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Chapter 8 Chemical Equations and Reactions 1 Section 8.1 Objectives: • List 3 observations that suggest that a chemical reaction has taken place. • List 3 requirements for a correctly written chemical reaction. • Balance a formula equation by inspection. 2 Chemical vs Physical Change • Physical change – a change in substance that does not change its chemical composition; ex: phase changes, size changes • Chemical change – a change in substance that results in entirely new substance with different chemical composition and properties; ex: burning, tarnishing, rusting, baking 4 Chemical Reactions • A chemical reaction is a process by which substances are changed into different substances (chemical change). • Chemical reactions DO NOT destroy or change actual atoms, but they do break and rearrange bonds between atoms often combining them into different compounds or freeing atoms to be in a pure state. 5 Indicators of a Chemical Reaction 1. Evolution of heat and/or light 2. Evolution of a gas (bubbles) 3. Formation of a precipitate Precipitate = a solid that is formed as a result of a chemical reaction in a solution Sometimes, Colors change (not a positive indication of a chemical reaction) 6 Indicators of a Chemical Reaction 1. Change in temperature (drop in temperatureor evolution of heat) and/or production of light 2. Evolution of a gas (bubbles) 3. Formation of a precipitate Precipitate = a solid that is formed as a result of a chemical reaction in a solution Elephant toothpaste Sometimes, Colors change indicates a reaction – but not always. Be careful what is actually changing color. If acid/base indicator, may not be a chemical reaction. 7 • Cold and Hot Packs - Chemical Reactions 8 • Chemical reactions are described by chemical equations - a written representation of a chemical reaction. 6H2O + 6CO2 C6H12O6 + 6O2 • The original substances are called reactants. • The resulting substances are called products. Reactants Products 9 Symbols used in chemical equations • + • means “and” or “reacts with” means “yields” or “produces” or “breaks down into” or “decomposes” 10 More Symbols used in chemical equations: • • • • • (s) solid (l) liquid (g) gas (aq) aqueous = dissolved in water Heat or Δ 11 Requirements for chemical equation: 1. Must include all reactants and products 2. Must contain correct formulas AND….. 12 3. Must satisfy Law of Conservation of Mass – mass (atoms) cannot be created or destroyed in ordinary chemical and physical changes - same number of atoms of each element must be on each side of equation Equation must be balanced – done with coefficients, whole numbers in front of formulas. 6CO2 + 6H2O C6H12O6 + 6O2 Coefficients represent the relative number of MOLES of that substance. If there is no coefficient it is assumed to be “1”. 13 How do you balance a chemical equation? Na2O + H2O NaOH Only by adding coefficients. 14 NEVER CHANGE A FORMULA! H2O H2O2 15 Assignment Using coefficients, balance the assigned equations on handout page 61. 16 Writing correct equations from sentences. 1st – identify reactants and products 2nd – write correct formulas for each reactant and product 3rd – add symbols for physical states, + and yield, catalysts, etc. 4th – balance with coefficients 17 Remember diatomic elements! • 7 diatomic elements • Always found as diatomic if alone in an equation. • H2, N2, O2, F2, Cl2, Br2, I2 • Sulfur should be written as S8 • Phosphorus should be written as P4 • When pure – do not include prefixes in name!!! 18 Iron reacts with water and oxygen in air to yield iron III oxide. 1st – identify reactants and products and write word equation with symbols for + and 2nd – write formula equation with correct formulas 3rd – add symbols for physical states, catalysts, etc. 4th – balance with coefficients 19 Write formula equation Solid sulfur reacts with oxygen gas to produce gaseous sulfur trioxide. 1st – identify reactants and products and write word equation with symbols for + and 2nd – write formula equation with correct formulas 3rd – add symbols for physical states, catalysts, etc. 4th – balance with coefficients 20 Dicarbon hexoxide reacts with oxygen to produce carbon dioxide and water. 1st – identify reactants and products and write word equation with symbols for + and 2nd – write formula equation with correct formulas 3rd – add symbols for physical states, catalysts, etc. 4th – balance with coefficients 21 Potassium chloride and phosphoric acid are produced when potassium phosphate reacts with hydrochloric acid. 1st – identify reactants and products and write word equation with symbols for + and 2nd – write formula equation with correct formulas 3rd – add symbols for physical states, catalysts, etc. 4th – balance with coefficients 22 Assignment Complete Handout page 62 – Word Equations. 23 Section 8.2 Objectives: • Define synthesis, decomposition, single-replacement, and double-replacement reactions. • Classify a reaction as synthesis, decomposition, singlereplacement, and double-replacement, or combustion. • Predict the products of simple reactions given the reactants. 24 Types of Chemical Reactions 1. 2. 3. 4. 5. Synthesis A + X AX Decomposition AX A + X Single-Replacement A + BX AX + B Double-Replacement AX + BY AY + BX Combustion CH4 + O2 CO2 + H2O 25 Synthesis A + X AX • Produces one product • Examples: • 2Fe + O2 2FeO • S8 + 8O2 8SO2 26 Decomposition AX A + X • One compound in the reactant side of equation. • Examples: • • • • • 2H2O 2H2 + O2 CaCO3 CaO + CO2 Ca(OH)2 CaO + H2O 2KClO3 2KCl + 3O2 H2SO4 SO3 + H2O 27 Single-Replacement A + BX AX + B • • • • Also called a single-displacement Elements replace “like” elements (+ + - -) A + BX AX + B or Y + BX BY + X Remember if hydrogen in water is being replaced that water is HOH. 2Na + 2H2O 2NaOH + H2 2Na + 2HOH 2NaOH + H2 28 Double-Replacement AX + BY AY + BX • Also called a double-displacement • compound + compound compound + compound • HCl + NaOH NaCl + HOH • Mg(NO3) + K3PO4 Mg3(PO4)2 + KNO3 29 Combustion • When a substance combines with oxygen and burns to produce carbon dioxide and water. • Hydrocarbons C3H8 + 5O2 3CO2 + 4H2O 30 On handout, write the type of reaction and, where indicated, write the balanced equation. 31 Knowing the type of reaction, you can predict products. 32 Predict product(s) and write the balanced chemical equation. Single replacement reaction between magnesium bromide and chlorine 33 Predict product(s) and write the balanced chemical equation. Single replacement reaction between aluminum and iron (III) oxide 34 Predict product(s) and write the balanced chemical equation. Double replacement reaction between silver I nitrate and zinc I chloride 35 Predict product(s) and write the balanced chemical equation. Double replacement reaction between sulfuric acid and sodium hydroxide 36 Predict product(s) and write the balanced chemical equation. Decomposition of HgO into mercury and oxygen 37 Predict product(s) and write the balanced chemical equation. Synthesis of carbon dioxide from carbon monoxide and oxygen. 38 Activity Series • Silently read page 271-272 • Write yourself an explanation of how to use the activity series chart. 39 What will be the products? • NaCl + F2 • FeCl3 + NaOH • Ag2O • AlBr3 + K2SO4 • C8H18 + O2 40 Use the Activity Series to predict the products of these possible reactions • Ni + AgNO3 • Ca + HCl • Ag + HCl • Br2 + NaI • I2 + NaBr 41