C_Unit_11B_Chemical_Reactions_2010-2011_K

C_Unit_11B_Chemical_Reactions_2010-2011_K - Unit 11B: Unit...

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Unformatted text preview: Unit 11B: Unit Chemical Equations Cypress Creek High School Chemistry 1K Chapter 10 Chemical Reactions Chemical When a substance undergoes a chemical change, it takes part in a chemical reaction. After it reacts, it no longer has the same chemical identity. What are the evidences that a chemical change has occurred? Chemical Reactions Chemical Example Remember the classic science fair project - the homemade volcano? It’s done by combining vinegar and baking soda (and some red food coloring for effect). These substances undergo a chemical reaction, evident by the overflow and bubbles. This reaction produces three new substances: water, carbon dioxide, and sodium acetate (used in hot packs) Reaction of baking soda and vinegar: http://www.metacafe.com/watch/706897/vinegar_grenade/ Chemical Equations Chemical – The simplest way to represent this reaction is by using words – Chemical equations replace chemical names with chemical formulas Reactants (LEFT) substances that undergo a reaction Yield sign (MIDDLE) represents the change/reaction Products (RIGHT) new substances formed in a reaction Chemical Equations Chemical Practice What is the chemical equation for the following reactions? What are the reactants and products? 1) Water and sulfur trioxide combine to form sulfuric acid 2 3 2 4 H O + SO → H SO reactants product 2) Potassium chlorite decomposes to form potassium chloride and oxygen 2 2 Review of Chemical Equations Review Label the following chemical equation: reactants products SnO2(s) + 2H2(g) → Sn(s) + 2H2O(g) subscript state of matter yield sign coefficient Chemical Equations Chemical States of Matter Some chemical equations indicate the state of matter of the substances (i.e. liquid) + + + Solid (s) - baking soda Liquid (l) - water Gas (g) - carbon dioxide Aqueous (aq) - vinegar and sodium acetate Aqueous means the substance is dissolved in water. For example, NaCl(aq) means table salt dissolved in water. Endothermic Chemical Equations Endothermic Energy is either released or absorbed during a chemical reaction. Can be many forms of energy (light, heat, mechanical, chemical, etc). Endothermic reactions absorb energy. Energy is a reactant Can feel cool to the touch Exothermic Chemical Equations Exothermic Energy is either released or absorbed during a chemical reaction. Can be many forms of energy (light, heat, mechanical, chemical, etc). Exothermic reactions release energy. Energy is a product Can feel warm to the touch Antoine-Laurent de Lavoisier Lived 1743-1794 Called the Father of Chemistry Performed experiment leading to the Law of Conservation of Mass Law of Conservation of Mass Law The LCM says matter is neither created nor destroyed The mass of the reactants is always the same as the mass of the products. The number of atoms of each element is always the same in the reactants and the products. Law of Conservation of Mass Law Examples Hydrogen gas and oxygen gas combine to create water. 2H2 + O2 → 2H2O The large numbers, known as coefficients, indicate how many moles of each compound there are. This helps to satisfy the LCM! They apply to every element in the compound (2H2O means 4 H’s & 2 O’s) They are the coefficients should be in the lowest whole number ratio Particles are rearranged, not created or destroyed! Law of Conservation of Mass Law Examples This should be a review - just count moles of atoms! How many carbon atoms are conserved? Hydrogen atoms? Oxygen atoms? LCM Examples LCM Calcium chloride and sodium sulfate combine to form calcium sulfate and sodium chloride. CaCl2 + Na2SO4 → CaSO4 + 2NaCl # atoms in the reactants Before and After: Does the mass change? Do the number of atoms change? # atoms in the products Ca 1 1 Cl 2 2 Na 2 2 S 1 1 O 4 4 Total 10 10 Balancing Chemical Equations Balancing Because the Law of Conservation of Mass (LCM) states that matter is not created or destroyed, the number of atoms of each element must be identical in the reactants and products. This means the chemical equation is balanced. Example 1: # atoms in the # atoms in the reactants products H 2 2 C 1 1 O 3 3 Total 6 6 • This equation is balanced because it satisfies the LCM • Writing coefficients is not necessary because each compound has a coefficient of 1 Balancing Chemical Equations Balancing Example 2: # atoms in the # atoms in the reactants products Na 1 2 O 3 4 H 1 2 C 1 1 Total 6 • This equation is not balanced because it does not satisfy the LCM • Writing coefficients is necessary 9 So we have to balance the equation… Balancing Chemical Equations Balancing Example 2 continued: • The process of balancing chemical equations requires trial and error. • You may not alter the subscripts because it changes the substance! • You may only place coefficients in front of the compounds. These act as multipliers to balance the number of atoms. # atoms in the reactants # atoms in the products Na 2 2 O 4 4 H 2 2 C 1 1 Total 9 9 • By placing a 2 in front of NaOH, the equation now satisfies the LCM and is balanced ***Tip: balance hydrogen and especially oxygen last - they often fall into place! Balancing Chemical Equations Balancing Practice Write a balanced chemical equation for the following reactions. Place coefficients in the blanks. Chemical equations never actually write the coefficient “1”, but for this activity write a number in every blank. A) ___ K(s) + ___ H2O(l) → ___ H2(g) + ___ KOH(aq) B) ___ CaCl2(aq) + ___ Na2CO3(aq) → ___ CaCO3(s) + ___ NaCl(aq) C) ___ N2(g) + ___ H2(g) → ___ NH3(g) Classifying Chemical Reactions Classifying Reactions are classified into several categories. Synthesis Decomposition Single Replacement Double Replacement Combustion A + B → AB AB → A + B A + BC → AC + B AB + CD → AD + CB CxHy + O2 → CO2+ H2O • By recognizing these patterns, you can classify a reaction and make predictions about its products. • Compare these reactions to dancing! Combination Reaction Combination Whenever two or more substances combine to form a single product, the reaction is called a synthesis reaction. When iron rusts, iron metal and oxygen gas combine to form one new substance, iron (III) oxide. Decomposition Reaction Decomposition Whenever a compound breaks down into two or more simpler substances, the reaction is called a decomposition reaction. When hydrogen peroxide spontaneously decomposes, it becomes water and oxygen gas. 2H2O2(aq) → 2H2O(l) + Decomposition Reaction Cont’d Decomposition In a decomposition reaction, a compound breaks down into two or more simpler substances. The compound may break down into individual elements, such as when mercury(II) oxide decomposes into mercury and oxygen. The products may be an element and a compound, such as when hydrogen peroxide decomposes into water and oxygen. The compound may break down into simpler compounds. Decomposition Exceptions Decomposition Carbonates and chlorates are special case decomposition reactions that do not go to the elements. • Carbonates (CO32-) decompose to carbon dioxide and a metal oxide Example: CaCO3 CO2 + CaO • Chlorates (ClO3-) decompose to oxygen gas and a metal chloride Example: 2 Al(ClO3)3 2 AlCl3 + 9 O2 • There are more exceptions!!!!!! Single-Displacement Reaction Single-Displacement Whenever one element takes the place of another, the reaction is called a singledisplacement reaction. This reaction will only occur if the more active metal will become part of a compound in the product. An iron nail soaking in copper (II) sulfate yields iron (II) sulfate and copper Activity Series Activity Li Rb K Ca Na Mg Al Mn Zn Fe Ni Sn Pb H2 Cu Hg Ag Pt Au Most Active • This feature is found on your handy helper. • Use it to determine whether a singledisplacement reaction will occur (react) or not. • Active metals have a low electronegativity. They bond with nonmetals more readily because they are less stable by themselves. • If a very active metal is competing with a less active metal to bond (with a nonmetal or negatively charged polyatomic ion), the more active metal will “win”. • Think of it as a competing suitors who want to date the same girl! Least Active Activity Series Activity Li Rb K Ca Na Mg Al Mn Zn Fe Ni Sn Pb H2 Cu Hg Ag Pt Au Most Active Example #1: Is this combination possible? Pb + MgSO4 → PbSO4 + Mg ♥ ♥ + → + Answer: No, because Mg is more active than Pb Example #2: Is this combination possible? NaNO3 + K → Na + KNO3 ♥ + Least Active → + Answer: Yes, because K is more active than Na ♥ Double-Displacement Reaction Double-Displacement Whenever two ionic compounds trade cations, the reaction is called a double-displacement reaction. One of the products will either be water, a gas, or most commonly a precipitate (insoluble in water). Lead (II) nitrate and potassium iodide form lead (II) iodide and potassium nitrate. Note the yellow precipitate is PbI2. Combustion Reaction Combustion Whenever hydrocarbons combine rapidly with oxygen (usually by fire), the reaction is called a combustion reaction. The products will be water and carbon dioxide. Burning methane (CH4) yields water and carbon dioxide This reaction contributes to our greenhouse gases and global warming. Identifying Chemical Reactions Identifying Identify each of the following chemical equations as synthesis, decomposition, single-displacement, doubledisplacement, or combustion reaction. A) B) C) Predicting Chemical Reactions Predicting Steps 1. Determine what type of reaction is being presented 2. Write the correct formulas for the product(s) 3. Balance the equation 2 3 Example: CaCl + Al(OH) → ? 1. Double-displacement reaction 2. __CaCl2 + __Al(OH)3 → __Ca(OH)2 + __AlCl3 3. 3CaCl2 + 2Al(OH)3 → 3Ca(OH)2 + 2AlCl3 Predicting Chemical Reactions Predicting Practice 2 Predict the chemical reaction: NaCl + Ca(OH) → ? 4 Predict the chemical reaction: Mg + CuSO → ? What is Equilibrium? What When a reaction results in complete conversion of reactants to products the reaction goes to completion. Most reactions, however, don’t. They appear to stop. The reason is that these reactions are reversible A reversible reaction is one that can occur in both the forward and the reverse directions. Chemical Equilibrium Chemical Chemical equilibrium occurs when two opposite reactions occurring at the same time and rate A + B ↔ AB Forward & Reverse reactions don’t stop at equilibrium, just looks that way because concentration remains constant Rateforward reaction = Ratereverse reaction Equilibrium Example Equilibrium You have a bridge between 2 cities. The number of cars going in either direction on the bridge are equal (at equilibrium) The populations of the cities on either side of the bridge do not have to be equal! Equilibrium Expressions Equilibrium Equilibrium expressions relate concentrations of reactants to those of the products Include only the gaseous or aqueous phases A (s) + 2B (g) ↔ 2C (g) + 3D (g) Keq = [C]2[D]3 [B]2 eq K = equilibrium constant (capital K) Numerical value of the ratio of product concentrations to reactant concentrations [ ] = concentration in M (mol/L) Equilibrium Expression Examples Equilibrium Write the equilibrium expressions for the following reactions: 2 [C ] [ D] 1. A (aq) + 2B (aq) ↔ 2C (aq) + D (aq) Keq = Keq = 1 [ A] [ B] 2 3 [ A][ B ] 2 Equilibrium Constant (Keq) Equilibrium Tells you whether the products or reactants are favored Keq > 1 Products are favored (forward rxn); lots of product is made Keq < 1 Reactants are favored (reverse rxn); not much product made Practice Practice N2O4 (g) ↔ 2NO2 (g) [N2O4] = 0.045M [2NO2] = 0.061M Write the equilibrium expression. Find Keq AB (s) ↔ A (g) + B (g) Write the equilibrium expression. If Keq = 1.44, what is [B]? Le Chatelier’s Principle Le Le Chatelier’s Principle states that if stress is added to a system at equilibrium, the reaction will speed up in the direction that will relieve the stress. Once the stress is relieved, equilibrium is re-established and no further changes are noticed. Stress Factors Stress 1. Changes in the volume Suppose the volume of the reaction vessel for the system is decreased, resulting in an increase in pressure. The equilibrium will shift to relieve the stress of increased pressure. Stress Factors Stress 2. Concentration Shifts away from an increase or addition Shifts toward a decrease or subtraction Stress Factors Stress 3. Temperature (treat heat as a reactant if endothermic or product if exothermic) Shifts away from an addition Shifts toward a subtraction Stress Factors Stress CO (g) + H2 (g) ↔ CH4 (g) + H20 (g) 4. Pressure (only effects gases) Increase pressure, shifts to side with lower total moles of gas Decrease pressure, shift to side with higher total moles of gas Stress Factor Practice Stress N2 (g) + 3H2 (g) ↔ 2NH3 (g) + heat Which way will the reaction shift if you: Add N2 Decrease pressure Remove NH3 Heat Add a catalyst The End. The Study for your test! ...
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This note was uploaded on 09/27/2011 for the course CHEM idk taught by Professor All during the Spring '11 term at University of Houston.

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C_Unit_11B_Chemical_Reactions_2010-2011_K - Unit 11B: Unit...

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