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Ch. 8: Chemical Equations

Ch. 8: Chemical Equations. The Chemical Equation Writing & Balancing Chemical Equations Information in a Chemical Equation. Modified from unknown author 2011 MO. Chemical Reactions.

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Ch. 8: Chemical Equations

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  1. Ch. 8: Chemical Equations The Chemical Equation Writing & Balancing Chemical Equations Information in a Chemical Equation Modified from unknown author 2011 MO

  2. Chemical Reactions • A chemical equation represents with symbols and formulas the identities and relative amounts of the reactants and products in a chemical reaction • Possible indicators of a chemical reaction include: • Change in heat and light (exothermic/endothermic) • Production of a gas • Formation of a precipitate • Color change

  3. 8.1 Word Equations To write a word equation, write the names of the reactants to the left of the arrow separated by plus signs; write the names of the products to the right of the arrow, also separated by plus signs (+). example: Iron + Oxygen  Iron(III)oxide The arrow ()in the word equation can be read as either: 1. yields 2. gives 3. reacts to produce The + can be read as…plus, combineswith, when reacted with (on right) Iron turns to red-brown rust in the presence of oxygen in the air

  4. 8.1 Skeleton Equations • A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. • Write the formulas of the reactants to the left of the arrow (yields sign) and the formulas of the products to the right. example: Fe + O2 Fe2O3

  5. 8.1 Balancing Chemical Equations • The small whole numbers that are placed in front of the formulas in an equation are called coefficients examples: 2H2 + (1)O2 2H2O • A chemical reaction is also described by a balanced equation in which each side of the equation has the same number of atoms of each element and mass is conserved. • To write a balanced chemical equation, first write the skeleton equation. Then, use coefficients to balance the equation so that it obeys the law of conservation of mass.  law of conservation of mass: in any chemical change, mass is conserved.

  6. 8.1 Balancing Equations * to add more information to an equation we can indicate the physical states of each substance. 1. ( l ) = liquid 2. (g) = gas 3. (aq) = aqueous solution 4. (s) = solid example: 4Fe(s) + 3O2 (g)  2Fe2O3 (s) • A catalyst is used in many reaction mixtures to speed up the reaction; because the catalyst is not a reactant or a product its formula is written above the arrow in a chemical equation. MnO2 example: 2H2O2 (aq)  2H2O (l) +2O2 (g)

  7. 8.1 Symbols used in Chemical Equations • + Used to separate two reactants or two products •  “Yields” separates reactants from products • Used in place of  for reversible reactions • (s) Designates a reactant or product in the solid state • (l) Designates a reactant or product in the liquid state • (g) Designates a reactant or product in the gaseous state • (aq) Designates an aqueous solution •  Indicates that heat is supplied to a reaction •  A formula written above the yield sign and it indicates a catalyst Pt

  8. Additional symbols used in chemical equations

  9. Balancing Chemical Equations Steps: • Write the unbalanced equation. • Inventory the atoms in the reactants and in the products. • Balance the equation. Strategies: • Balance elements left-to-right on the Periodic Table (metals then nonmetals) • Balance polyatomic ions that appear on both sides of the equation as single units. • Leave H and O until last.

  10. 1. Write the unbalanced equation. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __H2 (g)

  11. 2. Inventory reactants and products. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __H2 (g) 1 1 1 Zn Cl H 1 2 2

  12. BALANCED ! BALANCED ! 3. Balance the equation __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __H2 (g) 1 2 1 1 1 2 2 1 1 1 Zn Cl H 2 2

  13. Try to balance this reaction: 1 2 1 2 __CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) 1 4 2 C H O 1 2 3 4 4 4

  14. Try to balance this reaction: 2 2 3 __KClO3 (s)  __KCl(s) + __O2 (g) 2 1 1 3 K Cl O 1 1 2 2 2 2 6 6

  15. Try to balance this reaction: 2 3 1 6 __Al(OH)3(s) + __H2SO4(aq)  __Al2(SO4)3(aq) + __H2O(l) 2 1 1 5 7 Al S H O 2 3 2 13 3 12 12 8 18 10 18

  16. BALANCED ! What does a Chemical Equation mean? C3H8 (g) + 5 O2 (g)→3 CO2 (g)+4 H2O (l) In terms of particles: 1 molecule C3H8 + 5 molecules O2 →3 molecules CO2 + 4 molecules H2O or 3 C atoms + 8 H atoms + 10 O atoms → 3 C atoms+ 6 O atoms + 8 H atoms + 4 O atoms → (3 C + 8 H + 10 O) (3 C + 8 H + 10 O)

  17. BALANCED ! C3H8 (g) + 5 O2 (g)→ 3 CO2 (g)+4 H2O (l) In moles: 1 mole C3H8 4 moles H2O + 5 moles O2 → 3 moles CO2 + or (3)(12g)+ (8)(1g)44 g C3H8 (3)(12g)+ (6)(16g)132 g CO2 (8)(1g)+ (4)(16g)72 g H2O (5)x (32 g)160 g O2 + → + (44 g + 160 g = 204 g) → (132 g + 72 g = 204 g)

  18. 8.2 & 8.3 Types of Chemical Reactions • Synthesis Reaction - Two or more substances( elements or compounds) combine to form a single new substance.A + B → ABA+B+C → ABC • Decomposition Reactions - A single compound (one reactant) breaks down to form two or more smaller compounds or elements - Most decomposition reactions take place only when energy in the form of electricity (electrolysis), light or heat is added. Takes energy to break bonds. AB → A + B ABC → A+B+C • Single-Replacement Reactions - One element replaces a similar element in a compound - Many Reactions take place in aqueous solution. - More active metal replaces another. (Activity Series) -both reactant and product side have an element & a compound. - A + BX → AX + B Ex) 2Al(s) + 3Pb(NO3)2(aq) → 3Pb(s) + 2Al(NO3)3(aq) • Double-Replacement Reactions - a chemical change involving an exchange of positive ions between two compounds. - One of the compounds formed is usually a precipitate(solid that settles out of solution), an insoluble gas, or water. - The other compound remains dissolved in solution. AX + BY → AY + BX • Combustion Reactions - A substance combines with oxygen, releasing a large amount of energy in the form of light and heat.

  19. Synthesis Reaction (Combination) - Two or more substances( elements or compounds) combine to form a single new substance. A + B → AB A+B+C → ABC May be more than one product possible: nm +nm= Solid sulfur (S8) can combine with oxygen gas to produce sulfur dioxide or sulfur trioxide. m+nm= iron can combine with sulfur to produce ferrous sulfide or ferric sulfide. Often produces large amounts of energy: forming bonds produces energy

  20. a. Reactions with Oxygen & Sulfur - Element + Oxygen → Oxide ex)2Mg(s) + O2(g) → 2MgO(s) - Element + Sulfur → Sulfide ex)see slide before b. Reactions with Halogens - Metal + Halogen → ionic compound ex)2Na(s) + Cl2(g) → 2NaCl(s) Mg(s) + F2(g) → MgF2(s)

  21. Decomposition Reactions - A single compound (one reactant) breaks down to form two or more smaller compounds or elements - Most decomposition reactions take place only when energy in the form of electricity (electrolysis), light or heat is added. Takes energy to break bonds. AB → A + B ABC → A+B+C

  22. a. Decomposition of a Binary Compound into its element ex) 2H2O(l) → 2H2(g) + O2(g) b. Metal Carbonates - Metal Carbonate → Metal oxide + Carbon Dioxide gas ex) CaCO3(s) → CaO(s) + CO2(g) c. Metal Hydroxides (except those containing Group 1 metals) - Metal Hydroxide → Metal oxide + water ex) Ca(OH)2(s) → CaO(s) + H2O(g) d. Metal Chlorates - Metal chlorate → Metal chloride + Oxygen ex) 2KClO3(s) → 2KCl(s) + 3O2(g)

  23. Single-Replacement Reactions - One element replaces a similar element in a compound - Many Reactions take place in aqueous solution. - More active metal replaces another. (Activity Series) -both reactant and product side have an element & a compound. - A + BX → AX + B Ex) 2Al (s) + 3Pb(NO3)2(aq) → 3Pb(s) + 2Al(NO3)3(aq)

  24. Activity Series • Lists metals in order of decreasing reactivity • Activity of halogens decreases down group • An element can replace any element below it but not any above it. Ex. 1) 2Al(s) + 3ZnCl2(aq) → 3Zn(s) + 2AlCl3(aq) Ex. 2) Co(s) + 2NaCl(aq) → no reaction

  25. Double-Replacement Reactions - a chemical change involving an exchange of positive ions between two compounds. - One of the compounds formed is usually a precipitate(solid that settles out of solution), an insoluble gas, or water. - The other compound remains dissolved in solution. AX + BY → AY + BX

  26. a. Formation of a Precipitate - The cations of one reactant combine with the anions of another reactant to form an insoluble compound. ex) 2KI(aq) + Pb(NO3)2(aq) → PbI2(s) + 2KNO3(aq) b. Formation of a Gas ex) FeS(s) + 2HCl(aq) → H2S(g) + FeCl2(aq) c. Formation of Water ex) HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

  27. Combustion Reactions - A substance combines with oxygen, releasing a large amount of energy in the form of light and heat. X + O2(g) → XO + heat Ex) C3H8 (g) + 5O2(g) → 3CO2 (g) + 4H2O(g)

  28. CoCl2 6H2O Formulas of hydrates are written by first writing the formula for the anhydrous compound and then adding a dot followed by the number of water molecules present. 2-di 3-tri 4-tetra 5-penta 6-hexa 7- hepta 8-octa 9-nona 10-deca Solids that contain water as part of their crystalline structure are known as hydrates.

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