Chapter 7 Chemical Reactions



Chapter 7 Chemical ReactionsGA Performance StandardsSPS2. Students will explore the nature of matter, its classifications, and it system for naming types of matterd. Demonstrate the Law of Conservation of Matter in a chemical reaction.e. Apply the Law the Conservation of Matter by balancing the following types of chemical equations:SynthesisDecompositionSingle ReplacementDouble Replacement7.1 Describing ReactionsA. Chemical EquationsReactants are the substances that undergo change Products are the new substances formed as a result of the change.1. Using Equations to Represent ReactionsDuring a chemical reaction, the reactants change into productsReactants ProductsExample: Carbon + Oxygen Carbon dioxideChemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulasExample: C + O2 CO22. Conservation of MassThe law of conservation of mass states that mass is neither created nor destroyed in a chemical reactionNumber of atoms in reactants always equals the number of atoms in the products.B. Balancing EquationsIn order to show that mass is conserved during a reaction, a chemical equation must be balanced.Coefficients are then number that appear before the formula.When you change a coefficient, you change the amount of that reactant or product represented in the chemical equationWhen balancing a chemical equations NEVER change the subscripts in the chemical formulaSteps to balancing chemical equationALL elements in a chemical equation must balance!!!Balancing StepsCount (and write down) the number of atoms of each element on the reactants and product side of the equation (separately)Add coefficients in front of the compounds to balance each side. (NEVER change a subscript!!)Add coefficients in front of the elements to balance each side. (NEVER change a subscript!!)Check to make sure each side as the same number of atoms of each element. You may have to change a coefficient as you work through this processEXAMPLE Al + O2 Al2O3(step 1) 1 Al, 2 O 2 Al, 3O (common multiple between 2 & 3 = 6)(step 2) Al + 3O2 2Al2O3[does it balance] 1 Al, 6 O 4 Al, 6O [need to balance Al](step 3) 4Al + 3O2 2Al2O3(step 4) 4 Al, 6 O 4 Al, 6O {it balances so we are done]FINAL ANSWER 4Al + 3O2 2Al2O3C. Counting with molesbecause chemical reactions often involve large numbers of small particles, chemists use a counting unit called the mole to measure amounts of a substance.Mole is an amount of a substance that contains approximately 6.02 x 1023 particles of that substance.This number is known as Avogadro’s number, and a mole of Iron is 6.02 x 1023 atoms of iron.1. Molar MassMolar mass is the mass of one mole of a substance.For an element, the molar mass is the same as its atomic mass expressed in grams1 mole of carbon is 12.0 grams.2. Mole-Mass ConversionsOnce you know the molar mass of a substance you can convert moles of that substance into mass; or mass into moles.To find the molar mass of a compound add up the mass of each elementFor example: CO2 has molar mass of 44.0 grams { 12 + 16 +16 = 44.0]This relationship yield the following conversion factors 44.0 g CO2 1 mol CO2=1 mol CO244.0 g CO2So if I know I have 33.5 g of CO2 I can calculate the number of moles I have by33.5 g CO2 x 1 mol CO244.0 g CO2=0.76 mol CO2REMEMBER when using conversion factors that the old unit must cancel!!!Example 2: What is the mass of 2.35 moles of CO22.35 mol CO2 x 44.0 g CO21 mol CO2=103.4 g CO2D. Chemical CalculationsIn chemical reactions, the mass of a reactant or product can be calculated by using a balanced chemical equation and molar masses of the reactants and products.The chemical equation tells you how to relate amounts of reactants to amounts of productsMolar masses let you convert those amounts into masses1. Converting Mass to Moles.7.2 Types of ReactionsA. Classifying ReactionsSome general types of chemical reactions are synthesis reactions, decomposition reactions, single-replacement reactions, double-replacement reactions and combustion reactions.Each describes a different way that reactants form products.1. SynthesisSynthesis reaction is a reaction in which two or more substance react to forma single substance.the product synthesized is always a compoundthe general equation for a synthesis reaction is: A + B ABEXAMPLE: 2Na + Cl2 2NaCl2. DecompositionDecomposition reaction is a reaction in which a compound breaks down into two or more simpler substances.The reactant in a decomposition reaction is always a compoundThe general equation for a synthesis reaction is: AB A + B EXAMPLE: 2H2O 2H2 + O2 3. Single ReplacementSingle-replacement reaction is a reaction in which one element takes the place of another element in a compound.This reaction begins and ends with a different single elementThe general equation for a single-replacement reaction is: A + BC B + ACEXAMPLE: 2K + 2H2O H2 + 2KOH4. Double ReplacementDouble replacement reaction is where two different compounds exchange positive ions and form two new compounds.The general equation for a double-replacement reaction is: AB + CD AD + CBEXAMPLE: Ca(CO3) + 2HCL CaCL2 + H2(CO3)5. CombustionCombustion reaction is one in which a substance reacts rapidly with oxygen, often producing heat and light.the products of a combustion reaction is almost always CO2 and H2O.EXAMPLE: CH4 + 2O2 CO2+ 2H2O7.3 Energy Changes in ReactionsA. Chemical Bonds and EnergyHeat produced by a propane grill is a form of energy and can be shown as a productFor example: C3H8 + 5O2 3CO2+ 4H2O + HeatChemical 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.1. Breaking BondsBreaking chemical bonds requires energy.In the burning of propane the energy comes from a sparkThe eight C-H bonds and the 2 O=O bonds have to be broken2. Forming BondsIn the propane example above 6 C=O bonds and 8 O-H bonds are formedThe heat and light given off is a result from the formation of these new bondsB. Exothermic and Endothermic ReactionsDuring a chemical reaction, energy is either released or absorbed1. Exothermic ReactionExothermic reaction is a reaction that released energy to its surroundingsEnergy is EXITING the reactionExample: one mole of propane reacts with 5 moles of oxygen and releases 2220 kJ of heat.You can use this value in place of heat in the balanced chemical equation.C3H8 + 5O2 3CO2+ 4H2O + 2220kJ2. Endothermic ReactionsEndothermic reactions is a chemical reaction that absorbs energy from its surroundsEnergy is ENTERING the reactionThe energy (or heat) needed to complete the reaction can be show an a reactants in the balanced chemical equation.2 HgO + 181.7 kJ 2Hg + O2For every 2 moles of HgO that decomposes 181.7 kJ of heat must be absorbed.C. Conservation of EnergyIn an exothermic reaction, the chemical energy of the reactants is converted into heat plus the chemical energy of the productsIn an endothermic reaction heat plus the chemical energy of the reactants is converted into the chemical energy of the productsIn BOTH cases the total amount of energy before and after the reaction takes place is the same.The law of conservation of energy states that energy cannot be created or destroyed, it can only change forms. ................
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