Thermochemistry

Chem. Catalyst 6.1 Write a balanced molecular equation to describe the following reaction. One molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

Demonstration(CH4 and O2) Observe the reaction. What are some signs a change in energy is involved in this process? Where do you think this energy comes from?

Energy What is energy? Conservation of energy Energy is the capacity to do work or produce heat. In our study of energy we will focus on heat transfer Conservation of energy Energy can be converted from one form to another, however it cannot be created or destroyed. Two Types of Energy Potential Energy: Energy Of Position Chemical Energy (energy stored in chemical bonds) Kinetic Energy: Energy of Motion

How is Energy Measured? Joules (J) or KJ Calories or calories A 2 kg mass moving at the speed of 1 m/s possesses a kinetic energy of 1 J Calories or calories 1 calorie (cal) is the amount of energy required to heat 1 gram of water by 1 degree Celsius. 1 Cal = 1,ooo cal

Conservation of Energy

Chemical Energy Chemical energy is potential energy stored in chemical bonds. When chemical reactions occur, bonds are broken and reformed. This leads to a change in potential energy. Heat is released or absorbed in this process and that heat is called enthalpy (at constant pressure). Exothermic reactions are reactions that release energy (-DH) Endothermic reactions are reactions that absorb energy (+DH)

Energy in Chemical Reactions To determine if a chemical reaction is exothermic or endothermic, we monitor energy transfer. Surroundings System (chemical reaction) Endothermic Reaction Heat moves from surroundings to system Exothermic Reaction Heat moves from system to the surroundings

Practice Questions Are the following processes exothermic or endothermic? Explain. The combustion of gasoline Water condensing on a cold pipe CO2(s)  CO2(g) F2(g)  2F (g) C3H8 (l)  C3H8 (g) 1 mol O2  2 mol O 1 mol NH3 (l)  1 mol NH3(s)

Lab#1 Results Review: Exothermic and Endothermic Reactions

Exothermic vs. Endothermic Look at the following graphs comparing the change in temperature of the surroundings vs. time? Which reaction is endothermic? Exothermic? Graph 1 Temperature Change of the Surroundings Graph 2 Temperature Change of the Surroundings Time Temperature Time Temperature

Endothermic Reactions HEAT + citric acid + 3 NaHCO3 (s)  3 CO2 (g) + 3 H2O (l) + Na3C6H5O7 (aq) Reaction Time Temperature Reaction Time Energy Reactants Products Heat SURROUNDINGS!!! SYSTEM!!! Reaction A is endothermic because it is absorbing heat causing the surrounding temperature to decrease.

Enthalpy of an Endothermic Reaction Reaction Time Energy Reactants Products DH = Hp - Hr For this endothermic reaction…the products are higher in energy b/c of the heat absorbed from the system (Fig 6.3)

Exothermic Reaction Temperature Vs. Time Energy Vs. Time Heat Mg (s) + HCl (aq)  MgCl2 (aq) + H2 (g) + HEAT Temperature Vs. Time Energy Vs. Time Reaction Time Temperature Energy Reactants Heat Products SURROUNDINGS!!! SYSTEM!!! Reaction B is exothermic because it is releasing heat causing the temperature of the surroundings to increase.

Thermochemical Equations Thermochemical equations include the heat released or absorbed by a chemical reaction. DH = change in enthalpy Enthalpy is heat released or absorbed by a chemical system. SO3(g) + H2O (l)  H2SO4(aq) DH = -221 KJ

Thermochemistry Use the following equation to answer the questions below. H2O(s)  H2O(l) DH=6.01 kJ Is heat released or absorbed when water melts? How much heat is required to melt a 1.0 kg block of ice?

Example Problems The reaction, SO3(g) + H2O (l)  H2SO4(aq) Is the last step in the commercial production of sulfuric acid. The enthalpy change for this reaction is -227 kJ. In designing a sulfuric acid plant, is it necessary to provide for heating or cooling of the reaction mixture? Explain.

Example Problem Is the reaction exothermic or endothermic? 2 H2 (g) + O2 (g)  2 H2O (l) DH = -572 kJ Is the reaction exothermic or endothermic? How much heat is evolved for the production of 1.00 mol of H2O (l). How much heat is evolved when 4.03 g of hydrogen is reacted with excess oxygen. How much heat is released when 186 g of oxygen is reacted with excess hydrogen?

Specific Heat Capacity The specific heat capacity is the amount of energy required to raise the temperature of 1 gram of the substance by 1 oC.

Specific Heat Capacity The table to the right lists the heat capacities of common substances. What substance will absorb the most energy to increase the temperature by 1 degree Celsius? Which substance requires the lowest amount of energy to increase the temperature by 1 degree Celsius? Substance Heat Capacity (j/g oc) H2O (l) 4.18 H2O (s) 2.03 Al (s) 0.89 Fe (s) 0.45

Specific Heat Capacity Calculate the amount of heat required to raise 88 grams of water from 25 oC to 50 oC. q = s x m x DT Change in temperature (Tf – Ti) Heat (heat) released in joules mass Specific heat capacity (J/g oC)

Calorimetry A calorimeter is a device used to absorb all* heat released by a system to the surroundings. System Thermometer Surroundings Water Metal Coffee Cup

All the heat from the metal is released into the water causing the temperature of the water to increase. Therefore the q for the water and q for the metal are related by the following equation (q H2O + q Metal = 0) The q for the water is the same value but opposite in sign of q for the metal. ( q H2O = - q Metal ) Coffee Cup Water Metal Thermometer

Calorimetry Problem A 5.037g piece of iron heated to 100.0 oC is placed in a coffee cup calorimeter that contains 27.3 g of water at 21.2 oC. If the final temperature is 22.7 oC, what is the specific heat capacity of the iron?

2 HCl (aq) + Ba(OH)2 (aq)  BaCl2(aq) + 2 H2O(l) DH = -118 kJ Calorimetry Problem Consider the following reaction. 2 HCl (aq) + Ba(OH)2 (aq)  BaCl2(aq) + 2 H2O(l) DH = -118 kJ Calculate the heat released when 100.0 mL of 0.500 M HCl is mixed with excess Ba(OH)2. The initial temperature of the mixture is 25.0 oC. Calculate the final temperature of the mixture if the mixture has a mass of 400.0 g and a specific heat capacity of 4.18 J/g oC.

Calorimetry Problem Calculate the specific heat of a metal if it causes the temperature of 355 g of water to increase in temperature by 5.6 oC. The initial temperature of the metal is 100 oC and the initial temperature of the water is 26 oC. The mass of the metal is 2.3 grams.

Calorimetry Problem A 120 gram sheet of iron metal at 25.0 oC is placed on top of a 240 gram sheet of aluminum at 50.0 oC. Calculate the final temperature of the mixture assuming no heat is lost to the surroundings.

Chem Catalyst A 30.0 g sample of water at 280 K is mixed with 50.0 g of water at 330 K. Calculate the final temperature of the mixture assuming no heat is lost to the surroundings.

Hess’s Law In a chemical reaction, the enthalpy change can be determined by the sum of a series of stepwise reactions. Calculate the change in enthalpy for the following reaction. A  C DH = ? A  B DH= 25 kJ B  C DH= 10 kJ

1 Step Reaction 2 Step Reaction A  B DH= 25 kJ B  C DH= 10 kJ A  C DH = ? 1 Step Reaction 2 Step Reaction C C Energy (kJ) B B A A The change in H is the same regardless of the number of steps in the reaction

Hess’s Law Add the following reactions together to calculate DH for the following. A  C DH = ? B  A DH= -45 kJ B  C DH= -10 kJ

Hess’s Law 2 A  B DH = ? A  E DH= 45 KJ B  2E DH= 35KJ

SIP/Urban Plunge Problem St. Anthony’s dining hall serves approximately 2,600 meals each day.  Assuming each meal averages about 800 Calories to calculate the following. How much energy (in Kilojoules) does St. Anthony’s Dining Hall distribute on an annual basis?  Calculate the maximum mass of an iceberg that could be melted by this energy using the value for the molar heat of ice that you determined in your lab.  Use the nutrition information for skittles to determine how much energy St. Anthony’s distributes in bags of skittles. How much money would all those bags of skittles cost assuming a bag retails for 75 cents?

Hess’s Law 2NO (g) + O2 (g)  2NO2 (g) DH = -112 kJ Consider the following 1 step reaction N2 (g) + 2O2 (g)  2 NO2 (g) DH = 68 kJ DH for this reaction can also be calculated as the sum of the following reactions. N2 (g) + O2 (g)  2NO (g) DH = 180 kJ 2NO (g) + O2 (g)  2NO2 (g) DH = -112 kJ

Hess’s Law Calculate DH for the conversion of graphite to diamond C graphite (s)  C diamond (s) DH = ? C graphite (s) + O2 (g)  CO2 (g) DH = -394 kJ C diamond (s) + O2(g)  CO2 (g) DH = -396 kJ

Hess’s Law 2 B(s) + 3H2(g)  B2H6 DH = ? 2 B(s) + 3/2 O2(g)  B2O3 (s) DH = -1273 kJ B2H6(g) + 3O2(g)  B2O3(s) + 3H2O (g) DH = -2035 kJ H2(g) + ½ O2 (g)  H2O(l) DH =-286 kJ H2O (l)  H2O(g) DH = 44 kJ

Enthalpy Summary 2 ways to calculate or determine DH for a reaction Directly using a calorimeter Indirectly, using Hess’s Law 1 additional way Using heats of formation

Standard Enthalpies of Formation Consider the following reaction between hydrogen and oxygen. What bonds are being broken in this reaction? What bonds are being formed? Is energy released or absorbed (exothermic or endothermic)? What is the change in enthalpy? O2 + 2H2 Enthalpy 2 H2O

Standard Enthalpies of Formation (DH) can also be calculated using standard enthalpies of formation. Standard enthalpy of formation is the change in enthalpy that accompanies the formation of one mole of a compound from its elements in their standard states (table 6.2 and A-21). Standard states are a reference state and are defined on page 260. Elements in their standard states is defined to have an enthalpy of formation of zero.

Standard Enthalpies of Formation The change in enthalpy for this reaction can be calculated by subtracting the enthalpy of formation for the products from the enthalpy of formation of the reactants. DHof = (Sum Hof Products) – (Sum Hof reactants) O2 + H2 Enthalpy H2O

Standard Enthalpies of Formation Use the data in A-1 to calculate the change in enthalpy for the formation of H2O from O2 and H2. Problem 61 a.

Reaction Spontaneity In your own words, what does it mean for a chemical reaction to be spontaneous? What factors determine if a reaction is spontaneous?

Enthalpy Early chemists thought the change in enthalpy was the key to determine if a reaction was spontaneous. If a reaction released heat it was thought to be spontaneous. However, ice melts spontaneously at temperatures above 25 oC. This is an endothermic process.

Entropy Entropy is the amount of randomness or disorder in a system. How does the entropy of your room change after you clean it? In any spontaneous process there is always an increase in the entropy of the universe

Gibb’s Free Energy DGo = DHo – TDSo DG = DH – TDS DG= - reaction is spontaneous DG = + reaction is nonspontaneous What combinations of enthalpy and entropy will always cause reactions to be spontaneous? What combinations will produce reactions to be nonspontaneous?

Gibb’s Free Energy Given the following data for a chemical reaction, predict what temperature the reaction will be spontaneous. Reaction 1 DH = -18 kJ and DS = - 60 J/K Reaction 2 DH = +18 kJ and DS = -60 J/K

Gibb’s Free Energy

Sample Problem The reaction SO3(G) + H2O (l)  H2SO4(aq) Is the last step in the commercial production of sulfuric acid. The enthalpy change for this reaction is -227 kJ. In designing a sulfuric acid plant, is it necessary to provide for heating or cooling of the reaction mixture. Explain.

Sample Problem Are the following processes exothermic or endothermic. Explain. The combustion of gasoline Water condensing on a cold pipe CO2(S)  CO2(G) F2(g)  2F (g)

2H2(g) + O2(g)  2H2O(l) DH = -572 kJ Sample Problem Consider the following reaction 2H2(g) + O2(g)  2H2O(l) DH = -572 kJ How much heat is evolved for the production of 1.00 mol of H2O (l) How much heat is evolved when 4.03 g of hydrogen is reacted with excess oxygen? How much heat is evolved when 186 g of oxygen is reacted with excess hydrogen?

Sample Problem An unknown chemical is burned and the energy released causes the temperature of a 341 mL sample of water to increase from 25.0 oC to 31.5 oC. Is this reaction exothermic or endothermic? How much energy is released or absorbed? specific heat of H2O = 4.18 J/goC 1 mL of water = 1 gram of water