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Chapter 9 Chemical Reactions -orThe old element shuffle Review: Naming and writing formulas for ionic compounds. Ex1: FeCl3 Find the name for each ion using your ion sheet. Cl has a -1 charge and is chloride. Fe is iron and has more than 1 possible ion. Since we have 3 Cl’s and therefore 3 negative charges, the iron must be +3, so the ion is iron (III). Put the ion names together, cation first. iron (III) chloride Ex2: Barium nitrate Find the ion symbols: barium = Ba+2 nitrate = NO3Put them together so that the charges cancel: Ba(NO3)2 Practice: chromium (II) sulfate K2S All chemical reactions are written as: Reactants → Products Both reactants and products may be one or more elements or compounds. A written chemical reaction is a description of what really happens in nature. Two laws govern what happens in a chemical reaction: 1. The law of conservation of mass says that matter is not created or destroyed in a chemical reaction. This means that only atoms that you start with on the reactant side must be present on the product side. These atoms are rearranged to form new substances. This also means that the total mass of the reactants must equal the total mass of the products Ex: Diagramming the reaction of CH4 with O2. 2. The law of conservation of energy says that energy is not created or destroyed in a chemical reaction. Energy can only be transferred to or from the surroundings or transformed into a different form of energy. This means that the energy stored in the reactant chemicals plus any absorbed energy must equal the energy stored in the product chemicals plus any released energy. When considering a chemical reaction, the surroundings include everything that is not actually involved in the reaction ie. Solvents, beakers, air, your fingers. Ex: Diagram the reaction of CH4 with O2 in terms of energy. Energy diagrams: Exothermic Endothermic The amount of energy that is gained or released in a given chemical reaction can be measured. The amount of energy change for a reaction is specific to that reaction and is called the enthalpy of the reaction. It is given the symbol ΔH Ex: For the reaction 2H2 + O2 → 2H2O For every 1 mole of H2 that reacts, 68.15 kcal of heat are released to the surroundings. We write this as ΔH = -68.15 kcal/mol Note that ΔH is negative. This indicates that heat is removed from the system (exothermic). ΔH for an endothermic reaction will be positive (heat added to the system). Signs of a chemical reaction: 1. Heat (given off or absorbed) 2. Light emitted 3. Color change 4. Gas given off (bubbling, fizzing) 5. Forming a precipitate (solid forms from solution) There are 2 types of numbers present in chemical equations: 1. Subscripts. These describe the compositions of the substances in the reaction. They tell how many atoms of each element are present in a given molecule. These are fixed for a substance and cannot be changed. Ex: O2 Na2SO4 Al2O3 2. Coefficients. These are the larger font numbers that appear in front of a substance in a reaction. They tell how many of that substance take part in this particular reaction. These may vary from one reaction to another. Ex: 2H2 + O2 → 2H2O CH4 + 2O2 → CO2 + 2H2O Note that when the coefficient is 1, we don’t write anything. 1 is always understood. The coefficient multiplies the element or compound that comes after it. Ex: 2H2O means 2 water molecules, (H2O H2O) this means 4 H atoms and 2 O atoms. 3Na2SO4 + 2Fe → Fe2(SO4)3 + 6Na How many Na Na S S O O Fe Fe Other symbols that you may see in chemical equations: Subscripts: (s) = solid eg. Na(s) (l) = liquid eg. H2O(l) (g) = gas eg. Cl2(g) (aq) = aqueous (dissolved in water) eg. NaCl(aq) Ex: 2H2(g) + O2(g) → 2H2O(l) Sometimes there are symbols or numbers above or below the arrow. These indicate conditions for the reaction. 1. Reversible reaction – 2. Heat added – 3. Specific temperature – 4. Specific pressure – 5. Catalyst – Examples: Balancing chemical equations: Since a chemical equation is a description of what happens in a given reaction, the identities of the products and reactants cannot change. This means that you may never change a subscript when balancing an equation. To balance an equation you may only insert coefficients so that the number of each type of atom is the same on both sides of the equation. Ex: CH4 + 2O2 → CO2 + 2H2O 1C 4H 4O 1C 4H 4O (Java – balancing) How to balance equations: 1. Make an inventory of the number of atoms of each element on both sides of the arrow. Ex: C2H4 + O2 → CO2 + H2O CCHHOO2. Pick an element to balance first. Save an element that appears by itself until the end. Ex: Save O until the end. Balance C first. 3. Adjust your inventory to show changes. Work back and forth between reactants and products until all elements are balanced. 4. If it will not work out. Start over with a different element to begin. 5. Make sure that the coefficients are in the lowest ratio. Hints: Polyatomic ions that do not change in a reaction may be balanced as a group. Ex: H2SO4 + Cr → Cr2(SO4)3 + H2 6. Balance with a fraction and then multiply the whole equation to get rid of the fraction. Ex: C3H6 + O2 → CO2 + H2O Practice: CH4 + Cl2 → CCl4 + HCl KClO3 → KCl + O2 MgCl2 + FeN → FeCl3 + Mg3N2 Pb(NO3)2 + NaCl → NaNO3 + PbCl2 What a balanced equation can tell you – Charting a reaction 2C2H6 Moles Molecules Molar mass Mass (g) Total mass (g) + 7O2 → 4CO2 + 6H2O 2BF3 Moles Molecule s Molar mass Mass (g) Total mass (g) + 3Li2SO3 → B2(SO3)3 + 6LiF The coefficients can be considered as mole ratios. They tell you the relative numbers of moles of all products and reactants in the reaction. Types of chemical reaction 1. Synthesis. In these reactions , the reactants are elements (or very simple compounds) that combine to form a more complex compound. Form: A + B → AB (where A and B are elements and AB is a compound formed from the elements) Ex: 2H2 + O2 → 2H2O Predicting the products of a synthesis reaction: Use your ion sheets to tell what ions A and B will form. Put the ions together to form a neutral compound. Balance the equation. Ex: Mg + Cl2 → ???? Mg forms a +2 ion Cl forms a -1 ion. They would combine as 1 Mg and 2 Cl so the compound that forms isMgCl2. Fill in the product and balance. Mg + Cl2 → MgCl2 Ex2: Al + Br2 → Al form s a +3, Br forms a -1 so, AlBr3 Al + Br2 → AlBr3 balance!!!! 2. Decomposition. In these reactions , the reactants are compounds that break down into the elements (or simpler compounds) Form: AB → A + B (just the opposite of synthesis) Ex: 2H2O → 2H2 + O2 Predicting the products of a decomposition reaction: Break up the compound into elements (do not forget BOFINCH) Balance the equation. Ex: NO2 → N2 + O2 Balance!!! 3. Single displacement. In these reactions you start with a single element and a compound. The single element takes the place of (displaces) one of the elements in the compound. Form: A + Bx → Ax + B where A is an element and Bx is a compound. A takes the place of B with x. Ex: Al + CuCl2 → AlCl3 + Cu Predicting the products of a SD reaction: Find if the lone element on the reactant side forms a + or – ion. If it forms a + ion then it takes the place of the + ion in the compound. (- ion takes – ion’s place). Put A and x together to form a neutral compound. B ends up as the elemental form. Balance. Ex: Na + ZnCO3 → ??? Na forms a +1 ion. Zn is a +2 and CO3 is a -2. Na will take the place of Zn. The compound that forms will be Na2CO3 Zn will be left alone Ex2: Cl2 + AlBr3 → ??? Cl forms a -1 ion. Al is +3 and Br is -1 Cl will take the place of Br. The compound that forms will be AlCl3 Br will be left alone 4. Double displacement. In these reactions you start with 2 compounds. The + ions switch partners Form: Ax + By → Ay + Bx where A and B are + ions and x and y are -. A and B switch places. Ex: Na2CrO4 + Pb(NO3)2 → PbCrO4 + NaNO3 Predicting the products of a SD reaction: Switch the partners of the + ions. Figure out how each of the + ions will go with their new partners to form a neutral compound. Balance. Ex: Na3PO4 + Co(NO3)3 → ??? Co+3 and Na+ are the cations so they will switch partners. Co+3 and PO4-3 go together 1:1 CoPO4 Na+ and NO3- go together 1:1 NaNO3 5. Combustion. In these reactions you start with a CH compound. The compound reacts with O2 to form CO2 and H2O. Form: CxHy + O2 → CO2 + H2O The only thing that will change is the CH compound. The products are always the same. Ex: C2H6 + O2 → CO2 + H2O Predicting the products of a combustion reaction: Always the same: carbon dioxide and water Balance. Practice: For the following, give the type of reaction, predict the products and balance. HgO → C6H6 + O2 → Al + Cl2 → K + Al2(SO4)3 → NaBr + AgNO3 → Net Ionic Equations: these are equations that are simplified to shown only the elements or ions that undergo a change in the reaction. Ions that do not undergo a change (are the same on both sides of the equation) are dropped out. These are called spectator ions. Writing net ionic equations: Na2CrO4(aq) + Pb(NO3)2(aq) → PbCrO4(s) + 2NaNO3(aq) Ionic compounds dissociate into ions when dissolved so we can write: 2Na+(aq) + CrO4 -2(aq) + Pb+2 (aq) + 2NO3-(aq) → PbCrO4(s) + 2Na+(aq) + 2NO3-(aq) Since Na+ and NO3- do not change (spectator ions) they drop out of the equation: CrO4 -2(aq) + Pb+2 (aq) → PbCrO4(s) Practice: Write the net ionic equation for Na3PO4(aq) + Co(NO3)3(aq) → CoPO4(s) + 3NaNO3(aq) Activity series is a listing of elements and ions in order of decreasing reactivity. It is used to predict whether a reaction with certain reactants will take place or not. If element A is higher on The list than element B, Then A will replace B in a Single displacement Reaction. If A is lower Than B then no reaction Practice: Which of the following reactions will take place? 3Ca + 2AlCl3 → 3CaCl2 + 2Al H2 + ZnCl2 → 2HCl + Zn 2Na + Cu(NO3)2 → 2NaNO3 + Cu