Chapter 5: Unit 2. Classification of Reactions
Classifications of Reactions
Chemical reactions can be broadly classified in five categories.
1) Decomposition reaction: When one reactant breaks down to two more reactant, the reaction is called decomposition reaction
- Combination reaction: When two or more reactants join together to form one single product, te reaction is called combination or synthesis reaction. Here are some example of combination reaction.
3) Single replacement reaction:
- An active element replaces another element in a compound.
In the above example, Magnesium replaces Hydrogen from water to form Hydrogen gas( H2) and Magnesium Hydroxide is formed.(Mg(OH)2)
4) Double replacement Reactions: When the cations of two ionic compounds switch places to form products, the reaction is classified as
when Potassium Iodide(KI) is added to colorless Lead Nitrate solution ( Pb(NO3)2) , yellow precipitate of Lead Iodide ( PbI2) is formed.
Below is the generic representation of all the different types of reactions discussed .
5) Combustion reaction: When a hydrocarbon burns to produce Carbon dioxide and gaseous water, the reaction is called combustion reaction. A combustion reaction always produces heat energy along with the products.
There are many other categories of reactions like acid-base reactions, precipitation reactions, etc. These reactions will be described under respective chapters. Only redox reaction is described in this chapter.
Identifying Chemical Reactions
Following activity has been taken from AACT
Solubility rules and activity series of metals are required to complete the following activity.
Part 1: Determine if there is a chemical reaction
| ReactantsAppearance of reactant(s) | Rxn(Yes/No) | Proof (List all that apply):Appearance of Products | |
| 1. | Copper metal + hydrochloric acid | ||
| 2. | Zinc metal + hydrochloric acid | ||
| 3. | Potassium iodide + lead (II) nitrate | ||
| 4. | Potassium thiocyanate + iron (III) nitrate | ||
| 5. | Potassium iodide + aluminum nitrate | ||
| 6. | Ammonium dichromate | ||
| 7. | Magnesium + oxygen |
Part 2: If there is a chemical reaction then complete the word equation
- Copper metal + hydrochloric acid à
- Zinc metal + hydrochloric acid à
- Potassium iodide + lead (II) nitrate à
- Potassium thiocyanate + iron (III) nitrate à
- Potassium iodide + aluminum nitrate à
- Ammonium dichromate à
- Magnesium + oxygen à
Part 3: Convert complete word equations to balanced chemical equations
- Formula Equation:
Balanced equation:
- Formula Equation:
Balanced equation:
- Formula Equation:
Balanced Equation:
- Formula Equation:
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- Formula Equation:
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Balanced Equation:
Answer Key: Identifying Chemical Reactions
Part 1: Determine if there is a chemical reaction
| ReactantsAppearance of reactant(s) | Rxn(Yes/No) | Proof (List all that apply):Appearance of Products | |
| 1. | Copper metal + hydrochloric acid Cu: solid chunks, shiny, orange-red colorHCl: colorless, liquid | No | No reaction (NR) or no Rxn |
| 2. | Zinc metal + hydrochloric acid Zn: sliver chunk, with rugged edgesHCl: colorless, liquid | Yes | Gas produced – bubbles are produced in the test tube and zinc color changes and becomes darker/black. |
| 3. | Potassium iodide + lead (II) nitrate KI: liquid and pale-yellow colorPb(NO3)2: liquid and colorless | Yes | Change in color and production of a precipitate (ppt). when liquids are mixed a bright yellow solid substance is produced. |
| 4. | Potassium thiocyanate + iron (III) nitrate KSCN: colorless liquidFe(NO3)3: orange-yellow liquid | Yes | Change in color – when mixed the solution becomes dark red color and resembles fake blood. |
| 5. | Potassium iodide + aluminum nitrate KI: liquid and pale-yellow colorAl(NO3)3: | No | No reaction (NR) or no Rxn |
| 6. | Ammonium dichromate (NH4)2Cr2O7: bright orange fine solid crystals | Yes | Change in color, formation of a gases (reactions resemble eruption of a volcano) – sparks of light are produced, dark green fluffy solid is produced, water vapor is produced. Nitrogen gas is produced but is not seen. Larger quantity of dark green product compared to original solid. |
| 7. | Magnesium + oxygen Mg: flat dull silver ribbon | Yes | Production of light – bright white light is produced, and Mg ribbon turns to black ash. |
Part 2: If there is a chemical reaction then complete the word equation
- Copper metal + hydrochloric acid à No reaction
- Zinc metal + hydrochloric acid àzinc chloride + hydrogen gas
- Potassium iodide + lead (II) nitrate à potassium nitrate + lead (II) iodide
- Potassium thiocyanate + iron (III) nitrate à potassium nitrate + iron (III) thiocyanate
- Potassium iodide + aluminum nitrate àNo reaction
- Ammonium dichromate àchromium (III)oxide + nitrogen gas + water vapors
- Magnesium + oxygen à magnesium oxide
Part 3: Convert complete word equations to balanced chemical equations
- No reaction = No balanced chemical equations
- Formula Equation: Zn(s) + HCl (aq) à ZnCl2 (aq) + H2(g)
Balanced equation: Zn(s) + 2HCl(aq) à ZnCl2 (aq) + H2(g)
- Formula Equation: KI(aq) + Pb(NO3)2 (aq) à KNO3 (aq) + PbI2 (s)
Balanced Equation: 2KI(aq) + Pb(NO3)2 (aq) à 2KNO3 (aq) + PbI2 (s)
- Formula Equation: KSCN(aq) + Fe(NO3)3 (aq) à KNO3 (aq) + Fe(SCN)3 (aq)
Balanced Equation: 3KSCN(aq) + Fe(NO3)3 (aq) à 3KNO3 (aq) + Fe(SCN)3 (aq)
- No reaction = No balanced chemical equations
- Formula Equation: (NH4)2Cr2O7(s) àCr2O3(g) + N2 (g) + H2O(g)
Balanced Equation: (NH4)2Cr2O7(s) à Cr2O3(g) + N2 (g) + 4H2O(g)
- Formula Equation: Mg(s) + O2 (g) à MgO(s)
Balanced Equation: 2Mg(s) + O2(g) à2MgO(s)
Questions:
- Classify the following reactions:
- a) N2 + 3H2 —> 2NH3
- b) Na2SO3 + H3PO4 —> H2SO3 + Na3PO4
Ans: a) combination, b) double displacement