1. Equations for Chemical Reactions
Foothill Chemistry

2. What is a Chemical Reaction?
A process by which one or more substances are transformed into one or more different substances.
The equation for a chemical reaction represents the identities and relative molecular or molar amounts of the reactants and products using symbols and formulas.
Example:
(NH4)2Cr2O7(s)  N2(g) + Cr2O3(s) + 4H2O(g)

3. Indications of a Chemical Reaction
Change in energy as heat or light or cold
Production of a gas (bubbles)
Formation of a precipitate
Color change / smell change

4. Characteristics of Chemical Equations
Equation must represent known facts
Reactants and products must be identified
Through experimentation or reference sources
Equation must contain the correct formulas for the reactants and products
The chemical formulas have to be correct
The law of conservation of mass must be satisfied
Mass is not created or destroyed

5. Steps in Writing a Chemical Equation
Write a word equation – an equation in which the reactants and products are represented by words
Methane + Oxygen react to yield (make / produce / form) Carbon Dioxide + Water
Replace the names of the products and reactants with their proper chemical formulas
Use an arrow  to represent the “react to yield” statement.
CH4(g) + O2(g)  CO2(g) + H2O(g)
Add the necessary coefficients to balance the equation
This step meets the “Conservation of Mass” requirement
CH4(g) +2O2(g)  CO2(g) + 2H2O(g)

6. Symbols of Chemical Reactions
Explanation 
“yields”; points to results of the reaction
(l)
Liquid state of matter 
Reversible reaction (the reaction goes in both directions)
(aq)
Aqueous solution (dissolved in water)
(s)
Solid state of matter or precipitate
(g)
Gaseous state of matter 
Precipitate (not solid) 
Indicates a gaseous product

7. Symbols or Words above the 
Δ or the word, ”heat”, above the arrow means that the reactants are heated in the reaction
# atm, the number symbol will be an numeric symbol and will indicate the atmospheric pressure at which the reaction is carried out
The word, “pressure” indicates that the pressure at which the reaction is carried out is higher than normal atmospheric pressure
#oC – The temperature at which the reaction is carried out (e.g. 0oC)
(Chemical formula) – The formula of a catalyst. It may be an element or a chemical. The catalyst increases the reaction rate without being used in the reaction.

8. Reversible Reaction
A chemical reaction in which the products re-form the original reactants.
If that reaction occurs equally in both directions, the arrow lengths will be equal. If they occur greater in one direction or another, the arrows will show a longer arrow for the reaction that produces a greater amount of product.

9. Chemical Reaction Significance
The coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products
The relative masses of the reactants and products can be determined from the reaction’s coefficients.
The reverse reaction has the same relative amounts of substances as the forward reaction.

10. Balancing Chemical Equations
Identify names of the reactants and products and write a word equation.
Write a formula equation by substituting correct formulas for the reactants and products, along with any other necessary symbols.
Balance the formula equation according to the law of conservation of mass.
Balance the different types of atoms one at a time
Balance the atoms that appear only once on each side of the equation
Balance polyatomic ions that appear on both sides of the equation as single units
Balance H and O atoms after all of the other atoms have been balanced
Count the atoms to be sure the equation is in balance.

11. Word Problem Practice
zinc and lead(II) nitrate yield zinc nitrate and lead
aluminum bromide and chlorine yield aluminum chloride and bromine
sodium phosphate and calcium chloride yield calcium phosphate and sodium chloride
aluminum and hydrochloric acid yield aluminum chloride and hydrogen gas
potassium chlorate, when heated, yields potassium chloride and oxygen gas
calcium hydroxide and phosphoric acid yield calcium phosphate and water
copper and sulfuric acid yield copper(II) sulfate and water and sulfur dioxide
hydrogen and nitrogen monoxide yield water and nitrogen

12. Types of Chemical Reactions
Synthesis Reaction
Decomposition Reaction
Single Displacement (Replacement) Reaction
Double Displacement (Replacement) Reaction
Combustion Reaction

13. Make It and Break It Synthesis Reaction (Composition Reaction)
Two or more substances combine to form a new compound
A + X  AX
“Make It”
Decomposition
One substance reacts to form two or more simpler compounds
AX  A + X
“Break It”

14. Common Types of Reactions
Synthesis
Making a sulfide or oxide by reacting with sulfur or oxygen
16 Rb(s) + S8(s)  8Rb2S(s)
C(s) + O2(g)  CO2(g)
Reacting with Halogens (Group 17 or 7A)
2Na(s) + Cl2(g)  2NaCl(s)
Reacting with Oxides and Water
CaO(s) + H2O(l)Ca(OH)2(s)

15. Common Types of Reactions
Decomposition
Breaking a Binary Compound
Electrolysis (Breaking using Electricity)
2H2O(l)  2H2(g) + O2(g)
Metal Carbonates – Produces metal oxide +CO2
Metal Hydroxides – Produces metal oxide + water
Metal Chlorate – Produces metal chloride + O2
Acid Decomposition – Produces non-metal oxide and water (can be a dioxide or trioxide)
Electricity

16. Switcheroo Reactions
Single-displacement (sometimes single- replacement)
One element replaces a similar element in a compound
A + BX  AX + B (Cation replacement)
Y + BX  BY + X (Anion Replacement)
Double-Displacement (sometimes double- replacement)
The ions of two compounds exchange places in an aqueous solution to form two new compounds.
AX + BY  AY + BX

17. Common Types of Reactions
Single Displacement
Displacement of a metal in a compound by another metal (cations)
2Al(s) + 3Pb(NO3)2(aq)  2 Al(NO3)3 (aq) + 3 Pb(s)
Displacement of hydrogen in water by a metal (anions)
More reactive metals (farther left on table) form hydroxides and hydrogen gas
2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
Less reactive metals react with steam to form a metal oxide and hydrogen gas
3Fe(s) + 4H2O(g)  Fe3O4(s) + 4H2(g)

18. Common Types of Reactions
Single Displacement
Displacement of hydrogen in acid by metal (cations)
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Displacement of halogens (anions)
2NaBr(aq) + Cl2(g)  2NaCl(aq) + Br2(l)
** Halogens can only displace halides if they are higher on the periodic table than the one that is being replaced.
Br2 + KCl(aq)  No Reaction

19. Common Types of Reactions
Double Displacement
Formation of a Precipitate
2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
Formation of a Gas
FeS(s) +2HCl(aq)  H2S(g) + FeCl2(aq)
Formation of Water
HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)

20. BOOM! Combustion Reaction
A substance combines with oxygen, releasing a large amount of energy in the form of heat and light
Burning natural gas, propane, and wood are also examples of combustion reactions
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)