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Chemical Reactions

Chemical Reactions
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Dr.SamuelHunt,United Arab Emirates,Teacher
Published Date:21-07-2017
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Chapter 7 Chemical ReactionsExperiencing Chemical Change • Chemical reactions are happening both around you and in you all the time. • Some are very simple, others are complex.  In terms of the pieces—even the simple ones are interesting. • Chemical reactions involve atom exchanges, which change the structures of molecules. • What are some examples of chemical reactions you experience? Tro's "Introductory Chemistry", 2 Chapter 7Combustion Reactions • Reactions in which O is consumed by combining with 2 another substance are called combustion reactions.  Always release heat and/or other forms of energy.  Produce one or more oxygen-containing compounds. • Combustion reactions are a subclass of oxidation– reduction reactions.  aka redox reactions.  Involve the transfer of electrons between atoms. Reactants Products 3Precipitation Reactions • Some reactions involve the combining of ions resulting in formation of a material that is insoluble in water. These are called precipitation reactions.  Formation of ―bathtub-ring‖ • Precipitation reactions are generally done with the reactants dissolved in water to allow the ions to move more freely.  Allowing the ions to contact each other more frequently.  Resulting in the reaction occurring faster. Tro's "Introductory Chemistry", 4 Chapter 7Chemical Reactions • A chemical change produces a new substances. • Reactions involve rearrangement and exchange of atoms, producing new molecules. Elements do not transmute. Atoms combine to make new compounds. 1. Molecules can combine to make bigger molecules. 2. Molecules can decompose into smaller molecules or atoms. 3. Atoms can be exchanged between molecules or transferred to another molecule. 4. Atoms can gain or lose electrons, turning them into ions. Tro's "Introductory Chemistry", 5 Chapter 7Evidence of Chemical Reactions • Look for evidence of a new substance. • Visual clues (permanent). Color change. Precipitate formation. Solid that forms when liquid solutions are mixed. Gas bubbles. Large energy changes. Container becomes very hot or cold. Emission of light. • Other clues. New odor. Whooshing sound from a tube. Permanent new physical state. Tro's "Introductory Chemistry", 6 Chapter 7Evidence of Chemical Change Release or Absorption of Heat Emission of Light Color Change Formation of a Gas Formation of Solid Precipitate Tro's "Introductory Chemistry", 7 Chapter 7Evidence of Chemical Change, Continued • In order to be absolutely sure that a chemical reaction has taken place, you need to go down to the molecular level and analyze the structures of the molecules at the Is boiling water a chemical change? beginning and end. Tro's "Introductory Chemistry", 8 Chapter 7Practice—Decide Whether Each of the Following Involve a Chemical Reaction. Yes, CO and H O combine into carbohydrates • Photosynthesis 2 2 • Heating sugar until it turns black Yes, sugar decomposing No, molecules still same • Heating ice until it turns liquid Yes, food decomposing and combining • Digestion of food with stomach acid • Dissolving sugar in water No, molecules still same • Burning of alcohol in a flambé dessert Yes, alcohol combining with O to make CO and H O 2 2 2 Tro's "Introductory Chemistry", 9 Chapter 7Chemical Equations • Short-hand way of describing a reaction. • Provides information about the reaction. Formulas of reactants and products. States of reactants and products. Relative numbers of reactant and product molecules that are required. Can be used to determine masses of reactants used and products that can be made. Tro's "Introductory Chemistry", 10 Chapter 7Conservation of Mass • Matter cannot be created or destroyed. Therefore, the total mass cannot change. And the total mass of the reactants will be the same as the total mass of the products. • In a chemical reaction, all the atoms present at the beginning are still present at the end. If all the atoms are still there, then the mass will not change. Tro's "Introductory Chemistry", 11 Chapter 7An Example: Combustion of Methane • Methane gas burns to produce carbon dioxide gas and gaseous water. Whenever something burns it combines with O (g). 2 Tro's "Introductory Chemistry", 12 Chapter 7An Example: Combustion of Methane • Methane gas burns to produce carbon dioxide gas and gaseous water.  Whenever something burns it combines with O (g). 2 CH (g) + O (g)  CO (g) + H O(g) 4 2 2 2 What incorrect assumption was made when writing this equation? This equation reads ―1 molecule of CH gas combines with 1 4 O molecule of O gas to make 1 molecule of CO gas and 1 molecule 2 2 H H O of H O gas‖. 2 + + C O O C H H H H O 1 C + 4 H + 2 O 1 C + 2 O + 2 H + O 1 C + 2 H + 3 O We are assuming that all reactants combine 1 molecule : 1 molecule; and that 1 molecule of each product is made – an incorrect assumptionAn Example: Combustion of Methane • To show the reaction obeys the Law of Conservation of Mass the equation must be balanced.  We adjust the numbers of molecules so there are equal numbers of atoms of each element on both sides of the arrow. CH (g) + 2 O (g)  CO (g) + 2 H O(g) 4 2 2 2 O O O O H H H H C + + + + C H H O O O O H H 1 C + 4 H + 4 O 1 C + 4 H + 4 O Tro's "Introductory Chemistry", 14 Chapter 7An Example: Combustion of Methane CH (g) + 2 O (g)  CO (g) + 2 H O(g) 4 2 2 2 • CH and O are the reactants, and CO and H O 4 2 2 2 are the products. • The (g) after the formulas tells us the state of the chemical. • The number in front of each substance tells us the numbers of those molecules in the reaction.  Called the coefficients. Tro's "Introductory Chemistry", 15 Chapter 7An Example: Combustion of Methane CH (g) + 2 O (g)  CO (g) + 2 H O(g) 4 2 2 2 • This equation is balanced, meaning that there are equal numbers of atoms of each element on the reactant and product sides.  To obtain the number of atoms of an element, multiply the subscript by the coefficient. 1  C  1 4  H  4 4  O  2 + 2 Tro's "Introductory Chemistry", 16 Chapter 7Symbols Used in Equations • Symbols used to indicate state after chemical. (g) = gas; (l) = liquid; (s) = solid. (aq) = aqueous = dissolved in water. • Energy symbols used above the arrow for decomposition reactions. D = heat.  hn = light. shock = mechanical. elec = electrical. Tro's "Introductory Chemistry", 17 Chapter 7Writing Balanced Chemical Equations 1. Write a skeletal equation by writing the formula of each reactant and product. 2. Count the number of atoms of each element on each side of the equation.  Polyatomic ions may often be counted as if they are one ―element‖. 3. Pick an element to balance.  If an element is found in only one compound on both sides, balance it first.  Metals before nonmetals.  Leave elements that are free elements somewhere in the equation until last.  Balance free elements by adjusting the coefficient where it is a free element. Tro's "Introductory Chemistry", 18 Chapter 7Writing Balanced Chemical Equations, Continued 4. Find the least common multiple (LCM) of the number of atoms on each side.  The LCM of 3 and 2 is 6. 5. Multiply each count by a factor to make it equal to the LCM. 6. Use this factor as a coefficient in the equation.  If there is already a coefficient there, multiply it by the factor.  It must go in front of entire molecules, not between atoms within a molecule. 7. Recount and repeat until balanced. Tro's "Introductory Chemistry", 19 Chapter 7Example • When magnesium metal burns in air, it produces a white, powdery compound magnesium oxide. 1. Write a skeletal equation Mg(s) + O (g)  MgO(s) 2 2. Count the number of atoms on each side. Mg(s) + O (g)  MgO(s) 2 1  Mg 1 2  O  1 Tro's "Introductory Chemistry", 20 Chapter 7