Chemical Reactions

What are Chemical Reactions?

• nature and identity of initial substance changes
• so, whenever a change occurs, a chemical reaction occurs

When a chemical reaction takes place, the following happens

• change in state of the initial object (solid, liquid, gas)
• change in color
• evolution of a gas ( as a result of the reaction)
• change in temperature

What are Chemical Equations?

a shorter form of describing a chemical reaction

• the simplest way is a word-equation

       Magnesium + Oxygen --> Magnesium Oxide

2 parts of the equation

1. Left side -reactants
2. Right side - product

Using Chemical Formula

a shorter way of representing the equation

• use the chemical symbols of the reactants and products

         Mg + O2 --> MgO

Balanced Chemical Equations

Mass can neither be created nor destroyed

• the total mass of reactants = total mass of products
• no. of atoms remain the same - before and after the chemical reaction

Steps to balance equation

Fe + H2O --> Fe3O4 + H2

Step 1: Identify each formula in the equation

            Fe

            H2O

            Fe3O4

            H2 

Completing the equation

Follow these final steps:

1. Count number of atoms on both sides- they should be equal
2. Write the physical states of the compound -s,l,g
3. Add temp, pressure, catalyst if required

Types of Chemical Equation

There are the following types:

1. Combination Reaction: single product formed with two or more reactions
2. Decomposition Reaction: single, complex reactant gives simpler products
3. Displacement Reaction:  the reactant replaces a compound in another reactant
4. Exothermic chemical Reaction: reaction produces a lot of heat
5. Thermal decomposition: decomposition reaction in the presence of heat 
6. Endothermic reactions: reaction in which energy (heat, light, electricity) is absorbed
7. Double displacement reaction: displacement of reactant and exchange of ions as well
8. Precipitation reaction: any reaction that forms a precipitate (insoluble substance)
9. Oxidation Reaction: if a reactant gains oxygen during a reaction
10. Reduction Reaction: if a reactant loses oxygen during a reaction
11. Redox Reaction: if both oxidation and reduction occur

Examples

Combination Reaction

• CaO(s) + H2O(l) → Ca(OH)2(aq)
• C(s) + O2(g) --> CO2(g)
• 2H2(g) + O2 --> 2H2O
• 2Mg + O2 --> 2MgO

Effects of oxidation

• corrosion
• rancidity

What happens when....?

Burn magnesium ribbon in air

• Observation: magnesium ribbon burns with a dazzling white flame and changes into a white powder.
•  Conclusion: MgO is formed due to the reaction between magnesium and oxygen present in the  air.