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Balancing Equations

Balancing Equations. Chemical equation must obey the law of conservation of mass. According to the Law, mass cannot be created or destroyed BALANCED OR NOT BALANCED? Mass of reactants: molar mass of Fe + 2(molar mass of O)= 55.85 g + 2X 16 g = 56 g + 32 g= 88 g

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Balancing Equations

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  1. Balancing Equations

  2. Chemical equation must obey the law of conservation of mass. According to the Law, mass cannot be created or destroyed BALANCED OR NOT BALANCED? Mass of reactants: molar mass of Fe + 2(molar mass of O)= 55.85 g + 2X 16 g = 56 g + 32 g= 88 g Mass of products: 2X 56 g + 3X 16g= 160 g Mass of reactants < mass of products NOT BALANCED Let’s look at the following equation: Fe + O2 Fe2O3

  3. #R atom #P • Fe 2 • O 3 So how do we balance? Step 1. Create a RAP table that shows how many elements are there as reactants or products? For example: Fe + O2 Fe2O3

  4. #R atom #P 1 Fe 2 2 O 3 Rule 2. Go to the first atom that’s not balanced and balance it! Since Fe atoms are not balanced what do we need to do to balance it? Right! Multiply it by 2 (Only multiply) 2x

  5. #R atom #P 1 Fe 2 2 O 3 In step 2 we balanced the number of Fe atoms by multiplying the reactant side by 2. This now becomes the new coefficient in the chemical equation. Modify the equation to reflect the change 2Fe + O2 Fe2O3 2x Are all atoms balanced?

  6. 3. Move to the next unbalanced atom. What is it? #R atom #P #R atom #P 2 Fe 2 2 O 3 2 Fe 2 3x2 O 3x2 How can we balance the Oxygen? Multiply Reactants by 3 and Products by 2 Adjust the equation to reflect your changes 2Fe + 3O2 2Fe2O3 But notice that by changing Oxygen we also Changed Iron. We need to go back and fix this.

  7. 4. Write out the updated RAP table. How can we Balance the Iron? #R atom #P 2 Fe 4 6 O 6 Sure! Multiply the # of Reactant Fe atoms by 2 ! 2x Re-write the equation reflecting The new changes you’ve made. 4Fe + 3O2 2Fe2O3 Do we have a balanced Chemical Equation now? Yes we do!

  8. #R atom #P • 2 Ag 2 • 2NO32 • Mg 1 • 2 Cl 2 Polyatomics When an equation has Polyatomics in it, such as in this Balanced chemical equation 2AgNO3 + MgCl2 2AgCl + Mg(NO3)2 And the polyatomic appears on BOTH the reactant and product Side of the equation Count the polyatomic as an “ATOM” So the above reactant atoms would be: If the same polyatomic does not Appear on both sides break the Polyatomic down into atoms!

  9. Let's try solving some Problems

  10. #R atom #P • NA 1 • 1 OH 2 • Ca 1 • 2 Br 1 Is this equation balanced? NaOH + CaBr2  Ca(OH)2 + NaBr • What atoms do we have in • This equation? • Count atoms & Start • the RAP table 2) Do the #Reactant atoms = the # of Product atoms? 3) So pick the 1st unbalanced atom & begin balancing

  11. #R atom #P 1 Na 1 1 OH 2 1 Ca 1 2 Br 1 We’ll start with balancing Hydroxide NaOH + CaBr2 Ca(OH)2 +NaBr How can we make both Hydroxides equal? Sure we’ll multiply #R OH by 2 Next step> rewrite the modified eqn. 2x 2NaOH + CaBr2  Ca(OH)2 + NaBr Hydroxide is now balanced so let’s move to the next Unbalanced atom, which is? …

  12. #R atom #P 2 Na 1 2 OH 2 1 Ca 1 2 Br 1 What can we do to balance the Bromine? Sure! Multiply the #P Bromine by 2 Now adjust the table to reflect The changes and then rewrite the Eqn. x2 2NaOH + CaBr2 Ca(OH)2 + 2NaBr

  13. #R atom #P 2 Na 2 2 OH 2 1 Ca 1 2 Br 2 Let’s update the RAP table with the new #’s Based on our updated equation. 2NaOH + CaBr2 Ca(OH)2 + 2NaBr Are we now balanced? Sure!

  14. #R atom #P #R atom #P 2 C 1 6 H 2 2 O 3 2 C 2 6 H 2 2 O 5 Ok Try Balancing this equation: C2H6 + O2  CO2 + H2O Step 1. Total up the atoms Step 2. Balance the #P Carbon & Re-write the equation C2H6 + O2 2CO2 + H2O Are we done?

  15. #R atom #P #R atom #P 2 C 2 6 H 2 2 O 5 2 C 2 6 H 6 2 O 7 Step 3. Carbons are balanced now but Hydrogen isn’t. So, balance Hydrogen atoms next Multiply #P Hydrogen by 3 x3 Step 4. Re-write the eqn. & Retotal the number of atoms C2H6 + O2  2CO2 + 3H2O Carbon and Hydrogen are now balanced but oxygen isn’t.

  16. #R atom #P 2 C 2 6 H 6 2 O 7 Step 5. To balance Oxygen multiply Oby 3½ Step 6. Re-write the eqn. & Retotal the number of atoms 3 ½ x C2H6 + 3½ O2  2CO2 + 3H2O It looks like we’re balanced. But, are we? No! We can’t have 3 ½ Oxygen molecules! Only whole Numbers are allowed. So what do we need to do to fix this?

  17. #R atom #P 4 C 4 12 H 12 14 O 14 Step 7. Let’s clean this up by Multiply everything by 2 C2H6 + 3½ O2  2CO2 + 3H2O x 2 2C2H6 + 7 O2 4CO2 + 6H2O Step 8. Retotal #R and the #P atoms Are we balanced? YES!

  18. #R atom #P 1 NH41 1 OH 3 1 Fe 1 3Cl 1 Try this problem NH4OH + FeCl3 Fe(OH)3 + NH4Cl Start here. Recognize we Have polyatomics but they Appear on both sides of the Equation. OK … Now finish it up

  19. Answer to previous problem 3NH4OH + FeCl3 Fe(OH)3 + 3NH4Cl

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