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# Hess’ Law

Hess’s Law states that the total enthalpy change accompanying a particular chemical reaction is independent of the pathway between the initial and final states. This is just a restatement of the principle of the conservation of energy, where the total energy change should be the same if a reaction takes multiple steps to reach the final product, or if a reaction just takes one step. Enthalpy is a measure of the total energy of a thermodynamic system. Thus, the enthalpy change of a particular chemical reaction can be measured by the change of enthalpy of formation of the products minus the change of enthalpy of formation of the reactants. By knowing this, and applying Hess’ Law, the enthalpy change for a particular reaction within a set of reactions can be calculated even if it cannot be measured directly. Another application of Hess’ Law is measuring the total change of enthalpy of a chemical reaction, by summing up the individual reactions which make up the complete reaction. If the enthalpy of a reaction is negative, then the reaction is termed “exothermic,” while if the enthalpy of reaction is positive, then the reaction is termed “endothermic.” Knowing these values, through the application of Hess’ law and combining it with the value of entropy is crucial for determining whether a chemical reaction is spontaneous or if it requires any additional energy for the reaction to take place. © BrainMass Inc. brainmass.com February 16, 2020, 5:09 pm ad1c9bdddf

### Enthalpy of A Reaction Using Hess' Law

This solution will discuss how to determine the enthalpy of a chemical reaction primarily using Hess's law. The use of the general formula for enthalpy will also be addressed. These problems will be solved using both Hess's law and the general formula for enthalpy. A step by step example and answers are included.

### Calorimetry and Hess' Law

Detailed work would be very helpful; the math involved in this gets me confused, but I'm not sure where I have been going wrong. If work is handwritten, please do not write in cursive for I have problems reading it. 1. The mass of a substance is 200gm and its specific heat is 0.09. How much heat is required to raise the tempe

### Calculating Standard Enthalpy of Formation

Several reactions and their standard reaction enthalpies at 298.15k are given: CaC2(s) + 2H2O(l)--> (CaOH)2(s) + C2H2 (g) deltaH:-127.9kj/mol Ca(s) + 1/2 O2(g)--> CaO(s) delta

### Help with these two review problems

1. 2CLF (g) + O2(g) --> CL2O + F20 (Hrxn = 167.4 kj/mol) 2CLF3(g) + 2O2(g) --> Cl2O(g) + 3F2O(g) (Hrxn = 341.4 kj/mol) 2F2(g) + O2(g) --> 2F2O (Hrxn = -43.4 kJ/mol) Using Hess' law, what is the Hrxn for ClF +F2 --> ClF3 in kj/mol (assume temperature is the same)? 2. Given NO(g) + NO2(g) --> N2O3 H=-40.2kJ N2

### Enathalpy of the Reaction

1.) Calculate delta H for the following reaction: The reduction of aluminum oxide by hydrogen gas: Al2O3(s) + 3H2(g) ------- 2Al(s) + 3H2O(g) 2.) Solve for entalapy using Hess's Law. The standard heat of combustion of liquid ethyl alcohol is -227 kcal/mol and that of acetic acid is -209 kcal/mol. The equations are: C2

### Hess's Law Manipulating Equations

I am having trouble with a question in my homework. It states: Given the following thermochemical equations 2KCl(s) + 3O2(g) .......2KClO3(s) Change in H = 78.0kJ P4(s) + 6Cl2(g)..........4PCl3(g) Change in H = -1148.0kJ P4(s) + 2O2(g) + 6Cl2(g).........4POCl3(g)

### Hess Law/Oxidation

See attached and directions.

### Hess's Law - Temperature

Please answer step-by-step with solutions: A 150.0 iron bar at 330 degrees C is put in 1.00 kg water at 20.0 degrees C. What is the final temperature of the bar? (Hint: bar and water will be the same at the end.) Cs = 0.45J/(grams degree C) for iron Cs = 4.18J/(grams degree C) for water

### Hess' Law

Calculate the enthalpy change for this reaction, using the following combustion data: C2H4(g) + 3 O2(g) yields 2CO2(g)+ 2H2O(l) deltaH= -1401 kj C2H6(g) + 7/2 O2(g) yields 2CO2(g) + 3H2O(l) deltaH= -1550kj H2(g) + 1/2 O2(g) yields H2O(l) deltaH= -286kj.