* Elements, Compounds, Mixtures - Part 3 Writing Balanced Equations, deducing formula from valency at Doc B's
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3.1 THE
CONSTRUCTION OF CHEMICAL EQUATIONS
"How to write and understand chemical equations" (3rd draft) Seven equations are presented, but approached in the following way
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3.1a
Chemical
Symbols and Formula
For any reaction, what you start with are called the reactants, and what you form are called the products.
It is most important you read about formula in an earlier section of this page. Empirical formula and molecular formula are dealt with on another page. In the equations outlined below several things have been deliberately simplified. This is to allow the 'starter' chemistry student to concentrate on understanding formulae and balancing chemical equations. Some teachers may disagree with this approach BUT my simplifications are:
==> means the direction of change from reactants == to ==> products No symbols or numbers are used in word equations. Always try to fit all the words neatly lined up from left to right, especially if its a long word equation.
Writing the correct symbol or formula for each equation component.
Numbers before a formula double or treble it etc.
Using numbers if necessary to balance the equation. If all is correct, then the sum of atoms for each element should be the same on both side of the equation arrow .....
PRACTICE QUESTIONS - on words and symbol equations (on other web pages)
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3.1d EXAMPLES of CONSTRUCTING WORD or SYMBOL EQUATIONS 3.1d(1)
A single symbol means an
uncombined single atom of the element,
iron + sulphur ==> iron sulphide
on average one atom of iron chemically combines with one atom of iron forming one molecule of iron sulphide two elements chemically combining to form a new compound Fe(s) + S(s) ==> FeS(s) Atom balancing, sum left = sum right: 1Fe + 1S = (1Fe combined with 1S) All the reactants (what you start with) and all the products (what is formed) are all solids in this case. When first learning symbol equations you probably won't use state symbols like (s) at first (see end note). |
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3.1d(2)
sodium hydroxide + hydrochloric acid ==> sodium chloride + water
the reactants are one molecule of sodium hydroxide and one molecule of hydrochloric acid the products are one molecule of sodium chloride and one molecule of water all chemicals involved are compounds NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(l) atom balancing, sum left = right: (1Na + 1O + 1H) + (1H + 1Cl) = (1Na + 1Cl) + (2H + 1O) |
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3.1d(3)
magnesium + hydrochloric acid ==> magnesium chloride + hydrogen
one atom of magnesium reacts with two molecules of hydrochloric acid the products are one molecule of magnesium chloride and one molecule of hydrogen Mg and H-H are elements, H-Cl and Cl-Mg-Cl are compounds Mg(s) + 2HCl(aq) ==> MgCl2(aq) + H2(g) atom balancing, sum left = right: (1Mg) + 2 x (1H + 1Cl) = (1Mg + 2Cl) + (2H) |
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3.1d(4)
H2O (example 2)
copper carbonate + sulphuric acid ==> copper sulphate + water + carbon dioxide
the reactants are one formula of copper carbonate and one molecule of sulphuric acid the products are one formula of copper sulphate, one molecule of water and one molecule of carbon dioxide all molecules are compounds in this reaction CuCO3(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l) + CO2(g) balancing sum left = sum right: (1Cu + 1C + 3O) + (2H + 1S + 4O) = (1Cu + 1S + 4O) + (2H + 1O) + (1C + 2O)
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3.1d(5)
CO2 (see also example 4)
methane + oxygen ==> carbon dioxide + water
Using displayed formula the equation would look like this ...
... in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecules. one molecule of methane is completely burned by two molecules of oxygen to form one molecule of carbon dioxide and two molecules of water CH4(g) + 2O2(g) ==> CO2(g) + 2H2O(l) atom balancing, sum left = sum right: (1C + 4H) + 2 x (2O) = (1C + 2O) + 2 x (2H + 1O) |
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3.1d(6)
magnesium hydroxide + nitric acid ==> magnesium nitrate + water
one formula of magnesium hydroxide reacts with two molecules of nitric acid to form one formula of magnesium nitrate and two molecules of water (all compounds) Mg(OH)2(aq) + 2HNO3(aq) ==> Mg(NO3)2(aq) + 2H2O(l) atom balancing, sum left = sum right: (1Mg + 2O + 2H) + 2 x (1H + 1N + 3O) = (1Mg + 2N + 6O) + 2 x (2H + 1O) |
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3.1d(7)
aluminium oxide + sulphuric acid ==> aluminium sulphate + water
one formula of aluminium oxide reacts with three molecules of sulphuric acid to form one formula of aluminium sulphate and three molecules of water note the first use of numbers (3) for the sulphuric acid and water! so picture three of them in your head, otherwise the picture gets a bit big! Al2O3(s) + 3H2SO4(aq) ==> Al2(SO4)3(aq) + 3H2O(l) atom balancing, sum left = sum right: (2Al + 3O) + 3 x (2H + 1S + 4O) = (2Al + 3S + 12O) + 3 x (2H + 1O) GCSE-AS-A2-IB note: Aluminium sulfate is actually an ionic compound (Al3+)2(SO42-)3 |
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NOTE 1:
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Note 2 on the state symbols X(?) of reactants or products in equations (g) means gas, (l) means liquid, (s) means solid and (aq) means aqueous solution or dissolved in water e.g. carbon dioxide gas CO2(g), liquid water H2O(l), solid sodium chloride 'salt' NaCl(s) and copper sulphate solution CuSO4(aq) |
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3.1e IONIC EQUATIONS
(for higher GCSE and AS students)
In many reactions only certain ions change their 'chemical state' but other ions remain in exactly the same original physical and chemical state. The ions that do not change physically or chemically are called 'spectator ions'. The ionic equation represents the 'actual' chemical change and omits the spectator ions. Five types of examples of ionic equations are presented below including neutralisation, salt precipitation and redox equations. |
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(1) Acid-base reactions: Acids can be defined as proton donors. A base can be defined as a proton acceptor. e.g. any acid-alkali neutralisation involves the hydroxide ion is (base) and this accepts a proton from an acid.
(2) Insoluble salt formation: An insoluble salt is made by mixing two solutions of soluble compounds to form the insoluble compound in a process called 'precipitation'. A precipitation reaction is generally defined as 'the formation of an insoluble solid on mixing two solutions or a bubbling a gas into a solution'.
(b) Lead(II) iodide, a yellow precipitate (insoluble in water!) can be made by mixing lead(II) nitrate solution with e.g. potassium iodide solution.
(c) Calcium carbonate, a white precipitate, forms on e.g. mixing calcium chloride and sodium carbonate solutions ...
(d) Barium sulphate, a white precipitate, forms on mixing e.g. barium chloride and dilute sulphuric acid ...
(3) Redox reaction analysis: (a) magnesium + iron(II) sulphate ==> magnesium sulphate + iron
(b) zinc + hydrochloric acid ==> zinc chloride + hydrogen
(c) copper + silver nitrate ==> silver + copper(II) nitrate
(d) halogen (more reactive) + halide salt (of less reactive halogen) ==> halide salt (of more reactive halogen) + halogen (less reactive)
(4) Ion Exchange Resins: Ion exchange polymer resin columns hold hydrogen ions or sodium ions. These can be replaced by calcium and magnesium ions when hard water passes down the column. The calcium or magnesium ions are held on the negatively charged resin. The freed hydrogen or sodium ions do not form a scum with soap.
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| 3.2 VALENCY - COMBINING POWER - FORMULA DEDUCTION | ||
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3.2a Introduction The valency of an atom or group of atoms is its numerical combining power with other atoms or groups of atoms.
A group of atoms, which is part of a formula, with a definite composition, is sometimes referred to as a radical. In the case of ions, the charge on the ion is its valency or combining power (list below). To work out a formula by combining 'A' with 'B' the rule is:
However it is easier perhaps? to grasp with ionic compound formulae.
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using the combining power of ions (table left) valency = numerical charge value. Examples of covalent combining power of atoms (valencies below)
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| 3.2c Examples of working out covalent formulae | ||
| 'A' (valency) | 'B' (valency) | deduced formula of A + B |
| 1 of carbon C (4) | balances 4 of hydrogen H (1) | 1 x 4 = 4 x 1 = CH4 |
| 1 of nitrogen (3) | balances 3 of chlorine Cl (1) | 1 x 3 = 3 x 1 = NCl3 |
| 1 of carbon C (4) | balances 2 of oxygen O (2) | 1 x 4 = 2 x 2 = CO2 |
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The
diagram on the left illustrates the three covalent examples above for
methane CH4 nitrogen trichloride NCl3 carbon dioxide CO2 |
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| Examples of working out ionic formulae | ||
| numerically charge = valency of A or B to deduce the formula, 'molecular' or ionic style and compound name | ||
| 2 of Na+ balances 1 of O2- because 2 x 1 = 1 x 2 = Na2O or (Na+)2O2- sodium oxide | ||
| 1 of Mg2+ balances 2 of Cl- because 1 x 2 = 2 x 1 = MgCl2 or Mg2+(Cl-)2 magnesium chloride | ||
| 1 of Fe3+ balances 3 of F- because 1 x 3 = 3 x 1 = FeF3 or Fe3+(F-)3 iron(III) fluoride | ||
| 1 of Ca2+ balances 2 of NO3- because 1 x 2 = 2 x 1 = Ca(NO3)2 or Ca2+(NO3-)2 calcium nitrate | ||
| 2 of Fe3+ balances 3 of SO42- because 2 x 3 = 3 x 2 = Fe2(SO4)3 or (Fe3+)2(SO42-)3 iron(III) sulphate | ||
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3.3 KS3-GCSE note on naming compounds When combined with other elements in simple compounds the name of the non-metallic element changes slightly from ...??? to ...ide. Sulphur forms a sulphide (ion S2-), oxygen forms an oxide (ion O2-), fluorine forms a fluoride (ion F-), chlorine forms a chloride (ion Cl-), bromine a bromide (ion Br-) and iodine an iodide (ion I-). The other element at the start of the compound name e.g. hydrogen or a metal like sodium, potassium, magnesium, calcium, etc. usually remains unchanged in simple compounds at KS3-GCSE level. So typical compound names are, sodium sulphide, hydrogen sulphide, magnesium oxide, potassium fluoride, hydrogen chloride, sodium chloride, calcium bromide, magnesium iodide etc. However, even at GCSE level the complications will arise e.g.
GCSE 'name and formula' of a compound quizzes (1) pick the name given the formula * (2) pick the formula given the chemical name GCSE formula quiz given the name, type in the formula GCSE name quiz given the formula, type in the name
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| (indonesia) Doc Brown's Chemistry Revision Notes on Catatan Revisi Kimia Doc Brown pada UNSUR , SENYAWA, CAMPURAN dan pemisahan, KIMIA & PERSAMAAN REAKSI Bagian 3 Bagaimana menulis persamaan, valensi, mengerjakan rumus & senyawa nama (halaman ini) * Daftar menurut Abjad dari KATA KUNCI untuk Bagian 1-3: atom * menyeimbangkan persamaan (bekerja dengan cara Anda ke bagian hati-hati) * sentrifugal / pemusingan * reaksi kimia / perubahan * kromatografi (kertas / lapisan tipis) * senyawa * Kovalensi * kristalisasi * decanting / dekantasi * rumus ditampilkan * penyulingan ( sederhana / pecahan ) * elemen * persamaan * penguapan * filtrasi * formula * najis / murni * larut * persamaan ionik * ion valensi * besi belerang pemisahan dan percobaan pemanasan * magnet * campuran * molekul * penamaan senyawa dan ion * partikel gambar unsur / senyawa / campuran * perubahan fisik * curah hujan * produk * substansi murni * pemurnian * reaktan * pasir / pemisahan garam * corong pisah * memisahkan campuran * larut / solusi / pelarut / zat terlarut * ekstraksi pelarut * simbol (untuk elemen , formula , dalam persamaan ) * simbol negara * valensi * bekerja di luar rumus * (arabic) مستدات براون ملاحظات على مراجعة الكيمياء العناصر والمركبات والخلائط والانفصال ، والتفاعلات الكيميائية والمعادلات الجزء 3 كيفية كتابة المعادلات ، تكافؤ ، والعمل على صيغة المركبات اسم (هذه الصفحة * (french) Notes de révision sur la chimie composés, de mélanges et de la séparation, les réactions chimiques et EQUATIONS Partie 3 Comment faire pour écrire des équations, valence, le travail sur la formule et composés nom (cette page) équilibrage des équations (travailler votre chemin à la section attentivement) symboles (pour éléments , la formule , dans les équations ) * |
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