Chemical Reactions Essentials:

When the nature and the identity of the initial substance have somewhat changed it is said to be a chemical change and  whenever a chemical change occurs, a  Chemical Reaction takes place. These  Chemical Reactions can be represented as chemical equations.

Understanding  Chemical Reactions 

• The description of a  Chemical Reaction in sentence form is quite long. It can be written in a shorter form. 
• The simplest way to do this is to write it in the form of a word-equation.
• When a magnesium ribbon is burnt in oxygen, it gets converted to magnesium oxide. 
• It can be written in words as:

Magnesium + Oxygen → Magnesium oxide

        (Reactants)                         (Product)

• Reactants: The substances that undergo chemical change in the  Chemical Reactions are the reactants.
• Product: The new substance formed during the  Chemical Reactions is a product. 
  • A word-equation shows change of reactants to products through an arrow placed between them. 
  • The reactants are written on the left-hand side (LHS) with a plus sign (+) between them and products are written on the right-hand side (RHS) with a plus sign (+) between them. 
  • The arrowhead points towards the products, and shows the direction of the  Chemical Reactions.

 Chemical Reactions Representation: Magnesium Burning in Oxygen

• Chemical equations can be made more concise and useful if we use chemical formulae instead of words.
• A chemical equation represents a  Chemical Reactions. 
  • Burning of magnesium ribbon in oxygen can be written as –

Mg + O2 → MgO

Balanced Chemical Equations: Preserving Mass in  Chemical Reactions 

• Law of conservation of mass states that mass can neither be created nor destroyed in a  Chemical Reactions i.e the total mass of the elements present in the products of a  Chemical Reactions has to be equal to the total mass of the elements present in the reactants. 
  • In other words, the number of atoms of each element remains the same, before and after a  Chemical Reactions.
  • Hence, we need to balance a skeletal chemical equation. 

Example: Zn + H2SO4 → ZnSO4 + H2

• As the number of atoms of each element is the same on both sides of the arrow, the equation is a balanced chemical equation.

Fe + H2O → Fe3O4 + H2 

• Step I: List the number of atoms of different elements present in the unbalanced equation 

• Step II: Start balancing with the compound that contains the maximum number of atoms. 
  • It may be a reactant or a product. 
  • In that compound, select the element which has the maximum number of atoms. i.e. the compound is Fe3O4 and the element oxygen in it. To balance the oxygen atoms.

• Now the partly balanced equation becomes:

Fe + 4 H2O → Fe3O4 + H2 

• Step III: Now to equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS. 

• The equation would be: Fe + 4 H2O → Fe3O4 + 4 H2 
• Step IV: To equalise Fe, we take three atoms of Fe on the LHS . 

• The equation would be: 3 Fe + 4 H2O → Fe3O4 + 4 H2 
• Step V: Finally, the numbers of atoms of elements on both sides of equations are equal. 
  • This equation is now balanced. 
  • This method of balancing chemical equations is called the hit-and-trial method.

3Fe + 4H2O → Fe3O4 + 4H2  

• Step VI: To make a chemical equation more informative, the physical states of the reactants and products are mentioned. 
  • The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.

• Thus, the balanced Equation becomes: 3Fe (s) + 4H2O (g) → Fe3O4 (s) + 4H2 (g)
• Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the  Chemical Reactions indicated above and/or below the arrow in the equation. For example,

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