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8.1 Describing Chemical Reactions Chemical reaction- process by which one or more substances are changed into one or more different substances Reactants: original substance Products: resulting substance Chemical Equation: represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction. Indications of a Chemical Reaction 1. Evolution of heat and light 2. Production of a gas 3. Formation of a precipitate (a solid that is produced as a result of a chemical reaction in solution and that separates from the solution) 4. Color change Chemical equation gives no indication of whether a reaction will naturally occur or not, the speed of the reaction, or if the reaction would even take place. Formula equation- represents the reactants and products of a chemical reaction by their symbols or formulas Diatomic Molecules- Hydrogen, nitrogen, oxygen , fluorine, chlorine, bromine, iodine (H2,N2, O2, F2, Cl2, B2, I2) Balancing Chemical Equations 1. Write a formula equation by substituting correct formulas for the names of the reactants and the products Ex: H2O(l) → H2(g) + O2(g) 2. Balance the formula equation according to the law of conservation of mass Numbers of each type of atom are the same for both the products and the reactants, the equation is balanced Trial and error o Balance the different types of atoms one at a time o First balance the atoms of elements that are combined and that appear only once on each side of the equation o Balance polyatomic ions that appear on both sides of the equation as a single unit o Balance H atoms and O atoms after atoms of all other elements have been balanced Ex: 2H2O(l) → 2H2(g) + O2(g) 3. Count the atoms to be sure that the equation is balanced Ex: 2H2O(l) → 2H2(g) + O2(g) (4H + 2 O) = (4 H) + (2O) Note: Coefficients- small whole number that appears in front of a formula in a chemical equation If the coefficients do not represent the smallest possible whole-number ratio of reactants and products, divide by their greatest common factor **YOU NEVER BALANCE EQUATIONS BY CHANGING SUBSCRIPTS** 1. Sample Problems: Balance the following reactions: a. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) b. 2HNO3(aq) + Mg(OH)2(s) → Mg(NO3)2(aq) + 2H2O (l) c. Al4C3(s) + 12H2O(l) → 3CH4(g) + 4Al(OH)3 (s) 2. Write a balanced chemical equation for the following: Solid sodium combines with chlorine gas to produce solid sodium chloride. 2Na(s) + Cl2(g) → 2NaCl(s) 3. Write the chemical equation for the following: ammonium chloride + calcium hydroxide → calcium chloride + ammonia + water 2NH4Cl + Ca(OH)2 →CaCl2 + 2NH3 + 2H2O 8.1 Describing Chemical Reactions ___________________ _______________- process by which one or more substances are changed into one or more different substances _____________________: original substance _____________________: resulting substance ______________________ ________________: represents, with symbols and formulas, the identities and relative amounts of the reactants and products in a chemical reaction. Indications of a Chemical Reaction 1. Evolution of ___________ and _______________ 2. Production of a ____________ 3. Formation of a __________________________ (a solid that is produced as a result of a chemical reaction in solution and that separates from the solution) 4. __________________ change Chemical equation gives no indication of whether a reaction will naturally occur or not, the speed of the reaction, or if the reaction would even take place. _____________________ _______________- represents the reactants and products of a chemical reaction by their symbols or formulas ___________________ ________________- Hydrogen, nitrogen, oxygen , fluorine, chlorine, bromine, iodine (H2,N2, O2, F2, Cl2, B2, I2) Balancing Chemical Equations 1. Write a formula equation by substituting correct formulas for the names of the reactants and the products Ex: 2. Balance the formula equation according to the law of conservation of _________ Numbers of each type of atom are the same for both the products and the reactants, the equation is balanced ___________and _______________ o Balance the different types of atoms one at a time o First balance the atoms of elements that are combined and that appear only once on each side of the equation o Balance _____________________ __________ that appear on both sides of the equation as a single unit o Balance ______ atoms and ____ atoms after atoms of all other elements have been balanced Ex: 3. Count the atoms to be sure that the equation is balanced Ex: Note: _________________- small whole number that appears in front of a formula in a chemical equation If the coefficients do not represent the smallest possible whole-number ratio of reactants and products, divide by their _______________ _______________ ___________________ **YOU NEVER BALANCE EQUATIONS BY CHANGING _______________** Sample Problems: 1. Balance the following reactions: a. _____Zn(s) + _____HCl(aq) → ______ZnCl2(aq) + ______H2(g) b. _____HNO3(aq) + _____Mg(OH)2(s) → ____Mg(NO3)2(aq) + ___H2O (l) c. _____Al4C3(s) + _____H2O(l) → ______CH4(g) + _____Al(OH)3 (s) 2. Write a balanced chemical equation for the following: Solid sodium combines with chlorine gas to produce solid sodium chloride. 3. Write the chemical equation for the following: ammonium chloride + calcium hydroxide → calcium chloride + ammonia + water