Unit 3-8 Balancing equation

1. Unit 3-8 Balancing equation

2. Introduction to Chemical Reactions What is a Chemical Reaction? • A process in which at least one new substance is produced as a result of chemical change. • 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 and chemical equations describe a chemical reaction

3. All chemical reactions… • have two parts: • Reactants = the substances you start with • Products = the substances you end up with • The reactants will turn into the products. Reactants ® Products

4. - Page 321 Products Reactants

5. In a chemical reaction • Must obey the Law of Conservation of Mass. Atoms aren’t created or destroyed (according to the Law of Conservation of Mass) • number and kind of atoms in reactants= number and kind of atoms in products • total mass of reactants = total mass of products • A reaction can be described several ways: • In a sentence:every item is a word Copper reacts with chlorine to form copper (II) chloride. • In a equation:symbols and formula are used. It represents identities and relative amounts of reactants and products in the chemical reaction CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

6. Symbols Used in Equations • yields • reversible reaction • above arrow: • ∆orheat reactants are heated • MnO2 or Ptcatalyst is needed • 25°C specific temperature requirement • 2 atm specific pressure requirement • symbols used to indicate state • (g) = gas; (l) = liquid; (s) = solid • (aq) = aqueous = dissolved in water • NaCl(aq) is sodium chloride dissolved in water whereas NaCl(l) is sodium chloride that has been heated and is now in the liquid state • () = precipitate • () = escaping gas

7. What is a catalyst? • A substance that speeds up a reaction by reducing activation energy, without being changed or used up by the reaction. • Enzymes are biological or protein catalysts in your body.

8. Reading A Chemical Equations • CH4 and O2 are the reactants, and CO2 and H2O are the products • the (g) after the formulas tells us the state of the chemical • the number in front of each substance tells us the numbers of those molecules in the reaction • called the coefficients CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g)

9. Reading A Chemical Equations • this equation is balanced, meaning that there are equal numbers of atoms of each element on the reactant and product sides • to obtain the number of atoms of an element, multiply the subscript by the coefficient 1  C  1 4  H  4 4  O  2 + 2 • This equation means 1 mol methane molecules reacts with 2 mol oxygen to produce 1 mol CO2 and 2 mol of water vapor. CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g)

10. Subscripts and Coefficients Give Different Information • Subscripts tell the number of atoms of each element in a molecule • Coefficients tell the number of molecules

11. Balance Chemical Equations • A balanced equation has the same number of each element on both sides of the equation. • When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction • you maynotchange the subscripts. Why not? • Changing the subscripts changes the compound. Subscripts are determined by the formula.

12. Some Suggestions to Help You • Assume there is a coefficient “1” in front of each reactant and product. • Find an element that is not balanced and balance it. If an element is found in more than one location on either the product or reactant side, you can NOT start with this, as three coefficients would have to be placed to begin, and there are many combinations that you could use and some info is required before they can be balanced. Often this will occur with oxygen or hydrogen. Balance O and H at last! • If there are odd and even of a substance, start with it and make both sides equal by making the coefficient the number of the opposite.

13. Once you make your first “move” by placing 2 coefficients, the rest will follow or be forced. GAME ON! Only one coefficient is placed to balance subsequent atoms each time. • IF everything balances except for oxygen or H, and there is no way to balance it with a whole number, try to use fractions. • Polyatomic ions that appear on both sides of the equation can be balanced as independent units. That is, treat the entire polyatomic ion as a group and balance the group.

14. Example: start with ??? Fe:1Fe3O4 + H23 Fe +H2O do O next O:1Fe3O4 + H2 3 Fe +4H2O finish with H H: 1Fe3O4 + 4 H2 3 Fe +4H2O • Fe3O4 + H2 Fe +H2O

15. Example: NH3+ O2NO +H2O In this case, N is balanced … can’t start here! O occurs in three substances … can’t start here! H occurs in two substances and is unbalanced … start here! 2NH3 + O2NO +3H2O Now balance N. 2NH3 + O22NO + 3H2O Finally balance O 2NH3 + 5/2 O2 2NO + 3H2O Now double all coefficients to create whole numbers. 4NH3 + 5O2 4NO + 6H2O 4 N = 4 N 12 H = 12 H 10 O = 10 O

16. Example: Ba(NO3)2+ Na2CrO4BaCrO4 +NaNO3 There is a polyatomic ion found in both a reactant and product … in fact there are two, so treat them as a “group” and balance! • Start with NO3- as it is unbalanced. 1Ba(NO3)2 + Na2CrO4BaCrO4 +2NaNO3 • Now balance Na. 1 Ba(NO3)2 + 1Na2CrO4BaCrO4 +2NaNO3 • Finally balance Ba and CrO4-2 1Ba(NO3)2 + 1Na2CrO41BaCrO4 +2NaNO3 Check that all atoms, or polyatomic ions are balanced. 2 NO3- = 2 NO3- 1Ba = 1Ba 2 Na = 2 Na 1 CrO4-2 = 1 CrO4-2

17. Homework • Handout. Due tomorrow!