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Chapter 3a Formulas, Equations and Moles Lecture Notes Dr. Sammia Shahid

Chapter 3a Formulas, Equations and Moles Lecture Notes Dr. Sammia Shahid. Introduction. Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken

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Chapter 3a Formulas, Equations and Moles Lecture Notes Dr. Sammia Shahid

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  1. Chapter 3a Formulas, Equations and Moles Lecture Notes Dr. Sammia Shahid

  2. 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

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

  4. 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.

  5. Balancing Chemical Equations • A balanced chemical equation represents the conversion of the reactants to products such that the number of atoms of each element is conserved. reactants  products limestone  quicklime + gas Calcium carbonate  calcium oxide + carbon dioxide CaCO3(s)  CaO(s) + CO2(g)

  6. Balancing Chemical Equations CaCO3(s)  CaO(s) + CO2(g) The letters in parentheses following each substance are called State Symbols (g) → gas (l) → liquid (s) → solid (aq) → aqueous

  7. Balancing Chemical Equations A balanced equation MUST have the same number of atoms of each element on both sides of the equation. H2 + O2→ H2O Not Balanced H2 + ½O2→ H2O Balanced 2H2 + O2→ 2H2O Balanced

  8. Balancing Chemical Equations The numbers multiplying chemical formulas in a chemical equation are called: Stoichiometric Coefficients (S.C.) 2H2 + O2→ 2H2O Balanced Here 2, 1, and 2 are stoichiometric coefficients.

  9. 2Na(s) + Cl2(g) 2NaCl(s) Balancing Chemical Equations A balanced chemical equation shows that the law of conservation of mass is adhered to. In a balanced chemical equation, the numbers and kinds of atoms on both sides of the reaction arrow are identical. left side: 2 Na 2 Cl right side: 2 Na 2 Cl

  10. 2H2(g) + O2(g) H2(g) + O2(g) 2H2O(l) H2O(l) Balancing Chemical Equations Write the unbalanced equation using the correct chemical formula for each reactant and product. DO NOT TRY TO BALANCE IT YET! DO NOT CHANGE THE FORMULAS! Find suitable coefficients—the numbers placed before formulas to indicate how many formula units of each substance are required to balance the equation.

  11. 4H2(g) + 2O2(g) 2H2(g) + O2(g) 2H2O(l) 4H2O(l) Balancing Chemical Equations Reduce the coefficients to their smallest whole-number values, if necessary, by dividing them all by a common denominator. divide all by 2

  12. 2H2(g) + O2(g) 2H2O(l) Balancing Chemical Equations Check your answer by making sure that the numbers and kinds of atoms are the same on both sides of the equation. left side: 4 H 2 O right side: 4 H 2 O

  13. Steps to Balancing Equations Some of the Helpful Hints for balancing equations: • Take one element at a time, working left to right except for H and O. Save O for next to last, and H until last. • IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element) • (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units • MINOH: try Metals first, then Ions, then Nonmetals, and Oxygen, then Hydrogen last

  14. 2H2(g) + O2(g) H2(g) + O2(g) H2(g) + O2(g) H2O(l) 2H2O(l) H2O2(l) Balancing Chemical Equations Do not change subscripts when you balance a chemical equation. You are only allowed to change the coefficients. unbalanced Balanced properly Chemical equation changed!

  15. Balancing Chemical Equations Example 1: B2H6 + O2→ B2O3 + H2O Balance O last B is already balanced Start by changing S.C. of H2O: B2H6 + O2→ B2O3 + 3H2O

  16. Balancing Chemical Equations Example 1: B2H6 + O2→ B2O3 + 3H2O B and H are balanced Balance O by changing S.C. of O2 B2H6 + 3O2→ B2O3 + 3H2O BALANCED!

  17. Balancing Chemical Equations Example 2: MnO2 + KOH + O2→ K2MnO4 + H2O Balance O last Mn is already balanced Change S.C. of KOH to balance K MnO2 + 2KOH + O2→ K2MnO4 + H2O

  18. Balancing Chemical Equations Example 2: MnO2 + 2KOH + O2→ K2MnO4 + H2O Mn, K, and H are balanced (H was balanced by chance) Balance O MnO2 + 2KOH + ½O2→ K2MnO4 + H2O or 2MnO2 + 4KOH + O2→ 2K2MnO4 + 2H2O

  19. Balance the following equations: C6H12O6→ C2H6O + CO2 Fe + O2→ Fe2O3 NH3 + Cl2→ N2H4 + NH4Cl Balancing Chemical Equations

  20. Balance the following equations: C6H12O6→ C2H6O + CO2 Unbalanced C6H12O6→ 2C2H6O + 2CO2 Balanced Balancing Chemical Equations

  21. Balance the following equations: Fe + O2→ Fe2O3 Unbalanced 4Fe + 3O2→ 2Fe2O3 (balance O first) Balancing Chemical Equations

  22. Balance the following equations: NH3 + Cl2→ N2H4 + NH4Cl Unbalanced N:H is 1:3 on left, must get 1:3 on right! 4NH3 + Cl2→ N2H4 + 2NH4Cl Balanced Balancing Chemical Equations

  23. Learning Outcomes • The student will be able to: • describe and apply mass relationships in chemical reactions; • define and write chemical formulas; • write and balance chemical reactions and chemical equations.

  24. COUNSELING HOURS

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