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Transcript
Chemical Reactions I. Chemical Reactions A. Definition Chemical reaction- process where one or more substances are rearranged to form new substances B. Indications/Signs of a Chemical Reaction Sign of a Chemical Reaction: Precipitate Formation Precipitate is a solid that forms from two liquids - suspension Sign of a Chemical Reaction: Gas Formation Sometimes when two substances come in contact a reaction occurs producing a gas. Sign of a Chemical Reaction: Permanent Color Change Some substances, when combined, will turn another color Energy is produced in the form of light When sodium is placed in water, the reaction is so violent a fire results Sign of a Chemical Reaction: Temperature Change • Exothermic reactions feel warm • Endothermic reactions feel cold Sign of a Chemical Reaction: Temperature Change When some chemicals react they either: give off energy, exothermic reaction (feel warm to the touch) OR use energy, endothermic reaction, and get cooler C. Representing Chemical Reactions We use chemical equations as a way to represent reactions Substances that start the reaction are called reactants Substances that are formed are called the products C. Representing Chemical Reactions Reactant 1 + Reactant 2 Product 1 + Product 2 (the number of reactants and products will vary) C. Representing Chemical Reactions The large number in front of elements or compounds are called coefficients Cu + 2AgNO3 Cu(NO3)2 + 2Ag Determining States of Matter • Most states of matter will be provided • Acids are aqueous, unless stated otherwise • Metals are solid at room temperature, except mercury (liquid) Determining States of Matter • Most diatomics are gases at room temperature (bromine is liquid and iodine is solid) • For products that are ionic compounds in water: use the solubility rules on the back of your periodic table to determine the state of matter. Insoluble substances will exist as solids. II. Describing Chemical Changes A. Writing equations • Equations are more practical than words to express a chemical reaction • It is very important that you note that “brinclhof” elements can not exist alone (Ex. Cl2 & NaCl not NaCl2) A. Writing Equations If they are not combined with a different element, they will bond with themselves, forming a diatomic molecule. Br I N Cl HO F Bromine Iodine Nitrogen Chlorine Hydrogen Oxygen Fluorine Diatomics Br I N Cl HO F Symbols used in reactions These symbols must be memorized! NOTE: All acids will be aqueous unless stated otherwise Common Compounds to Know NH3 = ammonia CH4 = methane Word Equation Not in your notes! equation expressed in words Iron (s) + chlorine (g) iron (III) chloride (s) Equations that only show formulas of substances involved, but are not balanced are called skeletal equations Ex 1: Solid sodium hydrogen carbonate (bicarbonate) reacts with hydrochloric acid to produce aqueous sodium chloride, water, and carbon dioxide gas. (not balanced) NaHCO3(s) + HCl (aq) NaCl(aq) + H2O(l) + CO2 (g) All acids will be aqueous Ex: In your notes BaCl2 (aq) + Na2SO4 (aq) BaSO4 (s) + 2NaCl (aq) “A solution containing 1 mole of aqueous barium chloride reacts with an aqueous solution containing 1 mole of sodium sulfate to yield 1 mole of solid (or precipitated) barium sulfate and 2 moles of aqueous (dissolved) sodium chloride.” Practice Practice 1. Write equations for each chemical reaction. a. Sulfur burns in oxygen to form sulfur dioxide. S + O2 (g) SO2 No Δ !! b. Heating potassium chlorate solid in the presence of the catalyst manganese (IV) oxide produces oxygen gas. Potassium chloride is left as a solid. KClO3(s) MnO2 O2 (g) + KCl(s) Practice Practice 2. Write a sentence that describes each chemical reaction. a. KOH(aq) + H2SO4(aq) H2O(l) + K2SO4(aq) An aqueous solution containing 1 mole of potassium hydroxide reacts with 1 mole of sulfuric acid to yield 1 mole of water and 1 mole of aqueous potassium sulfate. b. Na(s) + H2O(l) NaOH(aq) + H2(g) One mole of solid sodium reacts with 1 mole of water to yield 1 mole of aqueous sodium hydroxide and 1 mole of hydrogen gas. Practice (Not in your notes) Hydrofluoric acid (HF) is used to etch grooves in glass because it reacts with the silica (SiO2(s)) in glass. The products of the reactions are aqueous silicon tetrafluoride and water. What is the coefficient of silica in the balanced equation for the reaction? HF(aq) + SiO2(s) SiF4 (aq) + H2O(l) III. Balancing Chemical Equations A. Law of Conservation of Mass • • • Mass can not be created nor destroyed Since atoms have mass, the number of each type of atom must be equal on both sides of a reaction The sum of the mass of the reactants must equal the sum of the mass of products Rules for balancing equations • When balancing an equation you can only add coefficients, never add or change the subscripts (Ex. For 3H2O, 3 is the coefficient and 2 is the subscript) Coefficient vs. Subscript Example: 3H2O H2O = formula for water molecule 1 molecule of water = 2 Hydrogens & 1 Oxygen Coefficient of 3 means there are 3 molecules of water 3 molecules of water = 6 Hydrogens & 3 oxygens Rules for balancing equations • The coefficient tells you the number of particles of the element or compound used/produced in the reaction. (Ex. In 3H2O, there are 3 molecules of water) Particles can be: atoms, molecules, formula units, or moles 4Fe(s) + 3O2(g) 2Fe2O3(s) 4 atoms of iron used 3 molecules of oxygen used 2 formula units of iron (III) oxide produced The coefficient also tells you the number of moles of the element or compound used/produced in the reaction. 4Fe(s) + 3O2(g) 2Fe2O3(s) 4 moles of iron used 3 moles of oxygen used 2 moles of iron (III) oxide produced • For the sake of space, we sometimes leave off the state of matter after each of the substances • When no coefficient is written, it is assumed to be 1. Not in your notes! How many grams of solid silver are produced Cu(s) + AgNO3(aq) CuNO3(aq) + Ag(s) 63.5g 148.3g 123.4g ? 88.4g A balanced equation CH4(s) + 2 O2(g) CO2(g) + 2 H2O(g) Reactants Products Carbon 1 1 4 Hydrogen 4 4 4 oxygen Ex 1: Write a balanced equation for the reaction of copper metal and an aqueous solution of silver nitrate to form copper (II) nitrate and silver. Correct Formulas for: silver nitrate & copper (II) nitrate Ag+ NO3- Ag (NO3 ) Parentheses not needed Cu+2 NO3- Cu(NO3)2 Parentheses needed Cu(s) + AgNO3(aq) Cu(NO3)2 (aq) + Ag(s) Example 1 2 2 __ Cu(s)+ __ AgNO3(aq) __ Cu(NO3)2(aq)+ __ Ag 1 Cu 1 2 1 Ag 1 2 21 N 2 63 O 6 Balanced! Example 1 again 2 2 __ Cu(s)+ __ AgNO3(aq) __ Cu(NO3)2(aq)+ __ Ag 1 Cu 1 2 1 Ag 1 2 2 1 NO3 2 Balanced! Ex 2: Sodium and chlorine combine to form sodium chloride. Na + Cl2 NaCl Ex 2: Balance the following reaction: Na(s) + Cl2(g) NaCl(s) 2 2 2 1 Na 1 2 2 Cl 1 2 Balanced! Molar Ratio between sodium and chlorine is 2:1, it takes 2 moles of sodium to completely react with 1 mole of Chlorine (2 moles of sodium chloride are produced) Extra Example Balance the following reaction: Al(s) + O2(g) Al2O3(s) 42 3 4 2 1 6 2 2 Al 2 4 O 3 6 Balanced! Ex 3: Sodium hydroxide and calcium bromide react to produce calcium hydroxide and sodium bromide. 2 NaOH + 1 CaBr2 1 Ca(OH)2 + 2 NaBr What is the molar ratio between sodium hydroxide and calcium bromide in this equation? 2:1 Tips for Balancing Chemical Reactions 1) Write a correct skeletal chemical equation (use oxidation number to make products) 2) Identify and Count the number of atoms of each element in the reactants and products. 3) Balance the elements one at a time by using coefficients. (NEVER CHANGE SUBSCRIPTS) • it’s usually easier to begin with elements that appear only once • save lone elements (i.e. O2) to balance last • many times you can treat polyatomics as one unit 4) Check each atom or polyatomic ion to be sure the equation is balanced • Sometimes it is helpful to think of water as HOH • Finally, make sure all the coefficients are in the lowest possible whole number ratio that balances. Practice Practice 1. Write balanced chemical equations. a. Sodium and water form aqueous sodium hydroxide and hydrogen gas. 2Na + 2H2O(l) 2NaOH(aq) + H2(g) b. Aqueous calcium hydroxide and sulfuric acid react to form aqueous calcium sulfate and water. Ca(OH)2(aq) + H2SO4(aq) CaSO4(aq) + 2H2O(aq) Practice Practice 2. Balance the following reactions. a. 2SO2 + b. O2 2SO3 Fe2O3 + 3H2 2Fe + 3H2O c. 4P + 5O2 d. 21Al + P4O10 6HCl 2 AlCl3 + 3H2 3 e. 2C2H6 + 7O2 4CO2 + 6H20 More Practice 2 2 2 4 4 2 42 2 Xe 1 2 F 1 2 4 H1 2 4 O2 ___ XeF2 + ___ H2O __ Xe + __ O2 + __ HF 1 2 2 1 Practice (Not in your notes) Hydrofluoric acid (HF) is used to etch grooves in glass because it reacts with the silica (SiO2(s)) in glass. The products of the reactions are aqueous silicon tetrafluoride and water. What is the coefficient of silica in the balanced equation for the reaction? 1 4 HF(aq) + SiO2(s) 41 H 41 F 1 Si 2 O SiF4 (aq) + 2 H2O(l) 24 4 1 12 IV. Types of Chemical Reactions A. Classifying Chemical Reactions There are 5 basic types of chemical reactions: • Synthesis • Decomposition • Single replacement • Double Replacement • Combustion 1. Synthesis • Two or more substances combine to create a more complex substance (synthesis means “to make”) • Two or more parts combine and make ONE product General Form: A + B AB EX: 2Na + Cl2 2NaCl Hey baby let’s get jiggy. Synthesis aka Combination Element + Element Compound Synthesis reaction of two elements Ex: 2 Al + ___ 3 Cl2 ___ 2 AlCl3 ___ Al 3+ Cl 2. Decomposition • A complex substance is broken down into two ore more simpler substances (decompose means “to break down”) Look for (usually) one reactant and more than one product to identify a DECOMPOSITION REACTION. General Form: AB A + B Break yoself fool! Decomposition Ex: 2 NaCl 2Na + Cl2 3. Single Replacement Reactions one element replaces another element in a compound to form new compound General Form: A + BX AX + B I’m gon’ dance with yo’ lady 3. Single Replacement Reactions Ex: aluminum + iron (III) oxide 2 Al + 1 Fe2O3 2 Fe + 1 Al2O3 Single Replacement For Single Replacement Reactions FYI: Not all single replacement reactions will occur. You must consider the activity series. Metals that are lower on the activity series are “less active” and will not displace more metals. Although Hydrogen is not a metal, it can act as a metal and is included in the series. 4. Double Replacement Ions from two ionic compounds switch places Basically: an exchange of cations between two ionic compounds (H2O and HOH are the same thing) 4. Double Replacement: ions from two ionic compounds switch places Double Replacement 4. Double Replacement General Form: AX + BY AY + BX switch 4. Double Replacement 4. Double Replacement Ex: Li1+ I- Ag1+ NO3- lithium iodide and aqueous silver nitrate react LiI + AgNO3 LiNO3 + AgI(s) 5. Combustion A substance combines with oxygen, releasing a large amount of energy in the form of light or heat On the reactant side, look for a hydrocarbon (a compound made of C and H) or an oxygenated hydrocarbon (a compound made of C, H and O), plus a diatomic oxygen. 5. Combustion Combustion of hydrocarbons ALWAYS produces CO2 and H2O General Form: CxHyO + O2 CO2 + H2O 5. Combustion Ex: Show combustion of propane (C3H8) gas 1 C3H8 + 5 O2 3 CO2 + 4 H2O I sell propane and propane accessories! Practice (Not in your notes) Tin was among the first metals used by humans. Elemental tin is produced by heating tin(IV) oxide, the principal ore of tin, with carbon. The products of this reaction are tin and carbon dioxide. When the equation is written and balanced, ID the type of reaction. SnO2 + C Sn (s) + CO2 (g) Sn4+ O2O2- Single Replacement V. Predicting Products for Replacement Reactions To predict products of chemical reactions, it is important that you recognize ions V. Predicting Products from chemical reactions a. Single Replacement Reactions (SR) To predict products of SR reactions, create a new compound with the lone metal ion and the anion from the reactant compound. The cation from the reactant compound becomes a lone element on the product side. •You must consider the activity series (on the back of your periodic table) •NR = no reaction Ex 1 Mg(s) + Zn(NO3)2(aq) Mg Zn2+ NO3- 2+ + Mg(NO3)2 + Zn Cation replaces cation Ex 1 Mg(s) + Zn(NO3)2(aq) Magnesium + Zinc nitrate Magnesium nitrate + Zinc Mg2+ NO3Zn Mg(NO3)2 + Zn Mg(s) + AgNO3(aq) Magnesium + silvernitrate Magnesium nitrate + silver Mg2+ NO3Ag Mg(NO3)2 + Ag Ag(s) + Mg(NO3)2(aq) silver + Magnesium nitrate NR Practice 1.Write a balanced reaction for each SR reaction. a. Zn(s) + H2SO4(aq) ZnSO + H 4 2 b. 2 Na(s) + 2H2O(l) 2 NaOH + H2 HOH c. Sn(s) + NaNO3(aq) NR b. Double Replacement Reactions and Precipitates to predict products of a double replacement reaction, simply switch ions, being very careful to create new formulas from the new combination (assume constant oxidation #’s) Ex 1 Pb(NO3)2(aq) + KI(aq) Lead (II) nitrate + Potassium iodide Lead (II) iodide + potassium nitrate PbI2 + KNO3 Ex 2 FeS(s) + HCl(aq) Iron (II) sulfide + hydrogen chloride Iron (II) chloride + Hydrogen sulfide FeCl2 +H2S Ex 3 HCl(aq) + NaOH(aq) hydrogen chloride + sodium hydroxide hydrogen hydroxide + sodium chloride HOH +NaCl Practice 1. Write balanced equations for these DR reactions. a. NaOH + Fe(NO3)3 Na(NO3) + Fe(OH)3 b. 3KOH (aq) + H3(PO4) (aq) K3(PO4) + 3HOH c. NaCl + KBr NaBr + KCl c. Combustion Reactions All combustion reactions will have products of CO2 & H2O. Ex 1: C2H4 + O2 ^All produce CO2 + H2O^ C2H4 + O2 CO2 + H2O Balanced: C2H4 + 3O2 2CO2 + 2H2O