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School Chemistry Notes: Reading & deducing formula, writing chemical equations
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3. How to write chemical equations & how to work out compound formulae from valencies (Suitable for AQA, Edexcel and OCR GCSE chemistry students) Sub-index for this page
Part 1 Definitions in Chemistry, Elements, Compounds & Mixture pictures & Physical & Chemical Changes Part 2 Methods of Separating Mixtures of substances Part 3 How to write equations, valence, work out formula & name compounds (this page) Find your GCSE science course for more help links to revision notes Use your mobile phone or ipad etc. in 'landscape' mode This is a BIG website, you need to take time to explore it [SEARCH BOX] PART 3 INTRODUCTION to writing equations How do we write chemical equations?, How do we construct equations from first principles? How do we balance chemical equations? Examples of writing word equations and balanced symbol equations, what is the valence of an element? What are ionic equations? How do we write ionic equations? Practice in balancing chemical equations How can we use valence to work out a formula? - index of keywords-terms-phrases below You need to know your: chemical symbols, state symbols, selected chemical formula and how to read a formula So you can go from balancing easy equations like
to balancing a bit more complicated equations like:
FINALLY: how to balance much more awkward equations like:
So, hopefully by the end of the page you can balance equations as difficult as the one above! In balancing equations make sure you do NOT change a formula, you ONLY change the 'balancing number' before the formula - which itself must be correct. State symbols used in chemical equations, to show the physical state of the reactants and products
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3.1 THE
CONSTRUCTION OF CHEMICAL EQUATIONS
"How to write and understand chemical equations" 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, but examples are explained on this page too. 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, this is a matter of 'trial and error'.
If all is correct, then the sum of atoms for each element should be the same on both side of the equation arrow .....
NEVER alter a formula to balance an equation! BUT use the CORRECT FORMULA and only put NUMBERS BEFORE THE FORMULA if needed to balance the number of atoms to balance the equation. You do need to be able to read and understand a formula and I've included some reminders and explanations in the examples of how to construct and balance the 7 chemical equations described and explained in detail below. PRACTICE QUESTIONS - on words and symbol equations (on other pages)
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3.1d EXAMPLES of CONSTRUCTING WORD or SYMBOL EQUATIONS Remember from the 'Law of Conservation of Mass' the mass of products = mass of original reactants, which means that the number of atoms of each element in the reactants must be equal to those in the products and that is the basis of writing a correctly balanced symbol equation, BUT don't forget, you must write the correct formula for each species in the equation, otherwise you may write a correctly balanced equation which is totally wrong! so beware! Balancing equations example 3.1d (1) iron + sulfur ==> A single symbol means an
uncombined single atom of the element,
iron + sulfur ==> iron sulfide
on average one atom of iron chemically combines with one atom of iron forming one molecule of iron sulfide two elements chemically combining to form a new compound
Atom balancing, sum left = sum right: 1Fe + 1S = (1Fe combined with 1S) There is no need for any balancing numbers in this equation For a balanced equation on both sides of the equation you should have 1 iron atom and 1 sulfur atom combined in their particular way in the reactants or products 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). |
Balancing equations example 3.1d (2) sodium hydroxide + hydrochloric acid ==>
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 the chemical reactants and products involved are compounds
atom balancing, sum left = right: (1Na + 1O + 1H) + (1H + 1Cl) = (1Na + 1Cl) + (2H + 1O) For a balanced equation on both sides of the equation you should have 1 sodium atom, 1 oxygen atom, 1 chlorine atom and 2 hydrogen atoms combined in their particular way in the reactants or products. Note the subscript 2 after the H in water means two atoms of that element. There is no need for any balancing numbers in this equation |
Balancing equations example 3.1d (3) magnesium + hydrochloric acid ==> Thinking sequence:
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 (H2) are elements, H-Cl (HCl) and Cl-Mg-Cl (MgCl2) are compounds
atom balancing, sum left = right: (1Mg) + 2 x (1H + 1Cl) = (1Mg + 2Cl) + (2H) For a balanced equation on both sides of the equation you should have 1 magnesium atom, 2 hydrogen atoms and 2 chlorine atoms combined in their particular way in the reactants or products. You can only get the balance here by putting a 2 in front of the HCl formula because you need 2 Cl's to make the MgCl2. |
Balancing equations example 3.1d (4) copper carbonate + sulfuric acid ==>
H2O (example 2)
copper carbonate + sulfuric acid ==> copper sulfate + water + carbon dioxide
the reactants are one formula of copper carbonate and one molecule of sulfuric acid the products are one formula of copper sulfate, one molecule of water and one molecule of carbon dioxide all molecules are compounds in this reaction
balancing sum left = sum right: (1Cu + 1C + 3O) + (2H + 1S + 4O) = (1Cu + 1S + 4O) + (2H + 1O) + (1C + 2O) For a balanced equation on both sides of the equation you should have 1 copper atom, 1 carbon atom, 7 oxygen atoms, 2 hydrogen atoms, 1 sulfur atom combined in their particular way in the reactants or products There is no need for any balancing numbers in this equation |
Balancing equations example 3.1d (5) burning a hydrocarbon e.g. methane + oxygen ==> Thinking sequence:
To convert the one C into CO2 and the 4 H's into H2O, you need 4 O atoms in the form of 2 O2 molecules. 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
atom balancing, sum left = sum right: (1C + 4H) + 2 x (2O) = (1C + 2O) + 2 x (2H + 1O) For a balanced equation on both sides of the equation you should have 1 carbon atom, 4 hydrogen atoms, 4 oxygen atoms combined in their particular way in the reactants or products. You can only get this to balance by having a 2 in front of the O2 and 2 in front of the CO2, you need an O2 to make CO2 and another O2 to convert the H4 into 2H2O Note the 2 before the O2 and H2O doubles the number of these molecules to balance the equation. Using the complete combustion of a hydrocarbon as an example, its worth mentioning the use of 'fractions' of a formula in balancing equations
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Balancing equations example 3.1d (6) magnesium hydroxide + nitric acid ==> Thinking:
Note that here the subscript 2 after the (NO3) in the magnesium nitrate means everything in the brackets is doubled.
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 reactants and products are compounds)
atom balancing, sum left = sum right: (1Mg + 2O + 2H) + 2 x (1H + 1N + 3O) = (1Mg + 2N + 6O) + 2 x (2H + 1O) For a balanced equation on both sides of the equation you should have 1 magnesium atom, 8 oxygen atoms, 4 hydrogen atoms, 2 nitrogen atoms combined in their particular way in the reactants or products to balance this equation you need a 2 in front of the HNO3 and a 2 in front of the H2O, the 2s come from the 2 OH becoming 2 H2Os |
Balancing equations example 3.1d (7) aluminium oxide + sulfuric acid Thinking:
aluminium oxide + sulfuric acid ==> aluminium sulfate + water
one formula of aluminium oxide reacts with three molecules of sulfuric acid to form one formula of aluminium sulfate and three molecules of water, all reactants and products are compounds note the first use of numbers (3) for the sulfuric acid and water! so picture three of them in your head, otherwise the picture gets a bit big!
atom balancing, sum left = sum right: (2Al + 3O) + 3 x (2H + 1S + 4O) = (2Al + 3S + 12O) + 3 x (2H + 1O) For a balanced equation on both sides of the equation you should have 2 aluminium atoms, 15 oxygen atoms, 6 hydrogen atoms, 3 sulfur atoms combined in their particular way in the reactants or products This is quite an awkward equation to balance, a bit of real trial and error, but two 3s in the right place will do it. The best clue here is that you need 3 x SO4 for the aluminium sulfate, so you need 3 of the H2SO4 GCSE-AS-A2-IB note: Aluminium sulfate is actually an ionic compound (Al3+)2(SO42-)3 |
Extra NOTE 1 Reversible Reactions The
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Extra NOTE 2 State Symbols The use state symbols X(?) of reactants or products in equations, used to denote the physical state of reactants and products (g) means gas, (l) means liquid, (s) means solid and (aq) means aqueous solution, which means the substance is dissolved in water e.g. carbon dioxide gas CO2(g), liquid water H2O(l), solid sodium chloride 'salt' NaCl(s) and copper sulfate solution CuSO4(aq) |
3.1e IONIC EQUATIONS (for higher GCSE and advanced level students) What is an 'ionic equation'? How do we construct and write ionic equations? 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 ALL of the spectator ions. Five types of examples of ionic equations are presented below including neutralisation, salt precipitation and redox equations. |
(1) Acid-base neutralisation 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 - salt precipitation reactions: 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'.
(3) Redox reaction analysis - reactions involving an oxidation and reduction: (a) magnesium + iron(II) sulfate ==> magnesium sulfate + 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.
(5) Scum formation with hard water: On mixing hard water with soaps made from the sodium salts of fatty acids, insoluble calcium or magnesium salts of the soap are formed as a grey precipitate ...
Revision notes on writing equations in chemistry, how to balance chemical equations, how to read and write formulae, word equations, balancing symbol equations, how to write and balance ionic equations, how to work out a formula from valencies, how to work out formula from the charges on the ions, help when revising for AQA GCSE chemistry A level chemistry, Edexcel GCSE chemistry A level chemistry, OCR GCSE gateway science chemistry, OCR 21st century science chemistry, OCR A level chemistry, Salters A level chemistry, GCSE 9-1 chemistry, examinations. |
3.2 VALENCE - VALENCY - COMBINING POWER - FORMULA DEDUCTION | |||
3.2a Introduction What is valence? How do you use valence to work out the formula of a compound? The valence of an atom or group of atoms is its numerical combining power with other atoms or groups of atoms. i.e. its numerical capacity to combine with other 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 valence 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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Selected combining power of ions (table left) valence = numerical ion charge value and examples of covalent combining power of atoms i.e. valencies (selection below).
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3.2c Examples of working out covalent formulae |
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'A' (valence) | 'B' (valence) | 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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Six more examples
of working out an ionic formula numerically charge = valence of A or B to deduce the formula valence or ionic charge = the combining power of the ion 'molecular' or ionic style of formula and compound name 1 of K+ balances 1 of Br- because 1 x 1 = 1 x 1 gives KBr or K+Br- potassium bromide2 of Na+ balances 1 of O2- because 2 x 1 = 1 x 2 gives Na2O or (Na+)2O2- sodium oxide 1 of Mg2+ balances 2 of Cl- because 1 x 2 = 2 x 1 gives MgCl2 or Mg2+(Cl-)2 magnesium chloride1 of Fe3+ balances 3 of F- because 1 x 3 = 3 x 1 gives FeF3 or Fe3+(F-)3 iron(III) fluoride 1 of Ca2+ balances 2 of NO3- because 1 x 2 = 2 x 1 gives Ca(NO3)2 or Ca2+(NO3-)2 calcium nitrate2 of Fe3+ balances 3 of SO42- because 2 x 3 = 3 x 2 gives Fe2(SO4)3 or (Fe3+)2(SO42-)3 iron(III) sulfate |
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Revision notes on writing equations in chemistry, how to balance chemical equations, how to read and write formulae, word equations, balancing symbol equations, how to write and balance ionic equations, how to work out a formula from valencies, how to work out formula from the charges on the ions, help when revising for AQA GCSE chemistry A level chemistry, Edexcel GCSE chemistry A level chemistry, OCR GCSE gateway science chemistry, OCR 21st century science chemistry, OCR A level chemistry, Salters A level chemistry, GCSE 9-1 chemistry, examinations. |
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3.3 KS3, GCSE, A Level note on naming compounds When combined with other elements in simple compounds the name of the non-metallic element changes slightly from ...??? to ...ide. Sulfur forms a sulfide (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 sulfide, hydrogen sulfide, magnesium oxide, potassium fluoride, hydrogen chloride, sodium chloride, calcium bromide, magnesium iodide etc. However, even at GCSE level the complications will arise e.g.
See other web pages for: [SEARCH BOX] and [Main INDEX]
QUESTIONS: GCSE balancing and completing equation exercises:
(1)
GCSE 'name and formula' of a compound quizzes (1) m/c quiz picking the name given the formula (2) m/c quiz picking the formula given the chemical name (3) Type in the formula quiz given the name (4) Type in the name quiz given the formula, type in the name (5) Multiple choice quiz on the naming and formula of chemical compounds (combination of (1) + (2))
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Doc Brown's Chemistry |
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