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Chemical Reactions
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  1. Chemical Reactions

  2. Empirical formula • The simplest ratio between the atoms. • The formulas for ionic compounds must be written as empirical formulas. • For molecular substances sometimes the ratio between the atoms in a molecule is not in the simplest ratio. When the molecular and empirical formula are different then the molecular formula is a multiple of the empirical formula.

  3. DO NOW • Which compound contains a greater % of Oxygen. Justify your answer with a calculation • Li2O, CaO, PbO2

  4. 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

  5. Products Reactants

  6. How to Describe a Reaction • A reaction can be described several ways: #1.In a sentence every item is a word Copper reacts with chlorine to form copper (II) chloride. #2.In a word equation some symbols used Copper + chlorine → copper (II) chloride #3.In a chemical equation only chemical equations are used Cu + Cl2→ CuCl2

  7. Symbols in Equations • (s) after the formula = solid: Fe(s) • (g) after the formula = gas: CO2(g) • (l) after the formula = liquid: H2O(l) • (aq) after the formula = dissolved in water, an aqueous solution: NaCl(aq) is a salt water solution

  8. Symbols used in equations • the arrow →separates the reactants from the products (arrow points to products) • Read as: “reacts to form” or yields or produces • The plus sign + means “and” • ↑ used after a product indicates a gas has been produced: H2↑ • ↓ used after a product indicates a solid has been produced: PbI2↓

  9. Symbols used in equations • double arrow indicates a reversible reaction (more later) • shows that heat is supplied to the reaction • is used to indicate a catalyst is supplied (in this case, platinum is the catalyst)

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

  11. The Skeleton Equation • All chemical equations are a description of the reaction. • A skeleton equation uses formulas and symbols to describe a reaction • but doesn’t indicate how many; this means they are NOT balanced

  12. Write a skeleton equation for: • Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. • Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water.

  13. Write a skeleton equation for: • Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. FeS(s) + HCl(g)→ FeCl2(s) + H2S(g) • Nitric acid dissolved in water reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate dissolved in water. HNO3(aq) + Na2CO3(g)→ CO2(g) + H2O(l)

  14. Now, read these equations: Fe(s) + O2(g)→ Fe2O3(s) Cu(s) + AgNO3(aq)→ Ag(s) + Cu(NO3)2(aq) NO2(g) N2(g) + O2(g)

  15. ANSWERS TO WORKSHEET • 3 • 2 • 3 • 2 • 2 • 2 • 3 • 1 • 3 • 2 • 24. Crystal is a hydrate. Heating removes the water of hydration • 25 Hydrated crystal is blue • Anhydrous crystal is white • 26 The anhydrous compound is pure. The hydrated crystal contains 50% water so it is more expensive (you are paying for water in the bottle!!!)

  16. Balanced Chemical Equations • According to the Law of Conservation of Mass: atoms aren’t created or destroyed in a chemical reaction, they are just rearranged. • All the atoms we start with in the reactants we must end up within the products (meaning: balanced!) • A balanced equation has the same number of each element on both sides of the equation.

  17. Rules for balancing: • Assemble the correct formulas for all the reactants and products, using “+” and “→” • Count the number of atoms of each type appearing on both sides • Balance the elements one at a time by adding coefficients (the numbers in front) where you need more - save balancing the H and O until LAST! (hint: I prefer to save O until the very last) • Double-Check to make sure it is balanced.

  18. Never • Never change a subscript to balance an equation (You can only change coefficients) • If you change the subscript (formula) you are describing a different chemical. • H2O is a different compound than H2O2 • Never put a coefficient in the middle of a formula; they must go only in the front 2NaCl is okay, but Na2Cl is not.

  19. Practice Balancing Examples • _AgNO3 + _Cu → _Cu(NO3)2 + _Ag • _Mg + _N2→ _Mg3N2 • _P + _O2→ _P4O10 • _Na + _H2O → _H2 + _NaOH • _CH4 + _O2→ _CO2 + _H2O

  20. Practice Balancing Examples • 2AgNO3 + _Cu → _Cu(NO3)2 + 2Ag • 3Mg + _N2→ _Mg3N2 • 4P + 5O2→ _P4O10 • 2Na + 2H2O → _H2 + 2NaOH • _CH4 + 2O2 _CO2 + 2H2O

  21. Types of Reactions • There are probably millions of reactions. • We can’t remember them all, but luckily they will fall into several categories. • We will learn: a) the 5 major types. • We will be able to: b) predict the products. • For some, we will be able to: c) predict whether or not they will happen at all. • How? We recognize them by their reactants

  22. A A B also called Combination Reactions 2 substances combine to make one compound Synthesis Reaction A B + The general equation is A + B  AB

  23. #1 Synthesis Reactions also called Combination Reactions • 2 substances combine to make one compound • the general equation is : A + B → AB • Ca + O2→ CaO element + element • SO2 + O2→ SO3 compound + element • CO2 + H2O → H2CO3compound + compound • We can predict the products, especially if the reactants are two elements. • Mg + N2→_______ Mg3N2

  24. HCl(g) + NH3(g) NH4Cl(s) Synthesis Reaction

  25. Complete and balance: • Ca + Cl2→ • Fe + O2 → (assume iron (II) oxide is the product) • Al + O2 → • Remember that the first step is to write the correct formulas – you can still change the subscripts at this point, but not later while balancing! • Then balance by changing the coefficients only

  26. Decomposition Reaction A A B A B + The general equation is : AB → A + B A reaction where a more complex molecule breaks down to form two or more simpler products

  27. Decomposition Reaction 2NH4NO3(s) 4H2O(g) + 2N2(g) + O2(g) + energy Timothy McVeigh bombing, 1995 Regular building demolition with ammonium nitrate explosives

  28. #2 - Decomposition Reactions • one reactant breaks apart into two or more elements or compounds. • the general equation is : AB → A + B • H2O H2 + O2 • CaCO3 CaO + CO2 • CuSO4•5H2O CuSO4 + 5H2O • 2NaHCO3(s) Na2CO3(s) + H2O(l) + CO2(g) • Note that energy (heat, sunlight, electricity, etc.) is usually required

  29. #2 - Decomposition Reactions • We can predict the products if it is a binary compound (which means it is made up of only two elements) • It breaks apart into the elements: • H2O • HgO mercury mercury (II) oxide cinnabar

  30. B A + + A A C B B C Single Displacement Reaction A + BC  AC + B • A reaction where an element displaces another element in a compound, producing a new compound and an element • A metal will replace a cation (metal or H) • A non-metal will replace an anion (non-metal)

  31. Zn(s) + 2 HCl (aq) → ZnCl2(aq) + H2(g) zinc metal and hydrochloric acid react to form zinc chloride and hydrogen gas in this single-displacement reaction.

  32. #3 - Single Displacement Reactions • One element replaces another • the reaction follows the form of: compound + element → compound + element • Reactants must be an element and a compound. • Products will be a different element and a different compound. • Na + KCl → K + NaCl (cations switched) • F2 + LiCl → LiF + Cl2(anions switched)

  33. #3 Single Displacement Reactions • Metals will replace other metals (and they can also replace hydrogen) • Zn(s) + 2 HCl (aq) → ZnCl2(aq) + H2(g) • Cu(s) + 2AgNO3 → 2Ag(s) + Cu(NO3)2 • Think of water as: HOH • Metals replace the first H, and then combines with the hydroxide (OH). • 2Na (s) + 2H2O(l) → 2 NaOH(aq) + H2(g)

  34. #3 Single Displacement Reactions • We can even tell whether or not a single displacement reaction will happen: • More ‘active’ element replaces less active • The Activity Series of Metals lists metals (and hydrogen) in order of activity. • Elements higher on the list replaces those lower on the list.

  35. Hg(NO3)2(aq) + 2Ag(s) Hg(s) + 2AgNO3(aq) Single Displacement Reaction Will this reaction occur? H2(g) + ZnCl2(aq) 2HCl(aq) + Zn(s) What about this reaction? Both of these reactions do not occur.

  36. The “Activity Series” of Halogens Higher Activity Halogens can replace other halogens in compounds, provided they are above the halogen they are trying to replace in the periodic table. Fluorine Chlorine Bromine Iodine Lower Activity 2NaF(s) + Cl2(g) 2NaCl(s) + F2(g) ??? MgCl2(s) + Br2(g) No Reaction! ???

  37. #3 Single Replacement Reactions Practice: • Fe + CuSO4→ • Pb + KCl → • Al + HCl →

  38. B B D A A D A A C + B B C + Double Displacement Reaction AD + BC  AC + BD Two compounds switch parts to make two new compounds the general equation is : AB + CD → AD + CB

  39. Double Displacement Reaction sodium chloride and silver fluoride react to form sodium fluoride and silver chloride in this double displacement reaction

  40. #4 - Double Replacement Reactions • Two compounds switch parts to make two new compounds • the reaction is: compound + compound → compound + compound • NaOH + FeCl3→ • The positive ions change place. • NaOH + FeCl3→→Fe+3OH- + Na+1Cl-1 = NaOH + FeCl3→→Fe(OH)3 + NaCl

  41. Double Displacement Reaction Pb(NO3)2(aq) + 2KI(aq) PbI2(s) + 2KNO3(aq)

  42. #4 - Double Replacement Reactions • Have certain “driving forces”, or reasons • Will only happen if one of the products: a) doesn’t dissolve in water and forms a solid (a “precipitate”), or b) is a gas that bubbles out, or c) is a molecular compound (which will usually be water).

  43. Complete and balance: • assume all of the following reactions actually take place: CaCl2 + NaOH → CuCl2 + K2S → KOH + Fe(NO3)3→ (NH4)2SO4 + BaF2→ NaOH(aq) + HCl(aq) → H2O(l) + NaCl(aq)

  44. How to recognize which type? • Look at the reactants: E + E OR C + C = Synthesis C = Decomposition E + C = Single displacement C + C = Double displacement

  45. Practice Examples: • H2 + O2 →→ • H2O →→ • Zn + H2SO4→ → • HgO → → • KBr + Cl2→ → • AgNO3 + NaCl →→ • Mg(OH)2 + H2SO3 →→

  46. XyOz + energy Combustion Reaction fuel O2 + A reaction of a fuel with oxygen, releasing energy in the form of heat and/or light

  47. Combustion Reaction

  48. 2Mg(s) + O2(g) 2MgO(s) + energy P4(s) + 5O2(g) P4O10(g) + energy Combustion Reaction Element + O2 “oxide” + energy

  49. #5 – Combustion Reactions • Combustion is a fast reaction of a substance with oxygen to make compounds called oxides. • the general equation is : fuel + oxygen → oxides + energy • the three things that must be present for combustion to happen are: • fuel • oxygen • spark / heat

  50. Combustion Reaction Examples: • What is the main purpose for which fuels are burned around the world? • The following equations show what happens when different carbon-based fuels are burned. • C(s) + O2(g) → CO2(g) + energy • CH4(g) + 2O2(g) → CO2(g) + 2H2O + energy • ethanol CH3CH2OH(l) + O2(g) → CO2(g) + 3H2O(l) • C6H12O6(s) + 6O2(g)→ 6CO2(g) + 6H2O(g) + energy