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Chemical Reactions Chapter 7

Chemical Reactions Chapter 7. Tro, 2 nd ed. SSME’s on SSTS Combustion Reaction. H 2 (white) & O 2 (red)  2 H 2 O Plus LOTs of energy!!! A typical chemical reaction powers the liftoff of a space shuttle, along with the more complex solid rocket boosters combustion.

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Chemical Reactions Chapter 7

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  1. Chemical ReactionsChapter 7 Tro, 2nd ed.

  2. SSME’s on SSTS Combustion Reaction H2(white) & O2(red)  2 H2O Plus LOTs of energy!!! A typical chemical reaction powers the liftoff of a space shuttle, along with the more complex solid rocket boosters combustion.

  3. Evidence of Chemical Reactions There are five main observations that denote a chemical reaction may have occurred. Try to list them yourself first: Color change Precipitate Gas or odor detected Heat evolved or absorbed Change in pH (later in semester)

  4. Chemical Equations for Chemical Reactions Chemical reactions always involve change Atoms, molecules or ions rearrange to form new substances: chemical bonds are broken and new bonds are formed Chemical equations describe reactions observed in the laboratory or in nature; we use the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction. Chemical equations provide us with the means to: - summarize the reaction - display the substances that are reacting (reactants) - show the products - indicate the amounts of all component substances in a reaction

  5. Chemical Reactions (Unbalanced) Chemical Equation Al + Fe2O3 Fe + Al2O3 reactants products iron oxygen aluminumoxygen bonds break bonds form

  6. Chemical Reactions Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting. 2Al + Fe2O3 2Fe + Al2O3 Coefficient Coefficient Conditions required to carry out the reaction may be placed above or below the arrow. D = Heat added.

  7. Law of Conservation of Mass 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 are still present at the end - if all the atoms are still there, then the mass will not change

  8. Symbols used in chemical equations Learn the state symbols: s, l, g, aq Delta, D, means heat  means “yields” or makes or produces

  9. THREE STEPS TO WRITING CHEMICAL EQUATIONS: 1. PREDICT WHAT PRODUCTS WILL BE If two elements react, you can decide what compound will form based on whether it’s ionic or covalent. EXAMPLE: Al metal reacts with Cl2 gas (nonmetal) to make an ionic cmpd. What ions are likely to form? Al3+ and Cl-, so the product will be AlCl3. If two compounds react, you need to know about types of chemical reactions (coming up soon).

  10. THREE STEPS TO WRITING CHEMICAL EQUATIONS: 2. WRITE CORRECT CHEMICAL FORMULA FOR EACH REACTANT AND PRODUCT **Remember the seven diatomic elements **Remember the naming rules This is the step where most of the mistakes are made!!!

  11. THREE STEPS TO WRITING CHEMICAL EQUATIONS: 3. BALANCE EQUATION USING STOICHIOMETRIC COEFFICIENTS **Never change formulas’ subscripts to mass-balance an equation! Start with the most complex formula or with the formula that contains the most of an atom **Treat polyatomic ions as single units unless they have broken up in the reaction

  12. THREE STEPS TO WRITING CHEMICAL EQUATIONS: An Example Write the chemical equation for the reaction of solid aluminum with chlorine gas to form aluminum chloride salt Predict the product – has already been given, but we’ll learn how to do this later Write the correct chemical formulas – keep working on this __Al(s) + __Cl2(g) __AlCl3(s)Not mass balanced Balance equation using the correct stoich coefficients 1 Al & 2 Cl  1 Al & 3 Cl 2 Cl vs. 3 Cl: Find least common denominator (6) ___Al(s) + 3 Cl2(g) 2 AlCl3(s) Not balanced yet 1 Al & 6 Cl  2 Al & 6 Cl 2 Al(s) + 3 Cl2(g) 2 AlCl3(s)Now it’s balanced!

  13. Practice writing and balancing chemical equations 1. Glucose and oxygen produce carbon dioxide and water 2. Sulfuric acid and potassium hydroxide produce potassium sulfate and water 1. C6H12O6(s) + 6 O2(g) 6 CO2(g) + 6 H2O(l) 2. H2SO4(aq) + 2KOH(aq)  K2SO4(aq) + 2 H2O(l)

  14. The meaning of a formulais context-dependent. The formula H2O can mean: • 2 H atoms and 1 O atom • 1 molecule of water • 1 mole of water • 6.0221 x 1023 molecules of water • 18.015 g of water

  15. In an equation formulas can represent units of individual chemical entities or moles. Formulas Number of molecules Number of atoms Number of moles Molar masses

  16. Aqueous Solutions Some chemicals in a reaction are dissolved in water; they are called aqueous solutions Dissolving chemicals in water helps them to react together faster - water separates chemicals into individual molecules or ions, called dissociation - the particles come in contact more frequently speeding the reaction up

  17. Dissociation When ionic compounds dissolve in water, the anions and cations are separated from each other - this is called dissociation - however not all ionic compounds are soluble in water! When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion

  18. Dissociation When potassium iodide dissociates in water into potassium cations and iodide anions: KI(aq) K+1(aq) + I-1(aq) Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions CuSO4(aq) Cu+2(aq) + SO4-2(aq) Potassium sulfate dissociates in water into potassium cations and sulfate anions K2SO4(aq) 2 K+1(aq) + SO4-2(aq)

  19. Electrolytes Electrolytes are substances whose water solution is a conductor of electricity All electrolytes have ions dissolved in water

  20. Electrolytes Strong electrolytes: all electrolyte “units” are separated into ions; includes soluble salts, strong acids, strong bases Nonelectrolytes: none are separated into ions; includes covalent solutes Weak electrolytes: a small percentage are separated into ions; includes weak acids & weak bases

  21. Types of Electrolytes Salts = ionic compounds; soluble salts are all strong electrolytes Acids = form H+ ions in water solution. Strong acids dissoc 100%  strong electrolytes; weak acids dissoc <10%  weak electrolytes Bases = water soluble metal hydroxides. Strong bases dissoc 100%  strong electrolytes; weak bases dissoc <10%  weak electrolytes

  22. When will a Salt Dissolve? Soluble: if it dissolves in a specified solvent - NaCl is soluble in water, but AgCl is not Insoluble: if a significant amount does not dissolve in the specified solvent - AgCl is insoluble in water (though there is a very small amount dissolved, but not enough to be significant)

  23. Solubility RulesCompounds that are Generally Soluble in Water

  24. Solubility RulesCompounds that are Generally Insoluble

  25. Five Main Types of Chemical Equations Combination Decomposition Single-Replacement Double-Replacement Combustion (I added this!) (Ignore sections 9 & 10 and use our lecture notes!)

  26. TYPES OF CHEMICAL REACTIONS: 1. COMBINATION (OR SYNTHESIS): A + B C 2 K + S  K2S 2 Na + Br2 2 NaBr MEMORIZE THESE TWO: H2O + CO2 H2CO3 NH3 + H2O  NH4OH

  27. TYPES OF CHEMICAL REACTIONS: 2. DECOMPOSITION: C  A + B 2 KClO3 2 KCl + 3 O2 2 H2O2 2 H2O + O2 Memorize these two: H2CO3 H2O + CO2 NH4OH  NH3 + H2O (What is similar about these two equations and the ones in the first type?)

  28. TYPES OF CHEMICAL REACTIONS: 3. SINGLE REPLACEMENT: A + BC  B + AC K(s) + ZnSO4(aq) Zn(s) + K2SO4(aq) Al(s) + FeCl3(aq) Fe(s) + AlCl3(aq) Replacement occurs only if solid metal element is more reactive than metal ion which is part of the compound ACTIVITY SERIES (partial): K Ca Mg Al Zn Fe Sn H Cu Ag Au

  29. An atom of an element in the activity series will displace an atom of an element below it from one of its compounds . Sodium (Na) will displace an atom below it from one of its compounds. MetalsKCaNaMgAlZnFeNiSnPbHCuAgHg increasing activity

  30. Magnesium is above lead in the activity series. EXAMPLES USING ACTIVITY SERIES: Metal Higher in Activity Series Displacing Metal Below It Mg(s) + PbS(s) MgS(s) + Pb(s) MetalsMgAlZnFeNiSnPb

  31. Silver is below copper in the activity series. Metal Lower in Activity Cannot Displace Metal Above It Ag(s) + CuCl2(aq)no reaction MetalsPbHCuAgHg

  32. Chlorine is above bromine in the activity series. EXAMPLES USING ACTIVITY SERIES: Halogen Higher in Activity Series Displaces Halogen Below It Cl2(g) + CaBr2(s) CaCl2(aq) + Br2(aq) HalogensF2Cl2Br2I2

  33. Using the Activity Series SEE IF THESE REACT: if they do, then determine the product formulas and balance the equations! ___Ca(s) + ___MgCO3(aq) ___Zn(s) + ___HCl(aq) ___Mg(s) + ___AgNO3(aq) ___Ca(s) + ___HOH(l) ___Cu(s) + ___Fe(NO3)3(aq)

  34. TYPES OF CHEMICAL REACTIONS: (continued) 4. DOUBLE REPLACEMENT: trades partners cation to anion! AB + CD  AD + CB A. Precipitation reactions: must learn how to use the solubility rules! NaCl(aq) + AgNO3(aq) NaNO3(aq) + AgCl(s) B. Gas-forming reactions: 2 HNO3(aq) + MgS(aq) H2S(g) + Mg(NO3)2(aq) C. Neutralization (acid-base) reactions: HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) IN EVERY DOUBLE REPLACEMENT REACTION, A SOLID, LIQUID OR GAS HAS FORMED!

  35. TYPES OF CHEMICAL REACTIONS: (continued) 4.A. A precipitate is two ions combining to make a compound that is not soluble in water MgCl2(aq) + 2 NaOH(aq) Mg(OH)2(?) + 2 NaCl(?) One of the two products will be insoluble Use tables and rules to determine which is insoluble and will precipitate.

  36. Types of Chemical Reactions (continued) Practice finding which product is insoluble __Pb(NO3)2(aq) + __KI(aq) __PbI2(?) + ___KNO3(?) Practice: what compound precipitates when aqueous solutions of silver nitrate and lithium bromide are mixed?

  37. TYPES OF CHEMICAL REACTIONS: (continued) 4.B. Gas-forming reactions: may be double replacement, but also others. Na2S(aq) + 2 HCl(aq) 2 NaCl(aq) + H2S(g) All carbonate compounds react with any acid to form CO2(g) If two compounds react to form H2CO3, then it decomposes: H2CO3(aq) H2O(l) + CO2(g) EXAMPLE: NaHCO3(aq) + HCl(aq) NaCl(aq) + H2CO3(aq)  NaCl(aq) + H2O(l) + CO2(g)

  38. TYPES OF CHEMICAL REACTIONS: (continued) 4.C. Acid-base (neutralization) reactions usually form water and a salt HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq) H2SO4(aq) + 2 KOH(aq) 2 HOH(l) + K2SO4(aq) You try: ___H3PO4(aq) + ___Ca(OH)2(aq) _____

  39. TYPES OF CHEMICAL REACTIONS: 5. COMBUSTION: reacting very rapidly and exothermally with O2 gas S(s) + O2(g) SO2(g) (+ HEAT) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) (+HEAT) 2 H2(g) + O2(g) 2 H2O(g) (+ HEAT) C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) (+HEAT) ***Memorize: all hydrocarbon combustion products are the same: CO2 and H2O

  40. PRACTICE: WRITE THE COMPLETE BALANCED CHEMICAL EQUATION& ID THE TYPE OF REACTION 1. Aqueous solutions of magnesium chloride & sodium hydroxide form magnesium hydroxide and sodium chloride (you decide on phases) 2. Decomp of HCl gas to its elements 3. Zinc metal reacts with aqueous nitric acid 4. Ammonia gas plus water (Hint: combination) 5. Aqueous solutions of sulfuric acid + calcium hydroxide react 6. Aqueous solutions of aluminum chloride and zinc bicarbonate react 7. Methane gas burns in air

  41. PRACTICE: WRITE THE COMPLETE BALANCED CHEMICAL EQUATION& ID THE TYPE OF REACTION 1. MgCl2(aq) + 2 NaOH(aq) Mg(OH)2(s) + 2 NaCl(aq) Double replacement - ppt 2. 2 HCl(g) H2(g) + Cl2(g) Decomposition 3. Zn(s) + 2 HNO3(aq) Zn(NO3)2(aq) + H2(g) Single replacement: Zn > H 4. NH3(g) + H2O(l) NH4OH(aq) Combination (memorize) 5. H2SO4(aq) + Ca(OH)2(aq) CaSO4(aq) + 2 H2O(l) Double replacement - neut 6. 2AlCl3(aq) + 3Zn(HCO3)2(aq) 2 Al(HCO3)3(s) + 3 ZnCl2(aq) Double replacement - ppt 7. CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) Combustion HEAT!

  42. NET IONIC EQUATIONS REFER TO HANDOUTS!! SUBSTITUTE “normal chemical equation” for “molecular equation in your textbook PRACTICE, PRACTICE, PRACTICE! Memorize the six strong acids (HCl, HBr, HI, HNO3, H2SO4, HClO4) and what makes a strong base (Group I and Group II oxides and hydroxides)

  43. Heat (Enthalpy) inChemical Reactions (take good notes, not in your chapter) Energy changes always accompany chemical reactions. When this occurs energy is released to the surroundings. The amounts of substances are expressed in moles. H2(g) + Cl2(g)→ 2 HCl(g) + 185 kJ (exothermic) N2(g) + O2(g) + 185 kJ→ 2 NO(g)(endothermic)

  44. Energy of Activation A certain amount of energy is always required for a reaction to occur. The energy required to start a reaction is called the energy of activation. CH4 + 2 O2→ CO2 + 2 H2O + 890 kJ This reaction will not occur unless activation energy is supplied. The activation energy can take the form of a spark or a flame.

  45. 8.1 8.2

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