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Chemical Reactions Chapter 7 Describing Chemical Reactions What type of change is happening in the picture to the left? Chemical Reaction New Products being formed Heat and Light being produced Chemical Equations What is present before and after a chemical change takes place? Reactants – the substances undergoing the chemical change Products – the new substances formed as a results of the chemical change In the case of the burning charcoal Reactants are carbon and oxygen and the product is carbon dioxide C + O2 → CO2 Using Equations to Represent Reactions Reactants → Products Chemical Equation - a representation of a chemical reaction in which the reactants and products are expressed as formulas Conservation of Mass The mass of the products is always equal to the mass of the reactants Called the Conservation of Mass Principal was establish by Antoine Lavoisier Mass is neither created nor destroyed in a chemical reaction Conservation of Mass Balancing Equations In rockets Hydrazine + Oxygen yields Nitrogen + Water N H + O → N + H O 2 4 2 2 2 This equation is not in balance Balancing Equations We balance equations by adding coefficients (a whole number) before the formula itself We never change subscripts when balancing an equations N2H4 + O2 → N2 + 2H2O Some Practice Balancing Equations H + O → H O 2 2 2 HgO → Hg + O2 Zn + HCl → ZnCl2 + H2 Counting with Moles Chemists use a counting unit called the mole 1 mole = 6.02 X 1023 particles of that substance Particles might be atoms, ions, molecules Molar Mass Molar mass – the mass of one mole of a substance the atomic mass expressed in grams 1 mole of Na = 23 grams 1 mole of Cl = 35 grams 1 mole of NaCl = 58 grams 1 Mole of Hydrogen = 1 gram 1 Mole of Oxygen = 16 grams 1 Mole of H2O = 18 grams Mole-Mass Conversions Express molar mass as a conversion factor ie: CO2 has a molar mass of 44 grams 1 mole of Carbon = 12 grams 2 moles of Oxygen = 32 grams (16 g x 2) 44.0g CO2/1 mole of CO2 1 mole of CO2/44.0 g CO2 How many moles of CO2 do I have if I have 55 grams of CO2? The Calculation 55.0 g CO2 X 1 mole of CO2/44 g CO2 = 1.25 moles of CO2 If you know how many grams of a substance you have ~divide by the molar mass to determine how many moles you have Types of Reactions Synthesis Reactions Decomposition Reactions Single-Replacement Reactions Double-Replacement Reactions Combustion Reactions Synthesis Reactions A synthesis reaction is a reaction in which two or more substances react to form a single substance – the reactants may be either elements or compounds A + B → AB 2 Na + Cl2 → 2 NaCl 2 H2 + O2 → 2 H2O This reaction generates electricity for satellites and spacecraft Decomposition Reactions A decomposition reaction is a reaction in which a compound breaks down into two or more simpler substances – The reactant in a decomposition reaction must be a compound – the products may be elements or compounds AB → A + B 2 H2O → 2H2 + O2 CaCO3 → CaO + CO2 Making cement 2NaN3 → 2 Na + 3 N2 (Air bags) Single-Replacement Reactions A Single-replacement reaction is a reaction in which one element takes the place of another element in a compound A + BC → B + AC (A and B have switched places) Cu + 2Ag NO3 → Cu(NO3)2 + 2Ag 2K + 2H2O → H2 + 2 KOH Double-Replacement Reactions A double-replacement reaction is one in which two different compound exchange positive ions and form two new compounds AB + CD → AD + CB Pb(NO3)2 + 2 KI → PbI2 + 2KNO3 CaCO3 + 2 HCl → CaCl2 + H2CO3 Combustion Reactions A combustion reaction is one in which a substance reacts rapidly with oxygen, often (not always) producing heat and light. One of the reactants must be oxygen CH4 + 2 O2 → CO2 + H2O 2H2 + O2 → 2H2O Reactions as Electron Transfers (Redox Reactions) Oxidation – A loss of electrons during a chemical reaction Reduction – a gain of electrons during a chemical reaction 2 Ca + O2 → 2 CaO Calcium is oxidized Oxygen is reduced Na + Cl →NaCl Sodium is oxidized Chlorine is reduced Energy Changes in Reactions C3H8 + 5O2 → 3CO2 + 4 H2O + Heat Chemical energy is the energy stored in the chemical bonds of a substance Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products Breaking Bonds Breaking bonds requires energy This is why we need a spark to light the propane because it doesn’t happen spontaneously Forming bonds requires energy During a chemical reaction energy is either released or absorbed Exothermic Reactions A chemical reaction in which energy is released to the surroundings Combustion reactions are extreme exothermic reactions Ex. 1 mole of Propane produces 2220 kilojoules of heat In an exothermic reaction the kilojoules would be written to the right of the arrow Endothermic Reactions A chemical reaction that absorbs energy from its surroundings is called an endothermic reaction 2HgO + Heat → 2 Hg + O2 In an endothermic reaction heat appears on the left side of the arrow Conservation of Energy The total amount of energy before and after a reaction must always be the same Reaction Rates The progress of a chemical reaction can be measured over time Different reactions have different durations TNT Leaves turning color Any reaction that takes places over time can be expressed as a rate A reaction rate is the rate at which reactants change into products over time – in other words how fast the reaction is going Factors affecting Reactions Rates Temperature Surface area Generally an increase in temperature will increase the reaction rate Increasing the surface area of a reactant tends to increase the reaction rate Stirring Stirring the reactants will generally increase the reaction rate Factors affecting Reactions Rates Concentration Increasing the concentration generally increases the reaction rate Catalysts A substance that affects the reaction rate without being used up in the reaction Generally speed up a reaction Equilibrium Equilibrium is a state in which the forward and reverse paths of a change take place at the same rate Physical Equilibrium H2O (l) ↔ H2O (g) When physical change does not go to completion, a physical equilibrium is established between the forward and reverse changes Equilibrium Most chemical reactions are reversible to some extent A reversible reaction is a reaction in which the conversion of reactants into products and the conversion of products into reactants can happen simultaneously 2 SO2 (g) + O2 (g) ↔ 2 SO3 (g) When a chemical reaction does not go to completion, a chemical equilibrium is established between the forward and reverse reactions Factors Affecting Chemical Equilibrium When a change is introduced to a system in equilibrium, the equilibrium shifts in the direction that relieves the change Discovered by Henri Le Chatlier N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat → This reaction is exothermic ← This reaction would be endothermic Temperature If you add heat to the system the equilibrium would shift in the direction that removes heat from the system (In the above reaction you would decrease the amount of ammonia produced Factors Affecting Chemical Equilibrium Pressure Increasing the pressure would shift the equilibrium in the direction that reduces the pressure More gas molecules = more pressure Increasing the pressure in the ammonia production would produce more ammonia because the right side of the equation has fewer ( 2 vs. 4 ) gas molecules N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat Factors Affecting Chemical Equilibrium Concentration If we removed ammonia from the system the reaction would favor producing more ammonia If we removed either of the reactants the reaction would favor breaking down the ammonia and producing more of the reactants N2 (g) + 3 H2 (g) ↔ 2 NH3 (g) + Heat