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Chemical Reactions

Reactants  Products. Chemical Reactions. Reactions involve chemical changes in matter resulting in new substances being formed. Reactions involve rearrangement and exchange of atoms to produce new substances. Evidence of Chemical Reactions. Look for evidence of a new substance

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Chemical Reactions

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  1. Reactants  Products Chemical Reactions • Reactions involve chemical changes in matter resulting in new substances being formed. • Reactions involve rearrangement and exchange of atoms to produce new substances.

  2. Evidence of Chemical Reactions • Look for evidence of a new substance • Visual clues (permanent) that indicate a reaction has occurred: • color change • precipitate formation • solid that forms when liquid solutions are mixed • gas bubbles • large energy changes • emission of light • Production of flame • Other indications: • new odor • whooshing sound from a tube • permanent new state • temperature change

  3. Describing chemical reactions by formulas • We want to put enough information into the formula so that we can understand exactly what happens. • Sodium reacts with water to give off hydrogen and produces sodium hydroxide solution. • Na(s) + H2O(l)  H2(g) + NaOH(aq) • We show that solid sodium reacts with liquid water to give off gaseous H2 and aqueous sodium hydroxide.

  4. Chemical Equation • Shorthand way to describe a reaction • Provides information about the reaction • Formulas of the reactants and products • Gives the “states” of the reactants and products • Relative numbers of reactant and product molecules that are required • Can be used to determine amounts of reactants used and amounts of products produced in the reaction • Matter cannot be created or destroyed • Therefore the total mass cannot change • and the total mass of the reactants will be the same as the total mass of the products • In a chemical reaction, all the atoms present at the beginning must still be present at the end of the reaction • if all the atoms are still there, then the mass cannot change

  5. Atoms are conserved in a chemical reaction!

  6. Law of Conservation of Mass Na(s) + H2O(l) H2(g) + NaOH(aq) The number of each type of element in the reactants is the same as the number of each type of element in the products. To achieve this we need to balance the chemical equation. In balancing a chemical equation we can change the number of reactants or products, but never their chemical composition (identities or formulas)

  7. Balancing a Chemical Equation Allowed: Balance the equation by adding coefficients to the left of the chemical formulas: Na(s) + H2O(l) H2(g) + 2 NaOH(aq) Not allowed: Change a chemical formula to make an element balance (balancing H) Na(s) + H3O(l) H2(g) + NaOH(aq) Not allowed: Add more chemical species for convenience (balancing all elements) Na(s) + H2O(l) + H H2(g) + NaOH(aq)

  8. Balanced Chemical Equation 2 Na(s) + 2 H2O(l) H2(g) + 2 NaOH(aq) # Atoms in reactants 2 4 2 # Atoms in products 2 4 2 Element Na H O

  9. Balancing Chemical Equations: “Formal Approach” To balance a chemical equation, do not change any molecules or atoms, but change the number of molecules KClO4 KCl + O2 1. Place a ‘1’ in front of the molecule with the greatest number of atoms. If two molecules have the same number of atoms, then choose the molecule with the greatest number of different elements (this number may change) 1 KClO4 KCl + O2

  10. Balancing Equations 2. Insert coefficients that balance elements that are in compounds with more than one different element 1 KClO41 KCl + O2 3. Place coefficients in front of all else to balance remaining elements 1 KClO41 KCl + 2 O2

  11. 4. Clear fractions, if any, and multiply/divide by a common denominator. If, for instance, we get the following balanced equation... 4 KClO44 KCl + 8 O2 ...then divide by 4 to get... 1 KClO41 KCl + 2 O2 5. Check to be sure the final equation is balanced ReactantsProducts 1 K, 1 Cl, 4 O 1 K, 1 Cl, 4 O Balanced Chemical Equation: KClO4 KCl + 2 O2

  12. Simple, Systematic Approach Balance the following equation: MgCO3 + HCl MgCl2 + CO2 + H2O 1. In how many compounds does each element appear? Choose the element with the lowest number. Element Number of compounds in equation Mg 2 C 2 O 3 H 2 Cl 2

  13. 2. After choosing the element that appears in the fewest number of compounds, balance the number of atoms of that element (i.e., balance the number of Mg, C, H, Cl atoms) 1 MgCO3 + HCl 1 MgCl2 + CO2 + H2O 1 MgCO3 + HCl 1 MgCl2 + 1 CO2 + H2O 1 MgCO3 + 2 HCl 1 MgCl2 + 1 CO2 + H2O Mg C H Note that you can ‘coincidentally’ balance more than one element at a time (such as Cl)

  14. MgCO3 + 2 HCl MgCl2 + CO2 + H2O 3. Always check to confirm that the equation is balanced ReactantsProducts 1 Mg, 1 C, 3 O, 2 H, 2 Cl 1 Mg, 2 Cl, 1 C, 3 O, 2 H Balanced Chemical Equation: MgCO3 + 2 HCl MgCl2 + CO2 + H2O

  15. Balance by Inspection Cu + S Cu2S 1. Determine the required number of compounds by visually inspecting the chemical equation. This is often the fastest method for simple equations. 2 Cu + S Cu2S 2. Always check to confirm that the equation is balanced ReactantsProducts 2 Cu, 1 S 2 Cu, 1 S

  16. Now Your Turn Balance the following chemical equations: CO + O2 CO2 P4 + Cl2 PCl3 P + O2 P2O3 C3H8 + O2 CO2 + H2O Notice that on at least one reaction, you may be able to balance the reaction just by inspecting it

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