Thermochemistry. Energy (and Thermochemistry) World of Chemistry Chapter 10. Energy. Energy

Transcription

1 Thermochemistry Thermodynamics is the science of the relationship between heat and other forms of energy. (and Thermochemistry) World of Chemistry Chapter 10 is defined as the ability to do work or produce heat. The Law of Conservation of can be converted from one form to another but can neither be created nor destroyed. can be in many forms: Radiant : Electromagnetic radiation Thermal : Associated with random motion of a molecule or atom. Chemical : stored within the structural limits of a molecule or atom. Explain the energy conversion in the following pictures: There are three broad concepts of energy: 1. Kinetic 2. Potential 3. Heat Flow We will look at each of these in detail. 1

2 1. Kinetic energy due to the motion of the object depends on the mass of the object and its velocity Kinetic Temperature is a measure of the average kinetic energy of the components of a substance. a) Celsius: melting point of water set as 0 C and the boiling point as 100 C Temperature b) Kelvin Scale: Celsius-sized degree, but the scale is reset so it starts at 0 Kelvins (absolute zero)» particle motions stop at 0 K C = K Potential energy due to position or composition STORED energy that is not doing work right now but has ability if released Ex) coiled springs, chemical bonds in batteries, objects at a height Potential Measurement: stored energy converted to kinetic energy and measured using a calorimeter Joule (J) is metric unit for potential energy (1000 J = 1 kj) calorie (cal) is amount of energy required to raise the temperature of 1 g water by 1 C (1 cal =4.184 J) 3. Heat Flow Heat is defined as the energy that flows into or out of a system because of a difference in temperature between the system and its surroundings. 2

3 3. Heat Flow Heat flows from where it is hot to where it s not. Heat Flow Examples: a) Open the front door of your warm house on a cold morning. Heat will flow outside. b) Place a hot pack on a sore muscle. The heat will flow to muscle. Heat transfer occurs until both objects are same temperature. Temperature vs. Heat Bozeman Law of Conservation of (4:08 min) Heat is the energy associated with the motions of atoms, molecules, and ions. Temperature is a measurement of the flow of heat. Heat of Reaction In chemical reactions, heat is often transferred from the system to its surroundings, or vice versa. The substance under study in which a change occurs is called the thermodynamic system. The surroundings are everything in the vicinity of the thermodynamic Heat of Reaction Heat is denoted by the symbol q. The sign of q is positive if heat is absorbed by the system. The sign of q is negative if heat is evolved by the system. 3

4 Heat of Reaction An exothermic process is a chemical reaction or physical change in which heat is evolved (-q). PE stored in a substance is released and converted into KE Exothermic Process Example: Burning a peanut releases heat that is absorbed by a mass of water in a calorimeter cup. Temperature of water increases. Heat of Reaction An endothermic process is a chemical reaction or physical change in which heat is absorbed (+q). KE from surrounding is absorbed by the change and converted into PE. Endothermic Process Example: An ice cube is placed in a measured mass of water in a calorimeter. Ice cube melts as it absorbs heat from the water. Temperature of water decreases. + q - q In which process, does the surroundings get hot? Endothermic or Exothermic? In which process, does the surroundings get cold? Endothermic or Exothermic? Problem to Consider: You squeeze a hot pack and activate the chemical reaction inside. You feel the heat of the pack in your hands. a) What is the system? b) What is the surrounding? c) Is the reaction exothermic or endothermic? 4

5 Heat Content of Food Measured in Calories (1000 calories or 1 kilocalorie) Determined by burning food in a bomb calorimeter capital C Enthalpy and Enthalpy Change Enthalpy, denoted H, is an extensive property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction. An extensive property is one that depends on the quantity of substance. Enthalpy is a state function, a property of a system that depends only on its present state and is independent of any previous history of the system. Enthalpy and Enthalpy Change The change in enthalpy for a reaction at a given temperature and pressure (called the enthalpy of reaction) is obtained by subtracting enthalpy of the reactants from the enthalpy of the products. H = H (products) H (reactants) Thermochemical Equations A thermochemical equation is the chemical equation for a reaction (including phase labels) in which the equation is given a molar interpretation, and the enthalpy of reaction for these molar amounts is written directly after the equation. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) ; H = kj Applying Stoichiometry and Heats of Reactions Consider the following reaction. How much heat could be obtained by the combustion of 10.0 grams CH 4? CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l); H 0 = kj per 1 mole 10.0 g CH 4 x 1 mol CH 4 x kj = kj g CH 4 1 mol CH 4 Problem to Consider: How much heat will be released if 1.0 grams of hydrogen peroxide, H 2 O 2, decomposes? 2 H 2 O 2 2 H 2 O + O 2 H = -190 kj 1.0 g H 2 O 2 x 1 mol H 2 O 2 x kj = kj g H 2 O 2 2 moles Minus sign tells you heat is released! 5

6 How much heat will be released when 6.44g sulfur reacts with excess oxygen according to the following equation? 2 S + 3 O 2 2 SO 3 H = kj Thermochemical Equations The following are two important rules for manipulating thermochemical equations: When a thermochemical equation is multiplied by any factor, the value of H for the new equation is obtained by multiplying the H in the original equation by the same factor. Multiply by ½ 2 H 2 O 2 2 H 2 O + O 2 H = -190 kj H 2 O 2 H 2 O + ½ O 2 H = -95 kj Thermochemical Equations When a chemical equation is reversed, the value of H is reversed in sign. 2 H 2 O 2 2 H 2 O + O 2 H = -190 kj 2 H 2 O + O 2 2 H 2 O 2 H = 190 kj Hess Law If a reaction is the sum of two or more reactions, H for the overall process is the sum of the H values of the reactions. change is a state function! It is independent of the path followed from initial conditions to final conditions. Problem to Consider: The following Opposite reaction sides is carried out in two steps: subtract! N 2 + O 2 2 NO H = 180 kj 2 NO + O 2 2 NO 2 H = -112 kj Same sides Calculate the H for the reaction add! N O 2 2 NO 2 H = 180 kj + (-112 kj) = 68 kj SO 2 is on wrong side Also, SO 2 is wrong so reverse equation magnitude so divide (and For the sign following of H) information: by 2 (and H) S + 3/2 O 2 SO 3 H = -395 kj 2 SO 2 + O 2 2 SO 3 H = -198 kj SO 3 SO 2 + 1/2 O 2 H = 99 kj Calculate H for the reaction S + O 2 SO 2 H = -296 kj Now add to find answer! 6

7 Measuring Heats of Reaction To see how heats of reactions are measured, we must look at the heat required to raise the temperature of a substance, because a thermochemical measurement is based on the relationship between heat and temperature change. Calculating Requirements Specific Heat Capacity The specific heat capacity (or specific heat ), s, is the heat required to raise the temperature of one gram of a substance by one degree Celsius. Different substances respond differently to being heated. Specific Heat Capacity What does a higher specific heat capacity (like water) indicate about a substance? Calculating Requirements required to change the temperature of a substance depends on three things: 1) Specific heat capacity (s) 2) Mass in grams of sample (m) 3) Changes in temperatures in C ( T) We can represent this by the following equation: q = s x m x T A Problem to Consider Calculate the heat absorbed when the temperature of 15.0 grams of water is raised from 20.0 C to 50.0 C. q = s x m x T Note: T = T final - T initial q = (4.184 J/g C ) x (15.0 g) x ( C ) q = 1.88 x 10 3 J 1. Find the heat required to raise the temperature of 193 g of liquid ethanol from 19.0 C to 35.0 C. q = 7530 J 2. A 120 g sample of an unknown metal absorbed 9612 J of energy and its temperature increased from 25 C to 115 C. Identify the metal. s = 0.89 J/g C; metal could be Al 7

8 Calorimetry Problems These problems are based upon the Law of Conservation of. Typical set-up: Hot metal is added to cold water in an insulated container. Cold water gains the heat lost by hot metal and both become the same temperature. Problem to Consider A 107g sample of iron at 500. C is mixed with 240g of water at 20.0 C. Find the final temperature of the system. - heat lost metal = + heat gained by water -(0.449 J/g C)(107g)(x C) = (4.18 J/g C)(240g)(x-20.0 C) T f = 41.9 C g of aluminum at 400. C is placed in 167g of water at 25.0 C. What will be the final temperature? g of water at 30.0 C is mixed with 83.8g of a solid metal at C. If the final temperature was 50.0 C, what is the specific heat of the metal? 8