Chemical Reaction Equations
And
Types of Reactions
A chemical reaction can be defined as:
A chemical reaction has two parts:
Elements involved –Reactant
Substances formed by the chemical reaction - Product
The French chemist Antoine Lavoisier (in 1700’s) stated:
Law of Conservation of mass:in ordinary chemical reactions, matter can be changed in many ways, but it cannot be created or destroyed.
Writing Chemical Equations:
You may have to make a Chemical Formula from words and then balance the equation.
- Example: Write a decomposition reaction for water:
Unbalanced equation: H2O H2 + O2
Changes to make it balanced will be in bold print
Balanced equation: 2H2O 2H2 + O2
Symbols are used in Chemical Equations
(s) not in your book and it means it is a solid
(Yield sign) means produces or forms
(l) liquid
+ plus
(g) gas
(cr) crystalline solid
(aq) aqueous, a solid is dissolved in water
heated
the reactants are heated
light
the reactants are exposed to light
elec.
an electric current is applied to the reactants
Coefficients represent the number amount of every substance in the chemical reaction. The reactant side must equal the product side to make a balanced equation.
- In the problem below there had to be the same amount of Hydrogens and Oxygens on both sides to become a balanced equation.
Unbalanced equation: H2O H2 + O2
Balanced equation: 2H2O 2H2 + O2
- The unbalanced equation had on the reactant side: 2 H’s and 1 O but on the product side: 2 H’s and 2 O.
- To balance the equation: The reactant side: 2 were added to the water molecule making 2 water molecules. This means there are now 4 H’s and 2 O on the reactant side. The product side: The O’s are fine since there are 2, H’s are not. Therefore, writing a 2 in front of the H2 allows the product now to have 4 H’s too. Now that there are the same amount of H’s and O’s on both sides so the problem is balanced.
(1) Note: H, N, and O by themselves are diatomic and must be written that way to be able to correctly balance the chemical equation.
- In addition, all Halogens are diatomic when by themselves: F, Cl, Br, and I
(2) Note: You must balance the equations with the lowest whole numbers possible.
(3) Note: Sometimes a Chemical equation will already be balanced
Other Examples:
(1) Unbalanced equation: Mg (s) + HCl (l) MgCl2 (s) + H2 (g)
Balanced equation: Mg + 2 HCl MgCl2 + H2
(2) Unbalanced equation: Fe (s) + S (s) FeS (s) Product is Iron (II) sulfide
Balanced equation: 2Fe (s) + 3S (s) Fe2S3 (s) True Product name is Iron (III) sulfide
In this case you need to remember that Iron has different charges (transition element) and that each compound must be balanced before balancing the equation as a whole.
- Sulfur is in Group 16 therefore has a -2 charge. So to balance the Fe and the S both must =6 in this case. So 3x2 =6 (Iron) and 2x3 =6 (S)
(3) Original equation: CaO (s) + H2 (l) Ca(OH)2 (aq) Product is Calcium Hydroxide
Balanced already
In this case, there was a metallic oxide that reacted with H2O it usually forms a base, [compound containing –OH (hydroxide)]
- Since –OH is a polyatomic ion ( ) must be placed around it if more than one in the compound. Ca has a +2 charge and –OH has a -1 charge so to balance the compound there have to be two –OH’s.
(4) Original equation: SO2 (g) + H2O (l) H2SO3 (aq) Sulfurous acid
Balanced already
In this case, there was a nonmetal oxide reacted with water so the product is an acid. It will produce a H+ in aqueous solution
(5) Original equation: CaCO3 (s) CaO (s) + CO2 (g)
Balanced already
This problem involves heat so heat must be indicated on top of the arrow.
MnO2
(6) Original equation: H2O2 (aq) H2O (l) + O2 (g)
Balanced already
This problem indicates that a catalyst was involved. The catalyst only speeds up the reaction. So the catalyst must be stated on top of the arrow but is not used by the reaction.
Try to balance following problems:
- K + Cl2 KCl
- S + O2 SO3
- SO3 + H2O H2SO4 (aq) Sulfuric acid
- Ca(OH)2 + HCl CaCl2 + H2O
- (A challenge) C6H6 + O2 CO2 + H2O
Types of Reactions
1.)Synthesis Reaction (Also called Combination): Two or more substances combine to form a single substance.
*Write the general formula: A + BAB
2.)Decomposition Reactions: One single compound is broken down into two or more products.
*Write the general formula: AB A + B
When heat is applied to this type of reaction it can produce explosive results.
3.)Single Displacement Reactions (Also called Single-Replacement): One element replaces a 2nd element in a compound.
*Write the general formulas: A + BCAC +B
OR D + BC BD +C
Whether one metal will displace another metal from a compound can be determined by the relative reactivity of the two metals.
A nonmetal can also replace another nonmetal from a compound, but it usually only occurs if it is a Halogen.
Metals from Li to Na will replace H from acids and water, from Mg to Pb will replace H from acids only.
☺Why can’t this reaction take place? Mg (s) + LiNO3 (aq) no reaction
4.)Double Replacement Reaction: Involves the exchange of positive ions between two ionic compounds in an aqueous solution.
*Write the general formula: A B+ CDAD +CB
One of two things usually occurs in this type of reaction:
(A) One product is a gas that bubbles out of the mixture
(B) One product is only slightly soluble and a precipitate forms
☺What is a precipitate?
Is an insoluble compound formed from a double replacement reaction
(Precipitate reactions occur in everyday life. Ex. Kidney stones, soap scum, limestone deposits)
5.)(This one is not in your book) Combustion reaction: An element or a compound reacts with Oxygen, often producing energy as heat and light. Commonly involves
Hydrocarbons (Compounds with only C and H) and often as products with CO2
and H2O
General formula: CxHy + (x + y/4)O2 xCO2 + (y/2)H2O
Example: 2C6H6 + 15O2 12CO2 + 6 H2O These are harder to balance (same as problem #5 but balanced)
Determine what type of Reaction is occurring in the following problems and balance those that are not balanced:
A.) ZnO + HCl ZnCl2 + H2O
B.) H2 + N2 NH3
C.) NaCl (aq)+ AgNO3 (aq) AgCl (cr) + NaNO3 (aq)
D.) HI H2 + I2
E.) Cl2 + O3ClO + O2
F.) Mg + AgNO3 Mg(NO3)2 + Ag
G.) C3H8 +O2 CO2 + H2O
- You need to be able to make chemical reaction formulas for Decomposition and Synthesis problems only. Try the next two:
H.) Make a decomposition reaction for Table salt
I.) Make a Synthesis reaction for Silver Nitrate.
Answer the following questions:
What is an Exothermic Reaction?
What is an inhibitor?
What is an Endothermic Reaction?
Why is electricity sometimes used in an Endothermic Reaction?
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