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Understanding Chemical Reactions
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- 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.
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Balanced Chemical Equations: Preserving Mass in Chemical Reactions
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- 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
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Number of atoms of different elements on both sides of the arrow
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Element
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Number of atoms in reactants (LHS)
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Number of atoms in products (RHS)
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Zn
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1
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1
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S
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2
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2
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H
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1
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1
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O
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4
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4
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- 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
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Number of atoms of different elements present in the unbalanced equation
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Element
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Number of atoms in reactants (LHS)
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Number of atoms in products (RHS)
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Fe
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1
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3
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H
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2
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2
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O
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1
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4
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- 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.
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Atoms of Oxygen
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Atoms of Oxygen
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In reactants
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In products
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Initial
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1 (in H2O )
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4 (in Fe3O4 )
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To balance
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1 x 4
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4
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- 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.
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Atoms of Hydrogen
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Atoms of Hydrogen
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In reactants
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In products
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Initial
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8 (in 4 H2O)
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2 (in H2)
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To balance
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8
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2 x 4
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- The equation would be: Fe + 4 H2O → Fe3O4 + 4 H2
- Step IV: To equalise Fe, we take three atoms of Fe on the LHS .
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Atoms of Iron
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Atoms of Iron
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In reactants
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In products
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Initial
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1 (in Fe)
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3 (in Fe3O4 )
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To balance
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1 x 3
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3
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- 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.
Note: The word aqueous (aq) is written if the reactant or product is present as a solution in water.
- 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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