BALANCE CHEMICAL EQUATION

A chemical equation is represent of a chemical reaction using symbols of elements and formula of compounds. It takes practice to be able to write balance equation. There are essentially 2 steps to the process.

1. Write the unbalance equation.

• Chemical formulas of reactants are listed on the left hand side of the equation.
• Products are listed on the right hand side of the equation.
• Reactants and products are separated by putting an arrow between them to show the direction of the reaction. Reactions at equilibrium will have arrows facing both directions.

2. Balance the equation.

• Start by balancing an element that appears in only one reactant and product.
• Once one element is balanced, proceed to balance another, and another, until all elements are balanced.
• Balance chemical formulas by placing coefficients in front of them. Do not add subscripts, because this will change the formulas.

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BALANCE REDOX EQUATION

For more complex redox reaction, use ion-electron method. Follow the steps systematically so that equations become easier to balance.

STEP 1: Write the overall equation

Cr₂O₇^2-  +  Cl^-  ®  Cr³⁺   +  Cl₂      

STEP 2: Devide the equation into two half reactions 

reduction:  Cr₂O₇^2-  ® Cr³⁺         

oxidation:  Cl^-  ®  Cl₂                  

(**you must know which pair undergo reduction and oxidation by counting their oxidation numbers**)

STEP 3: Balance the half equation separately. 

i) balance the element other than oxygen and hydrogen

reduction:  Cr₂O₇^2- ®  2Cr³⁺         

oxidation:  2Cl^-  ®  Cl₂         

ii) balance the oxygen atom by adding H₂O, balance the hydrogen atom by adding H⁺ 

reduction: 14H⁺ + Cr₂O₇^2- ®  2Cr³⁺ + 7H₂O        

oxidation:  2Cl^-  ®  Cl₂         

iii) balance the cherge by adding electrons to the side with the greater overall positive charge.

reduction:  6e + 14H⁺ + Cr₂O₇^2- ®  2Cr³⁺ + 7H₂O 
                           ____+12_____              _____+6_____
        

oxidation:  2Cl^-  ®  Cl2 +  2e  

STEP 4: Balance electrons for both half-reactions

Multiply each half-reaction by an integer, so that number of electrons lost in one reaction equals the number gained in the other reaction.

reduction:   6e + 14H⁺ + Cr₂O₇^2- ®  2Cr³⁺ + 7H₂O 
     

oxidation:  ( 2Cl^-  ®  Cl2 +  2e ) X3
                   6Cl^-  ®  3Cl2 +  6e 

STEP 5: Combine both half equations

Cancelling same species appearing on both sides of the equation.

eduction:   6e + 14H⁺ + Cr₂O₇^2- ®  2Cr³⁺ + 7H₂O      

oxidation:                            6Cl^-  ®  3Cl2 +  6e 
_______________________________________________
Overall: 14H⁺ + Cr₂O₇^2- + 6Cl^-  ®  2Cr³⁺ + 7H₂O + 3Cl2 
____________________________________________________________

STEP 6: Verification

Check the equation to make sure that there are the same number of atoms of each kind and the same total charge on both sides.