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Balancing Chemical Equations

Learn the steps to balance chemical equations, understand the Law of Conservation of Mass, and practice balancing equations with this interactive guide. Discover how to represent chemical reactions and ensure all atoms are balanced.

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Balancing Chemical Equations

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  1. Balancing Chemical Equations What goes in must come out!

  2. Balancing Chemical Equations • Balancing a chemical equation is much like the work of an accountant who has to show every penny that comes in and where it has gone to.

  3. Objectives • Learn the steps to balancing chemical equations. • Take notes to help you understand. • Test yourself with a set of equations to balance. • Enter your own equations to see if they balance.

  4. Law of Conservation of MassYou need to remember this law! • The Law of Conservation of Mass states: that mass is neither created nor destroyed in any chemical reaction. Therefore balancing of equations requires the same number of atoms on both sides of a chemical reaction. • The number of atoms in the Reactants must equal the Number of atoms in the Products

  5. Lavoisier, 1788 Chemical Equations Because of the principle of the Conservation of Matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides.

  6. Law of Conservation of Mass • The mass of all the reactants (the substances going into a reaction) must equal the mass of the products (the substances produced by the reaction). • Reactant + Reactant = Product

  7. A simple equation, such as the synthesis of Iron (II) sulfide, • iron + sulfur Iron (II) sulfide • Fe + S FeS • Note that in a chemical equation, by convention, we use the arrow “ " instead of the equals “ = ".

  8. The last stage is to put in state of matter symbols, (s, l, g, aq), as appropriate (solid, liquid, gas, aqueous or dissolved in water) • Fe(s) + S(s) FeS(s)

  9. Balancing Equations ___ Al(s) + ___ Br2(l) ---> ___ Al2Br6(s) 2 3

  10. Chemical Reactions are Everywhere Cooking Respiration

  11. Chemical Reactions are Everywhere Hair Dye Auto Fuel

  12. How do you know when a chemical reaction takes place? Color Change Precipitate Formation

  13. How do you know when a chemical reaction takes place? Gas Formation Odor

  14. How do you know when a chemical reaction takes place? Temperature Change Change in Acidity

  15. Representing Chemical Reactions • Chemists observe chemical reactions and have come up with a way to represent or model what is happening. • Making NaCl • Solid Sodium combines with Chlorine gas to make solid Sodium Chloride: 2Na (s) + Cl2 (g)  2NaCl

  16. Chemical Equations are different from Numerical Equations • Numerical Equation: 3x + 2y = 47 • Chemical Equation 2Na + Cl2  2NaCl • ReactantA + Reactant B  Product • The reactants are used up in forming the product • The arrow  shows the direction of the reaction

  17. Symbols used in Chemical Equations

  18. If a chemical equation does not obey the law of conservation of mass the equation is said to be what? NOT BALANCED So Let’s look at the steps we need to take to BALANCE chemical equations Let’s work with the following equation: Fe + O2 Fe2O3

  19. #R atom #P • Fe 2 • O 3 Step 1. Create a RAP table (what’s a RAP table ??) A table that shows us what atoms are present in this reaction, how many there are and are they reactants or products? For example: Fe + O2 Fe2O3

  20. #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

  21. #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?

  22. 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.

  23. 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!

  24. #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!

  25. Then all you do is list the atoms that are involved on each side of the arrow Mg + O2 MgO First you need an equation with the correct “formulae” ………. You’ll probably be given this in the question Just like this one Mg + O2 MgO Mg O Mg O

  26. Mg + O2 MgO Mg + O2 MgO Mg O Mg O 1 1 2 1 Then start balancing: [1] Just count up the atoms on each side 1 1 2 1 [2] The numbers aren’t balanced so then add “BIG” numbers to make up for any shortages 2 2 2 And adjust totals

  27. 2 2 1 Mg + O2 MgO 2 2 Mg O But the numbers still aren’t equal, so add another “BIG” number 2 2 And adjust totals again NOW BOTH SIDES HAVE EQUAL NUMBERS OF ATOMS WE SAY THAT THE EQUATION IS BALANCED!!

  28. Try to balance these equations using the same method: [1] Na + Cl2 NaCl [2] CH4 + O2 CO2 + H2O [3] Li + HNO3  LiNO3 + H2 [4] Al + O2 Al2O3

  29. Here are the answers: [1] 2 Na + Cl22 NaCl [2] CH4 + 2 O2 CO2 + 2 H2O [3] 2 Li + 2 HNO3 2 LiNO3 + H2 [4] 4 Al + 3 O22 Al2O3

  30. Let's try solving some complex Problems

  31. #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

  32. #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? …

  33. #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

  34. #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!

  35. #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

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

  37. Example • NH3 + O2 NO + H2O ReactantsProducts • N appears once on both sides in equal numbers, so the coefficient for NH3 is the same as for NO.

  38. Example: NH3 + O2 NO + H2O • Next look at H which appears only once on each side but has different numbers of atoms, 3 on the left and 2 on the right. The least common multiple of 3 and 2 is 6, so rewrite the equation to get 6 atoms of H on both sides: • 2NH3 + O2 NO + 3H2O

  39. Example: 2NH3 + O2 NO + 3H2O • There are 2 oxygen atoms on the left and 5 on the right — the least common multiple of 2 and 5 is 10, so rewrite the equation as: • 2NH3 + 5O24NO + 6H2O

  40. Now count the atoms on each side: • 2NH3 + 5O2 4NO + 6H2O • Write them out keeping them on the appropriate side of the chemical equation • 2 N (nitrogen atoms)4 N (nitrogen atoms) • 6 H (hydrogen atoms) 12 H (hydrogen atoms) • 10 O (oxygen atoms)10 O (oxygen atoms) • This shows the equation not to be balanced “YET”

  41. Check the number again: • If you double the N and H on the left the equation will be balanced: • 4NH3 + 5O2 4NO + 6H2O

  42. Double-check: • 4NH3 + 5O2 4NO + 6H2O • 4 N (nitrogen atoms)4 N (nitrogen atoms) • 12 H (hydrogen atoms)12 H (hydrogen atoms) • 10 O (oxygen atoms)10 O (oxygen atoms) • The equation is Balanced

  43. Balancing Practice • For more help go to: http://richardbowles.tripod.com/chemistry/balance.htm#part0 • For some fun balancing equations go to: • http://www.mpcfaculty.net/mark_bishop/balancing_equations_tutorial.htm

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