Chemistry - Thermodynamics - Quick Guides

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Study GuideChemistry–Thermodynamics1. Enthalpy1.1What Is Enthalpy?Scientists discovered that almost all chemical reactions eitherabsorb heatorrelease heat. Becauseof this, we say that substances containheat energy.The heat content of a substance is calledenthalpy.When a chemical reaction occurs, the heat change of the reaction is thedifference between theenthalpy of the products and the reactants.We write this as:Here:•Δ (delta)meanschange•Hstands forenthalpySo,ΔH tells us how much heat is gained or lost during a reaction.Wecan think of enthalpy aschemical energy that usually appears as heat.1.2Using Enthalpy to Calculate Heat ChangeLet’s look at a real example to see how enthalpy calculations work.Example: Decomposition of Ammonium NitrateThe reaction is:To calculateΔH, we need the enthalpy values of each substance.

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Study Guide1.3CalculatingΔH for the ReactionFirst, add the enthalpy values of theproducts:•N₂O:+82 kJ•2 H₂O:2 × (−242 kJ) = −484 kJTotal products = 82 + (−484) = −402 kJNext, subtract the enthalpy of thereactant:Final Answer:This means:•36 kJ of heat is released•The reaction isexothermicImportant Note:The enthalpy of H₂O is doubled because the reaction produces2 moles of water.

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Study Guide1.4What Does the Sign ofΔH Mean?•AnegativeΔHmeans heat is released•ApositiveΔHmeans heat is absorbed1.5What Happens If the Reaction Is Reversed?If we reverse the reaction:Thesign ofΔH also reverses:This means:•Heat must beadded•The reaction is nowendothermicSo,36 kJ of energy is requiredto form ammonium nitrate from nitrous oxide and water vapor.1.6Standard Enthalpies of FormationScientists measure enthalpy values under standard conditions:•25°C•1 atm pressureThese values are calledstandard enthalpies of formationand are written as:

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Study GuideImportant rules:•Thestandard enthalpy of any element in its natural form is 0oExample: O₂(g), N₂(g), Al(s)•Compound values depend on theirphysical stateoExample: H₂O(l) and H₂O(g) have different values

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Study Guide1.7Using Standard Enthalpy TablesTo find the enthalpy change of a reaction, use this formula:What Does This Mean?•1124 kJ of heat is released•The reaction isexothermicPractice ProblemsUseTable 3to solve the following:1.CalculateΔH and classify the reaction:2.Calculate the heat releasedwhen100 g of CaOreacts with liquid water to formCa(OH)₂(s).2. Quiz: Enthalpy1.QuestionThe difference in the heat contents of the products and the reactants is calledAnswer Choices•temperature•heat of a reaction•heat of calorimetry

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Study GuideCorrect Answerheat of a reactionWhy This Is CorrectThe heat of a reaction refers to the energy change that occurs when reactants are converted intoproducts.It represents the difference between the heat content (enthalpy) of the products and the reactants, nottemperature or the measurement method (calorimetry).2.QuestionThe heat of a given reaction is called the change inAnswer Choices•enthalpy•entropy•free energyCorrect AnswerenthalpyWhy This Is CorrectEnthalpy (ΔH) measures the heat energy absorbed or released during a chemical reaction at constantpressure.Entropy and free energy describe different thermodynamic concepts, not heat content directly.3.QuestionThe change in enthalpy is the change in heat energy of a reaction and is calculated by

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Study GuideAnswer Choices•Σ(heat content of the reactants) −Σ(heat content of the products)•Σ(heat content of the products) −Σ(heat content of the reactants)•heat content of products × heat content of the reactantsCorrect AnswerΣ(heat content of the products) −Σ(heat content of the reactants)Why This Is CorrectΔH is calculated by subtracting the total enthalpy of the reactants from the total enthalpy of theproducts.This equation shows whether heat is absorbed or released during the reaction.4.QuestionIf the reaction is endothermic, the sign ofΔH will beAnswer Choices•negative•neutral•positiveCorrect AnswerpositiveWhy This Is CorrectEndothermic reactions absorb heat from their surroundings.Because energy is gained, the change in enthalpy (ΔH) has a positive value.

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Study Guide5.QuestionStandard enthalpies occur at what set of conditions?Answer Choices•25°C and 1.0 atm•100°C and 1.0 atm•25°C and 2.0 atmCorrect Answer25°C and 1.0 atmWhy This Is CorrectStandard enthalpy values are defined at standard conditions:25°C (298 K) and a pressure of 1.0 atmosphere.6.QuestionThe standard enthalpy for an element in its reference form is defined asAnswer Choices•–2.0 kJ/mol•0 kJ/mol•2.0 kJ/molCorrect Answer0 kJ/molWhy This Is CorrectBy definition, elements in their most stable (reference) form at standard conditions have a standardenthalpy of zero.

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Study Guide7.QuestionWhen looking up enthalpy values for a set of reaction species, the enthalpy depends on theAnswer Choices•physical state of the species•color of the species•concentration of the speciesCorrect Answerphysical state of the speciesWhy This Is CorrectEnthalpy values differ depending on whether a substance is solid, liquid, gas, or aqueous.Color and concentration do not affect standard enthalpy values.8.QuestionIf theΔH of a reaction is negative, the reaction isAnswer Choices•endothermic and releasing heat•exothermic and releasing heat•exothermic and absorbing heatCorrect Answerexothermic and releasing heatWhy This Is CorrectA negativeΔH indicates that heat is released to the surroundings.This release of energy defines an exothermic reaction.

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Study Guide9.QuestionWhen 1.0 mole of ZnO(s) decomposes, theΔH = 348 kJ/mol of heat energy. What does this tell youabout the formation of ZnO(s)?Answer Choices•the formation of ZnO(s) is endothermic•the formation of ZnO(s) does not require energy•the formation of ZnO(s) is exothermicCorrect Answerthe formation of ZnO(s) is exothermicWhy This Is CorrectIf decomposition requires energy (endothermic), then the reverse process—formation—must releaseenergy.Therefore, forming ZnO(s) is an exothermic process.10.QuestionWhat is theΔH value for O₂(g)?Answer Choices•142 kJ/mol•0 kJ/mol•–142 kJ/molCorrect Answer0 kJ/mol

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