Transcript Chapter 9
Reminders • End of semester Fri. Jan 24th (15 school days) • Finals Jan 22-24 • Chp 6 E.C. (Passed out today/ due Mon. Jan 13th) One cool thing you did over break… Today • Chp 9 Learning Targets • Unexpected Changes Lab • Introduction to Chp 9 • Handout E.C. assignment Chapter 9 (Last one for the semester!) Chemical Reactions and Writing Equations 1/6/14 Learning Targets • Describe the characteristics of a chemical reaction. • Explain how a chemical equation describes what happens in a chemical reaction. Important Terms: • Chemical Change (Reaction)- a process in which pure substance(s) are converted into different pure substance(s) • Conservation of Matter- in chemical and physical reactions, matter is neither created nor destroyed Examples Combustion of Ethanol Oxidation of Iron Precipitation of Silver Formation of Coral Reefs (from Calcium Carbonate) How do we describe/represent what happens in a chemical reaction? Reactants- substances which are present before a chemical reaction Products- substances which are present after a chemical reaction What causes chemical reactions? • What do we know about the importance of valence electrons? Chemical reactions provide atoms the opportunity to achieve a full set of valence electrons and become more stable (more energetically favorable) Since some elements/molecules are already stable, energy sometimes has to be supplied to initiate a chemical reaction (think back to the flame test lab) (strike a match!) Reminders • End of semester Jan 24 (14 school days) • Finals Jan 22-24 1/7/14 Learning Targets • Explain how a balanced chemical equation illustrates the law of conservation of matter. • Understand the five general types of reactions and how to balance them. Types of Chemical Reactions 1) 2) 3) 4) 5) Synthesis Decomposition Single Replacement Double Replacement Combustion By knowing the type of reaction that is occurring, you can predict the products that will be formed. Chemical EquationsHow we describe what is happening Using WordsHydrogen reacts with oxygen to form (or yield) water Using Formulas and Symbols 2H2 + O2 2H2O Breaking down the Notation of Chemical Equations 2H2(g) + O2(g) 2H2O(l) Describing the physical state of each substance ■ Solid (s) ■ Liquid (l) ■ Gas (g) ■ Aqueous (aq) means dissolved in water Coefficient • Like in math, the coefficient is the number placed in front of a formula. This number indicates the quantity of each molecule or atom in the reaction. • Ex. 2Ca(s) + O2(g) 2CaO(g) Symbols used in Chemical Equations • Double arrow indicates a reversible reaction • Shows that heat is supplied to the reaction • Used to indicate a catalyst is supplied, in this case, platinum. All of these are special conditions Balancing Chemical Equations • Since the conservation of mass says matter is neither created nor destroyed, we have to make sure our equation agrees • # reactant atoms must = # product atoms Step 1: Balancing Equations • Write the word equation that describes the reaction. iron reacts with oxygen to yield iron oxide Or Iron + Oxygen Iron Oxide Step 2: Balancing Equations 2. Replace the words in the equation with symbols and formulas. Fe + O2 Fe2O3 Do we have the same numbers of each element on both sides of arrow? Does this follow the law of conservation of matter? Step 3: Balancing Equations 3. Count the # of atoms of each element on both sides of the equation. Fe + O2 Fe2O3 Step 4: Balancing Equations 4. Starting with elements that only occur in one substance on each side of the equation, make sure that each side of the equation has an equal # of that element. Proceed with all elements. Remember that changing the # of one element may alter elements that have already been balanced. Fe + O2 ―› Fe2O3 Let’s try: CH4 +2O2 CO2 + 2H2O ! • Never change a subscript to balance an equation. If you change the formula you are describing a different reaction. ■ H2O is a different compound than H2O2 ■ • Never put a coefficient in the middle of a formula ■ 2 NaCl is okay, Na2Cl is not. Step 5: Check Your Work 5. Make sure that the equation is properly balanced. CH4 + 2O2 CO2 + 2H2O Balancing Equations: Examples ■ H2 ■ Co + ■ + O2 → O2 → H2O Co2O3 Pb(NO3)2 + K2S → PbS + KNO3 Balance the following iron(II) chloride + sodium phosphate → sodium chloride + iron (II) phosphate FeCl2 + Na3PO4 → NaCl + Fe3(PO4)2 Today ■ ■ Look at 3 different types of reactions. Begin “Single-Replacement Lab” set-up. Five General Types of Chemical Reactions • • Direct Combination (Synthesis) • • • • Decomposition Single-Replacement Double-Replacement Combustion By knowing the type of reaction that is occurring, you can predict the products that will be formed. I. Direct Combination Reactions (also called synthesis reactions). General form: A + B → AB (two reactants make a single product) A, B = elements or compounds AB = compound consisting of A and B ■ This is the only type of chemical reaction in which there is a single product formed. This single product is always more complex than the reactants. Examples of Synthesis Reactions ■ ■ calcium + oxygen yields calcium oxide 2Ca + O2 → 2CaO ■ carbon dioxide + water yields carbonic acid CO2 ■ ■ + H2O → H2CO3 Notice: All equations show two (or more) reactants, but only one product. http://www.ric.edu/ptiskus/reactions/Index.htm II. Decomposition Reactions General form: AB → A + B (one reactant makes two or more products) AB = compound A, B = elements or simpler compounds ▪ This is the only type of chemical reaction in which there is a single reactant. This single reactant is always more complex than the products. Decomposition Reactions: Examples ■ water yields hydrogen and oxygen 2H2O ■ ■ 2H2 + O2 marble (calcium carbonate) yields calcium oxide and carbon dioxide CaCO3 ■ → → CaO + CO2 Notice: all equations show a single reactant decomposing into two (or more) products. http://www.ric.edu/ptiskus/reactions/Index.htm III. Single-Replacement Reactions General Form: A + BX → AX + B One element and one compound recombine (switch partners) AX, BX = ionic compounds A, B = Metals X = ion that switches partners *Metal ‘A’ must be more reactive than ‘B’ for this to occur Single-Replacement Examples ■ Copper metal and silver nitrate: Cu + AgNO3 → CuNO3 + Ag ■ Notice: In the reaction, an copper combines with silver nitrate to create copper nitrate and silver ■ Zn + HCl → ZnCl2 + H2 IV. Double-Replacement Reactions General form: AX + BY → AY + BX (Positive ions in two compounds are exchanged) A,B = positive ions X,Y = negative ions ■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products. Double Replacement Examples ■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid CaCO3 + 2HCl → CaCl2 + H2CO3 ■ Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds www.ric.edu/ptiskus/reactions/Index.htm IV. Double-Replacement Reactions General form: AX + BY → AY + BX (Positive ions in two compounds are exchanged) A,B = positive ions X,Y = negative ions ■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products. Double Replacement Examples ■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid CaCO3 + 2HCl → CaCl2 + H2CO3 ■ Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds www.ric.edu/ptiskus/reactions/Index.htm Rules of Double-Replacement Reactions ■ ■ Reactants must be dissolved in water (releasing the ions). Will occur if one of the products : • • • is a molecule (covalent), a precipitate (solid comes out of solution), or an insoluble gas. V. Combustion Reactions General Form: CxHy + O2 → H2O + CO2 (hydrocarbon and oxygen react to form carbon dioxide and water) ■ This is the only type of chemical reaction where something reacts with oxygen and forms carbon dioxide and water Combustion Examples ▪ Methane reacts with oxygen: CH4 (methane) + O2 → H2O + CO2 ▪ Gasohol reacts with oxygen: C2H5OH (ethanol) + O2 → H2O + CO2 ▪ Notice: in both cases, water and carbon dioxide are the products. www.ric.edu/ptiskus/reactions/Index.htm 1. Write the word equation 2. Write the balanced formula equation ■ Solid iron (III) sulfide reacts with gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas. 1. Write the word equation 2. Write the balanced formula equation ■ Nitric acid reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate.