CHEMISTRY YEAR 9-10. Writing and Balancing Equations. Writing and Balancing Equations. A chemical equation is a shorthand expression for a chemical change or reaction. word - equation : states in words mercury (II) oxide + heat mercury + oxygen bulky and cumbersome 2 HgO 2Hg + O 2.
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Writing and Balancing Equations
word - equation : states in words
mercury (II) oxide + heat mercury + oxygen
bulky and cumbersome
2 HgO 2Hg + O2
1. The reactants are separated from the products by an arrow that indicates the direction of the reaction.
2. The reactants are placed to the left and the products to the right of the arrow. A plus sign (+) is placed between reactants and between products when needed.
3. Conditions required to carry out the reaction may be placed above or below the arrow. A delta indicates heat is applied.
4.Coefficients2 H2O are placed in front to balance the equation. One is never placed there, it is just understood.
5. The physical state of a substance is indicated by the following symbols:
(s) solid (l) liquid (g) gaseous (aq) aqueous: means water solution
H2 gas evolved solid precipitate
1. First you must have the equation in symbol format, you cannot balance a word equation.
a. Count and compare the number of atoms of each element on each side of the equation and determine those that must be balanced.
b. Balance each element one at a time, by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element.
It is usually better to balance in this order: metals, nonmetals, hydrogen, oxygen.
2H2SO4 = 4H’s 2S’s 8O’s
c. re-check each time you balance an element to see if anything else has become unbalanced. Make adjustments as needed.
d. Do a final check making sure that each element is balanced and that the smallest possible set of whole number coefficients has been used:
4HgO 4Hg + 2O2 incorrect
2HgO 2Hg + O2 correct
Magnesium + oxygen Magnesium Oxide
Potassium Chlorate Potassium Choride + Oxygen
Aluminum hydroxide + sulfuric acid
Aluminum sulfate + water
C4H10 + O2 CO2 + H2O
1. Combination or Synthesis Reaction:
Two reactants combine to give one product.
A + B AB
2Mg + O2 2MgO
A single substance is decomposed or broken down to give two or more different substances:
AB A + B
2PbO2 2PbO + O2
2 Na HCO3 Na2CO3 + H2O + CO2
3. Single - Displacement Reaction:
One element reacts with a compound to take the place of one of the elements of that compound.
A + BC B + AC
Zn + 2HCl H2 + ZnCl2
Note: Activity series table helps to predict which substances (elements) will be able to replace various other substances.
4. Double - Displacement or Metathesis:
Two compounds exchange partners with each other to produce two different compounds.
AB + CD AD + CB
NaCl + KNO3 NaNo3 + KCl
Energy changes always accompany chemical reactions. One reason why reactions occur is that the products attain a lower, more stable energy state than the reactants.
Reactions are either exothermic or endothermic.
Exothermic reactions liberate heat:
H2 + Cl2 2HCl + 185kJ (exothermic)
Endothermic reactions absorb heat:
N2 + O2 + 181kJ 2NO (endothermic)