Balancing Chemical Reactions

1. Balancing Chemical Reactions

2. Balancing Chemical Reactions Reactants: Zn + I2 Product: Zn I2

3. Balancing Equations:At the conclusion of our time together, you should be able to: List the various parts of a chemical equation and define each Count the number of atoms on each side of an equation Use conservation of matter to balance a chemical equation.

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5. Chemical Equations Depict the kind of reactants and productsand their relative amounts in a reaction. 4 4 Al (s) + 3 O2(g)---> 2 Al2O3 (s) The blue numbers are called subscripts. The numbers in the front are called coefficients. The letters (s) and (g) are the physical states of compounds.

6. Introduction Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction

7. Parts of a Reaction Equation • Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). • A + sign separates molecules on the same side • The arrow is read as “yields” • Example 2H2 (g) + O2 (g)  2H2O (l) This reads “hydrogen gas plus oxygen gas reacts to yield dihydrogen monoxide liquid.”

8. The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O2 CO2, contains the same information as the English sentence but has quantitative meaning as well.

9. 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.

10. Symbols Used in Equations • Solid (s) • Liquid (l) • Gas (g) • Aqueous solution (aq) • Catalyst H2SO4 • Escaping gas () • Change of temperature ()

11. Balancing Equations • When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. • Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)

12. Subscripts vs. Coefficients • The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

13. Balancing Equations:Let’s see if you can: List the various parts of a chemical equation and define each Count the number of atoms on each side of an equation

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15. Balancing Chemical Reactions II

16. Balancing Equations

17. Balancing Equations:At the conclusion of our time together, you should be able to: Use conservation of matter to balance a chemical equation.

18. Chemical Equations 4 Al(s) + 3 O2(g) ---> 2 Al2O3(s) This equation means 4 Al atoms + 3 O2 molecules ---produces---> 2 molecules of Al2O3 AND/OR 4 moles of Al + 3 moles of O2 ---produces---> 2 moles of Al2O3

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20. Ag + Cl2 AgCl Total atoms 1 2 1 1 Both sides must be equal for ALL atoms.

21. 2 Ag + Cl2 AgCl Total atoms 1 2 1 1 2 2 Both sides must be equal for ALL atoms.

22. 2 2 Ag + Cl2 AgCl Total atoms 1 2 1 1 2 2 2 Conservation of Matter Both sides must be equal for ALL atoms.

23. CH4 + O2 CO2+ H2O 1 2 1 2 4 2 1

24. CH4 + O2 CO2+ H2O 2 1 2 1 2 4 2 1 2 4

25. CH4 + O2 CO2+ H2O 2 2 1 2 1 2 4 2 1 2 4 4

26. Steps to Balancing Equations There are four basic steps to balancing a chemical equation. 1. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side.

27. Steps to Balancing Equations 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4. Check your answer to see if: • The numbers of atoms on both sides of the equation are now balanced. • The coefficients are in the lowest possible whole number ratios. (reduced)

28. Check Out This Actual Sign!!

30. Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l) This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H2) combines with one of the oxygen atoms from an oxygen molecule (O2) to form H2O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H2 molecule) to make a second H2O molecule.

31. Balancing Equations ___ Al(s) + ___ Br2(l) ---> ___ AlBr3(s) 2 3 2

32. Some Suggestions to Help You • Look for singleton elements in compounds and determine the relationships. • Do the hardest element to balance first keeping in mind the relationships from #1. • Balance the lone elements last.

33. Balancing Equations ____C3H8(g) + _____ O2(g) ----> _____CO2(g) + _____ H2O(g) 1 5 A 3 4 3A 4A 2 11 ____B4H10(g) + _____ O2(g) ----> ___ B2O3(g) + _____ H2O(g) A 4 10 2A 5A

34. Balancing Practice ProblemsChallenges #1 ____C7H6O2+ _____ O2----> _____CO2(g) + _____ H2O(g) 2 15 A 14 6 7A 3A

35. Balancing Equations:Let’s see if you can: Use conservation of matter to balance a chemical equation.

36. Balance Each of the Following Equations#6 ____Al4C3 + _____ H2O ----> _____CH4 + _____ Al(OH)3 1 12 A 3 4 3A 4A

37. Balance Each of the Following Equations#3 ____H3AsO4 ----> _____As2O5 + _____ H2O 2 2A 1 3 A 3A

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39. Balancing Chemical Reactions from Words

40. Balancing Equations:At the conclusion of our time together, you should be able to: List the various parts of a chemical equation and define each Count the number of atoms on each side of an equation Use conservation of matter to balance a chemical equation. Go from a word equation to a balanced chemical equation.

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42. MAGIC NUMBERS What number do you have?? Look familiar???? _________________ - ________ Your house number Your age

43. Converting and Balancing Word Equations Sodium phosphate + iron (III) oxide  sodium oxide + iron (III) phosphate Na3PO4 + Fe2O3 ----> Na2O + FePO4 2 3 2

44. Converting Word Equations into Chemical Equations #10 Strontium iodide + Lead (II) phosphate  Strontium phosphate + lead (II) iodide SrI2 + Pb3(PO4)2 ----> Sr3(PO4)2 + PbI2 3 3

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