CHEMICAL REACTIONS Chemistry I Chapter 7 1 Chemical Equations Their Job: Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients 2 Reactants: Zn + I 2 Product: Zn I 2 The letters (s), (g), and (l) are the physical states of compounds. Introduction Chemical reactions occur when bonds between the outermost parts of atoms are formed or broken Chemical reactions involve changes in matter, the making of new materials with new properties, and energy changes. Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction 3 Parts of a Reaction Equation Chemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A + sign separates molecules on the same side The arrow is read as yields Example C + O 2 CO 2 This reads carbon plus oxygen react to yield carbon dioxide 4 Chemical Equations 6 Symbols Used in Equations 7 Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788 Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) H Catalyst 2 SO 4 Change of temperature ( )
Balancing Equations 8 Balancing Equations 9 2 H 2 (g) + O 2 (g) ---> 2 H 2 O(l) What Happened to the Other Oxygen Atom????? When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combines with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule. you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent) Chemical Equations 10 Subscripts vs. Coefficients 11 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---produces---> 2 molecules of Al 2 O 3 AND/OR 4 moles of Al + 3 moles of O 2 ---produces---> 2 moles of Al 2 O 3 The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound. 12 13 Steps to Balancing Equations There are four basic steps to balancing a chemical equation. 1. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4. Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced) Some Suggestions to Help You Some Helpful Hints for balancing equations: Take one element at a time, working left to right except for H and O. Save H for next to last, and O until last. IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element) (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units
14 15 Balancing Equations 2 Al(s) + 3 Br 2 (l) ---> Al 2 Br 6 (s) Balancing Equations 16 Balancing Equations Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphate 17 C 3 H 8 (g) + 5 O 2 (g) ----> CO 3 2 (g) + 4 H 2 O(g) 2 B 4 H 10 (g) + 11 O 2 (g) ----> 2 Na 3 PO 4 + Fe 2 O 3 ----> 3 Na 2 O + 2 FePO 4 42 B 2 O 3 (g) + 10 5 H 2 O(g) Chemical Reactions Chemistry I Honors Chapter 8 Types of Reactions There are five types of chemical reactions we will talk about: 1. Synthesis reactions 2. Decomposition reactions 3. Single displacement reactions 4. Double displacement reactions 5. Combustion reactions You need to be able to identify the type of reaction and predict the product(s)
Steps to Writing Reactions Some steps for doing reactions 1. Identify the type of reaction 2. Predict the product(s) using the type of reaction as a model 3. Balance it Don t forget about the diatomic elements! (BrINClHOF) For example, Oxygen is O 2 as an element. In a compound, it can t be a diatomic element because it s not an element anymore, it s a compound! 1. Synthesis reactions Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.) reactant + reactant 1 product Basically: A + B AB Example: 2H 2 + O 2 2H 2 O Example: C + O 2 CO 2 Synthesis Reactions Here is another example of a synthesis reaction Practice Predict the products. Write and balance the following synthesis reaction equations. Sodium metal reacts with chlorine gas 2 Na (s) + Cl 2(g) 2NaCl (s) Solid Magnesium reacts with fluorine gas Mg (s) + F 2(g) MgF 2(s) Aluminum metal reacts with fluorine gas 2 Al (s) + 3 F 2(g) 2 AlF 3(s) 2. Decomposition Reactions Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds 1 Reactant Product + Product In general: AB A + B Decomposition Reactions Another view of a decomposition reaction: Example: 2 H 2 O 2H 2 + O 2 Example: 2 HgO 2Hg + O 2
Decomposition Exceptions Carbonates and chlorates are special case decomposition reactions that do not go to the elements. Carbonates (CO 3 2- ) decompose to carbon dioxide and a metal oxide Example: CaCO 3 CO 2 + CaO Chlorates (ClO 3- ) decompose to oxygen gas and a metal chloride Example: 2 Al(ClO 3 ) 3 2 AlCl 3 + 9 O 2 There are other special cases, but we will not explore those in Chemistry I Practice Predict the products. Then, write and balance the following decomposition reaction equations: Solid Lead (IV) oxide decomposes PbO 2(s) Pb (s) + O 2(g) Aluminum nitride decomposes 2 AlN (s) 2 Al (s) + N 2(g) Practice Identify the type of reaction for each of the following synthesis or decomposition reactions, and write the balanced equation: N 2(g) + O 2(g) Nitrogen 2 NO (g) monoxide Synthesis BaCO 3(s) BaO (s) + CO 2 (g) Decomposition 2 Co (s) + 3 S (s) (make Co 2 S 3 Co (s) be +3) 2 NH 3(g) + H 2 CO 3(aq) (NH 4 ) 2 CO 3(s) Synthesis Synthesis 2 NI 3(s) N 2 (g) + 3 Decomposition I 2 (s) 3. Single Replacement Reactions Single Replacement Reactions occur when one element replaces another in a compound. A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). element + compound product + product A + BC AC + B (if A is a metal) OR A + BC BA + C (if A is a nonmetal) (remember the cation always goes first!) When H 2 O splits into ions, it splits into H + and OH - (not H+ and O -2!!) Single Replacement Reactions Another view: Single Replacement Reactions Write and balance the following single replacement reaction equation: Zinc metal reacts with aqueous hydrochloric acid Zn (s) + 2 HCl (aq) ZnCl 2 + H 2(g) Note: Zinc replaces the hydrogen ion in the reaction
Single Replacement Reactions Sodium chloride solid reacts with fluorine gas 2 NaCl (s) + F 2(g) 2 NaF (s) + Cl 2(g) Note that fluorine replaces chlorine in the compound Aluminum metal reacts with aqueous copper (II) nitrate 2 Al (s) + 3 Cu(NO 3 ) 2(aq) 3 Cu (s) + 2 Al(NO 3 ) 3(aq) 4. Double Replacement Reactions Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound Compound + compound product + product AB + CD AD + CB Double Replacement Reactions Think about it like foil ing in algebra, first and last ions go together + inside ions go together Example: AgNO 3(aq) + NaCl (s) AgCl (s) + NaNO 3(aq) Another example: K 2 SO 4(aq) + Ba(NO 3 ) 2(aq) 2 KNO 3(aq) + BaSO 4(s) Practice Predict the products. Balance the equation 1. HCl (aq) + AgNO 3(aq) HNO 3(aq) + AgCl (s) 2. 3CaCl 2(aq) + 2Na 3 PO 4(aq) Ca 3 (PO 4 ) 2(s) + 6 NaCl (aq) 3. Pb(NO 3 ) 2(aq) + BaCl 2(aq) PbCl 2(s) + Ba(NO 3 ) 2(aq) 4. FeCl 3(aq) + 3NaOH (aq) Fe(OH) 3(s) + 3 NaCl (aq) 5. H 2 SO 4(aq) + 2NaOH (aq) 2 H 2 O (l) + Na 2 SO 4(aq) 6. 2KOH (aq) + CuSO 4(aq) K 2 SO 4(aq) + Cu(OH) 2(s) 5. Combustion Reactions Combustion reactions occur when a hydrocarbon reacts with oxygen gas. Combustion Reactions In general: C x H y + O 2 CO 2 + H 2 O Products in combustion are ALWAYS carbon dioxide and water. (although incomplete burning does cause some byproducts like carbon monoxide)
Combustion Example C 5 H 12 + 8 O 2 5CO 2 + 6 H 2 O Write the products and balance the following combustion reaction: 2C 10 H 22 + 31O 2 10 20 CO 2 + 11 22 H 2 O Mixed Practice State the type, predict the products, and balance the following reactions: 1. BaCl 2 + H 2 SO 4 BaSO 4 + 2HCl 2. C 6 H 12 + 9O 2 6 CO 2 + 6H 2 O 3. Zn + CuSO 4 ZnSO 4 + Cu 4. 2 Cs + Br 2 2 CsBr 5. FeCO 3 FeO + CO 2 Total Ionic Equations (HONORS ONLY) Once you write the molecular equation (synthesis, decomposition, etc.), you should check for reactants and products that are soluble or insoluble. We usually assume the reaction is in water We can use a solubility table to tell us what compounds dissolve in water. If the compound is soluble (does dissolve in water), then splits the compound into its component ions If the compound is insoluble (does NOT dissolve in water), then it remains as a compound Solubilities Not on the Table! Gases only slightly dissolve in water Strong acids and bases dissolve in water Hydrochloric, Hydrobromic, Hydroiodic, Nitric, Sulfuric, Perchloric Acids Group I hydroxides (should be on your chart anyway) Water slightly dissolves in water! (H+ and OH-) For the homework SrSO 4 does NOT dissolve in water There are other tables and rules that cover more compounds than your table! Total Ionic Equations Molecular Equation: K 2 CrO 4 + Pb(NO 3 ) 2 PbCrO 4 + 2 KNO 3 Soluble Soluble Insoluble Soluble Total Ionic Equation: 2 K + + CrO 4-2 + Pb +2 + 2 NO 3- PbCrO 4 (s) + 2 K + + 2 NO 3 - Net Ionic Equations These are the same as total ionic equations, but you should cancel out ions that appear on BOTH sides of the equation Total Ionic Equation: 2 K + + CrO -2 4 + Pb +2 + 2 NO 3- PbCrO 4 (s) + 2 K + + 2 NO - 3 Net Ionic Equation: CrO -2 4 + Pb +2 PbCrO 4 (s)
Net Ionic Equations Try this one! Write the molecular, total ionic, and net ionic equations for this reaction: Silver nitrate reacts with Lead (II) Chloride in hot water (Lead (II) chloride WILL dissolve in hot water, but not in cold!). AgNO 3 + PbCl 2 Molecular: 2 AgNO 3 + PbCl 2 2 AgCl + Pb(NO 3 ) 2 Total Ionic: 2 Ag + + 2 NO 3- + Pb +2 + 2 Cl - 2 AgCl (s) + Pb +2 + 2 NO 3 - Net Ionic: Ag + + Cl - AgCl (s)