This preview shows page 1. Sign up to view the full content.
Unformatted text preview: HEAT IN CHEMICAL REACTIONS REACTIONS
Thermodynamics / Thermodynamics Thermo chemistry Thermo Unit 8 Ch 17/18 Exothermic and Endothermic Reactions Endothermic Heat: (q) energy that transfers from one Heat: object to another because of a temperature difference between them. temperature Joule: The SI unit for energy and heat. Bond breaking vs. bond forming. Thermo chemistry: The study of the Thermo changes in heat within chemical reactions. reactions. Exothermic Exothermic
Exothermic reactions release heat. release Heat is a product. Combustion reactions. Examples: Examples: – MRE’s – Hot hands Endothermic Endothermic
Endothermic reactions absorb absorb heat. heat. Heat is a reactant. Example:
– Cold pack (p 525; figure 17.11) The enthalpy of a substance is its energy The Enthalpy (ΔH) Enthalpy plus a small-added term that includes the pressure and temperature of the substance. substance. When the pressure remains constant, the When heat absorbed or released during a chemical reaction is equal to the enthalpy change for the reaction. change Heat transfer and the sign of the Heat enthalpy change: enthalpy
– Positive – Negative Endothermic Exothermic Heat absorbed Heat released Enthalpy of Reactions Enthalpy
ΔHrxn = Hproducts - Hreactants ΔH The enthalpy change for a reaction is equal to The the heat absorbed or gained during the reaction. reaction. Enthalpy change is the enthalpy of the products Enthalpy minus the enthalpy of the reactants. minus The amount of heat absorbed / released is The dependent on the quantity (mole). dependent Standard enthalpy change (ΔHo): at 1atm and ): 250C. 25 Enthalpy of Reactions Enthalpy
Sample Problem Sample
How much heat will be released if 1.0 g of How hydrogen peroxide decomposes in a bombardier beetle to produce a steam spray. 2H2O2 2H 2H2O + O2 ΔH = -190 kJ ΔH Enthalpy and the Spontaneous Process Spontaneous Enthalpy diagrams. Spontaneous Process: Spontaneous a process that proceeds on its own, without any outside intervention. intervention. “The Chemist’s Questions”:
– – – What direction is spontaneous? What is the extent of the reaction? What is the speed of the reaction? Spontaneous vs. Exothermic No catalyst Energy Non-Spontaneous Endothermic Catalyst of Activation: The difference between the energy of an activated complex and the energy of the reactants of a chemical reaction.
– Demonstrate with an enthalpy diagram. Although most spontaneous reactions are exothermic, some are endothermic. Why? Ch 18 Order vs. Disorder. Relate the states of matter and the degree Relate of order. of Entropy: A quantitative measure of the Entropy: disorder, or randomness, in the substances involved in a reaction. substances Entropy (ΔS) Entropy Entropy Changes: Entropy ΔSrxn = Sproducts - Sreactants Entropy Increases: Sproducts > Sreactants Entropy Decreases: entropy increases: Sproducts < Sreactants Predicting
– Gases are formed from liquids or solids. – Solutions are formed from liquids and solids. – There are more molecules of gas as products than there are as reactants. – The temperature of a substance is increased. Entropy Criterion Entropy In any spontaneous process, the overall entropy In of the universe always increases. of Entropy of the Universe:
ΔSuniverse = ΔSreaction + ΔSsurroundings
A reaction is spontaneous when ΔSuniverse is positive. Change in entropy of the reaction (see predicting). Change in entropy of surroundings (endo vs. exo)
If ΔH is (-), then ΔSsurroundings is (+). If ΔH is (+), then ΔSsurroundings is (-). Hess’s Law Hess’s
• • • If a series of reactions are added together, If the enthalpy change for the net reaction will be the sum of the enthalpy changes for the individual steps. individual ΔHnet = ΔH1 + ΔH2 Two Rules:
• If the coefficients of an equation are multiplied/divided by a factor, the enthalpy change for the reaction is multiplied/divided by the same factor. • If an equation is reversed, the sign of ΔH changes also.
• Sample problems. Practice Problem Practice
1. The combustion of sulfur can produce SO2 as well as SO3 depending upon the supply of oxygen. From the following reactions and their enthalpy changes, 2SO2(g) + O2(g) 2SO3(g) H = -196kJ 2S(s) + 3O2(g) 2SO3(g) H = -790kJ calculate the standard enthalpy change for the combustion of sulfur to produce SO2. S(s) + O2(g) SO2(g) Calorimetry Calorimetry
Ch 17 The study of heat flow and heat The measurement. measurement. How do you measure an enthalpy How change of a reaction? change Calorimetry experiments determine Calorimetry the enthalpy changes of reactions by making accurate measurements of temperature changes produced in a calorimeter. calorimeter. Simple Calorimeter Simple See pages 511-512 Exothermic reactions release heat to Exothermic Heat and Temperature Heat surroundings. The size of the temperature increase depends on the amount of heat released and the heat capacity of the surroundings. surroundings. Heat capacity: The amount of heat needed to Heat raise the temperature of the object 1oC. raise Heat capacity depends on the objects mass and Heat its composition. its Specific heat (C): The heat capacity of 1 gram Specific of a substance. of C of H2O = 4.184 J/g-oC. Calculations Calculations
1. 2. 3. Qsur = m x C x (Tf – Ti) Qrxn = -Qsur Q = m x C x ΔT Calculations Calculations
1. 2. 2. 3. 4. Calculate Qsur Convert to Qrxn Convert grams to moles ΔH = Qrxn / moles ΔH Entropy and Gibbs Free Energy (G) Gibbs Spontaneity depends on entropy and Spontaneity enthalpy. enthalpy. Gibbs proposed a thermodynamic Gibbs concept to simultaneously incorporate the concepts of entropy and enthalpy. the ΔG = ΔH - TΔS Free Energy & Spontaneity Free If ΔG If is negative, the reaction is spontaneous and can proceed on its own. If ΔG is positive, the reaction is not spontaneous and requires a sustained input of energy to make it occur. If ΔG is zero, the reaction is at equilibrium. Free Energy and Work Free Spontaneous reactions release free Spontaneous energy that can perform work. energy ΔG represents the maximum work that a ΔG spontaneous process can perform. spontaneous ΔG for a nonspontaneous reaction is the ΔG minimum amount of work that must be performed to make a reaction occur. performed ΔG & Spontaneity ΔG
Case ΔSrxn ΔHrxn
1 2 3 4 + + + + ΔG
+/+/+ Reaction Spontaneity
Spontaneous at all T Spontaneous at high T Spontaneous at low T Nonspontaneous at all T ...
View Full Document