ionic or molecular to decide whether Greek prefixes belong in the name.

Comparar 3

Acids and oxoanions

  • Oxygen atoms in bromine oxoacidsUse the hypo-, -ous, -ic and per- pattern to rank a halogen's oxoacids by oxygen count without being given formulas.

Hydrates and ionic vs molecular

  • Rank hydrates by percent waterConvert hydrate names into formulas and rank them by mass percent of water.

Binary molecular compounds

  • Oxygen atoms in nitrogen oxidesDecode the prefixes of several nitrogen oxide names and rank them by oxygen atoms per molecule.

Calcular 9

Acids and oxoanions

  • Charge of arsenateWork out the charge of an oxoanion from the number of acidic hydrogens in its neutral acid.
  • Oxygen atoms in iron(III) perchlorateCombine a Roman numeral and a per- oxoanion into a formula, then count the atoms it implies.

Binary molecular compounds

  • Atoms in tetraphosphorus decoxideDecode both prefixes of a molecular name to count the atoms in one molecule.
  • Molar mass from a prefix nameConvert a prefix name into a molecular formula and use it to compute a molar mass.

Hydrates and ionic vs molecular

  • Water of hydration in barium chlorideDetermine the number of waters in a hydrate from the mass lost on heating.
  • Hydrogen atoms in a heptahydrateWrite a hydrate formula from its name and count the atoms contributed by the water of hydration.
  • Percent water in cobalt(II) chloride hexahydrateCalculate the mass percent of water in a hydrate named with a Roman numeral and a hydrate prefix.

Ionic compounds and metal charges

  • Roman numeral in Cr₂(SO₄)₃Find the charge of a transition-metal cation from the charge of the anions in its formula.
  • Ions in one formula unit of Al₂(SO₄)₃Read a formula containing parentheses to count the ions in one formula unit.

Encontre o erro 3

Acids and oxoanions

  • Fix the name of HClO₃(aq)Find the error when a binary-acid prefix is applied to an oxoacid.

Binary molecular compounds

  • Fix the name of Cl₂O₇Find the error in a student's prefix name for a binary molecular oxide.

Ionic compounds and metal charges

  • Spot the mismatched nameMatch common polyatomic ions to their formulas and find a misnamed salt.

Interprete as evidências 8

Ionic compounds and metal charges

  • Name CuClName a binary ionic compound of a metal with two common charges using a Roman numeral.
  • Name FePO₄Name a transition-metal salt with a one-to-one formula, deducing the Roman numeral from the anion charge.
  • Name Mg(NO₃)₂Name an ionic compound of a group 2 metal and an oxoanion from its formula.

Hydrates and ionic vs molecular

  • Name CuSO₄·5H₂OName a hydrate of a transition-metal salt, including the Roman numeral and the hydrate prefix.

Acids and oxoanions

  • Name HBr dissolved in waterName a binary acid from the formula of its aqueous solution.
  • Name HNO₂Name an oxoacid from its formula by identifying its -ite anion.
  • Name NaIO₄Name a salt of a less familiar halogen oxoanion by analogy with the chlorine series.

Binary molecular compounds

  • Name SF₆Name a binary molecular compound from its formula with Greek prefixes.

Trabalhe de trás para frente 1

  • From sodium selenate to iron(III) selenateWork backwards from a known salt to the charge of an unfamiliar polyatomic ion, then use it in a new formula.

Explique por quê 3

Hydrates and ionic vs molecular

  • Why is NH₄Cl ionic?Explain why an ammonium salt is classified and named as ionic although it contains only nonmetals.

Binary molecular compounds

  • Why OF₂ but Cl₂O?Explain the order of elements in binary molecular formulas and names using electronegativity.

Acids and oxoanions

  • Hydrogen chloride or hydrochloric acid?Explain why the same formula, HCl, has one name as a gas and another in water.

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