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lions Review the Backpround section Atung Eqations A-Cin such a way that they add up to Equation L.

lions Review the Backpround section Atung Eqations A-Cin such a way that they add up to Equation L.

Chemistry: The Molecular Science
Chemistry: The Molecular Science
5th Edition
ISBN:9781285199047
Author:John W. Moore, Conrad L. Stanitski
Publisher:John W. Moore, Conrad L. Stanitski
Chapter4: Energy And Chemical Reactions
Section: Chapter Questions
Problem 89QRT
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4:52 PM Mon Dec 6 7 O 15% O Introduction The reaction of magnesium metal with air in a Bunsen burner flame provides a dazzling demonstration of a combustion reaction. Magnesium burns with an intense flame that produces a blinding white light. This reaction was utilized in the early days of photography as the source of "flash powder" and later in flashbulbs. It is still used today in flares and fire- works. How much heat is produced when magnesium burns? Concepts • Heat of reaction • Heat of formation • Hess's Law • Calorimetry Background Magnesium reacts with oxygen in air to form magnesium oxide, according to Equation 1. Mgis) + 20,0 → MgO(s) + heat Equation 1 As mentioned above, a great deal of heat and light are produced-the temperature of the lame can reach as high as 2400 °C. The amount of heat energy produced in this reaction cannot be measured directly in the high school lab. It is possible, however, to determine the amount of heat produced by an indirect method, using Hess's Law. The heat or enthalpy change for a chemical reaction is called the heat of reaction (AH. The enthalpy change-defined as the difference in enthalpy between the products and reactants-is equal to the amount of heat transferred at constant pressure and does not depend on how the transformation occurs. This definition of enthalpy makes it possible to determine the heats of reaction for reactions that cannot be measured directly. According to Hess's Law, if the same overall reaction is achieved in a series of steps, rather than in one step, the enthalpy change for the overall reaction is equal to the sum of the enthalpy changes for each step in the reaction series. There are two basic rules for calculating the enthalpy change for a reaction using Hess's Law.
Transcribed Image Text:4:52 PM Mon Dec 6 7 O 15% O Introduction The reaction of magnesium metal with air in a Bunsen burner flame provides a dazzling demonstration of a combustion reaction. Magnesium burns with an intense flame that produces a blinding white light. This reaction was utilized in the early days of photography as the source of "flash powder" and later in flashbulbs. It is still used today in flares and fire- works. How much heat is produced when magnesium burns? Concepts • Heat of reaction • Heat of formation • Hess's Law • Calorimetry Background Magnesium reacts with oxygen in air to form magnesium oxide, according to Equation 1. Mgis) + 20,0 → MgO(s) + heat Equation 1 As mentioned above, a great deal of heat and light are produced-the temperature of the lame can reach as high as 2400 °C. The amount of heat energy produced in this reaction cannot be measured directly in the high school lab. It is possible, however, to determine the amount of heat produced by an indirect method, using Hess's Law. The heat or enthalpy change for a chemical reaction is called the heat of reaction (AH. The enthalpy change-defined as the difference in enthalpy between the products and reactants-is equal to the amount of heat transferred at constant pressure and does not depend on how the transformation occurs. This definition of enthalpy makes it possible to determine the heats of reaction for reactions that cannot be measured directly. According to Hess's Law, if the same overall reaction is achieved in a series of steps, rather than in one step, the enthalpy change for the overall reaction is equal to the sum of the enthalpy changes for each step in the reaction series. There are two basic rules for calculating the enthalpy change for a reaction using Hess's Law.
4:52 PM Mon Dec 6 7 O 15% O 2/3 Heals of Redon and HesS Lowo z Consider the following three reactions: Equation A Mgls) + 2HCI(ag) - MgCl,lag) + H,() MgO(a) + 2HCI(ag) + MgCI,lag) + H00) Equation B Equation C It is possible to express the combustion of magnesium (Equation 1) as an algebraic sum of Equations A, B, and C. Applying Hess's Law, therefore, it should also be possible to deter- mine the heat of reaction for Equation 1 by combining the heats of reaction for Equations A-C in the same algebraic manner. Note: Chemical equations may be combined by addition, subtraction, multiplication, and division. Experiment Overview The purpose of this experiment is to use Hess's Law to determine the heat of reaction for the combustion of magneslum (Equation 1). The heats of reaction for Equations A and B will be measured by calorimetry. The heats of reaction for these two reactions will then be combined algebraically with the heat of formation of water (Equation C) to calculate the heat of reaction for the combustion of magnesium. Pre-Lab Questions 1. Review the Backpround section. Atange Equations A-C in such a way that they add up to Equation 1. 2. Use Hess's Law to express the heat of reaction for Equation 1 as the appropriate algebraic sum of the heats of reaction for Equations A-C. 3. The heat of reaction for Equation Cis equal to the standard heat of formation of water. The heat of formation of a compound is defined as the enthalpy change for the preparation of one mole of a compound from ita respective elements in their standard states at 25 "C. Chemical reference sources contain tables of heats of formation for many compounds. Look up the heat of formation of water in your textbook or in a chemical reference source such as the CRC Handbook of Chemistry and Physics. Materials
Transcribed Image Text:4:52 PM Mon Dec 6 7 O 15% O 2/3 Heals of Redon and HesS Lowo z Consider the following three reactions: Equation A Mgls) + 2HCI(ag) - MgCl,lag) + H,() MgO(a) + 2HCI(ag) + MgCI,lag) + H00) Equation B Equation C It is possible to express the combustion of magnesium (Equation 1) as an algebraic sum of Equations A, B, and C. Applying Hess's Law, therefore, it should also be possible to deter- mine the heat of reaction for Equation 1 by combining the heats of reaction for Equations A-C in the same algebraic manner. Note: Chemical equations may be combined by addition, subtraction, multiplication, and division. Experiment Overview The purpose of this experiment is to use Hess's Law to determine the heat of reaction for the combustion of magneslum (Equation 1). The heats of reaction for Equations A and B will be measured by calorimetry. The heats of reaction for these two reactions will then be combined algebraically with the heat of formation of water (Equation C) to calculate the heat of reaction for the combustion of magnesium. Pre-Lab Questions 1. Review the Backpround section. Atange Equations A-C in such a way that they add up to Equation 1. 2. Use Hess's Law to express the heat of reaction for Equation 1 as the appropriate algebraic sum of the heats of reaction for Equations A-C. 3. The heat of reaction for Equation Cis equal to the standard heat of formation of water. The heat of formation of a compound is defined as the enthalpy change for the preparation of one mole of a compound from ita respective elements in their standard states at 25 "C. Chemical reference sources contain tables of heats of formation for many compounds. Look up the heat of formation of water in your textbook or in a chemical reference source such as the CRC Handbook of Chemistry and Physics. Materials
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