Thermochemical equations changes

Remember that multiplying a thermochemical equation also multiplies its AH by the same factor. Reversing a thermochemical equation changes the sign of its AH . 

The thermochemical equation allows us to relate the enthalpy change to amounts of reactants and products, leading to conversion factors such as... 

The standard enthalpy change, Aff°, for a given thermochemical equation is equal to the sum of the standard enthalpies of formation of the product compounds minus the sum of the standard enthalpies of formation of the reactant compounds. 

STRATEGY The heat released by the reaction at constant pressure is calculated from the temperature change multiplied by the heat capacity of the calorimeter. Use the molar mass of one species to convert the heat released into the reaction enthalpy corresponding to the thermochemical equation as written. If the temperature rises, the... 

Calculate energy changes from calorimetry data and write a thermochemical equation (Examples 6.4 and 6.7). 

The overall enthalpy change is the same as that indicated earlier. An useful consequence of Hess s law is that thermochemical equations can be added and subtracted just like algebraic equations. This facilitates the calculation of enthalpy changes for reactions which cannot be studied experimentally. 

There are three different ways to represent the enthalpy change of an exothermic reaction. The simplest way is to use a thermochemical equation a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents. For example, consider the exothermic reaction of one mole of hydrogen gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen gas that reacts, 285.8 kj of heat is produced. Notice that the heat term is included with the products because heat is produced. 

In section 5.1, you learned about the energy changes that accompany physical changes, chemical reactions, and nuclear reactions. You learned how to represent energy changes using thermochemical equations and diagrams. In the next section, you will determine the enthalpy of a reaction by experiment. 

Use your results to determine the enthalpy change of the neutralization reaction, in kj/mol of NaOH. Write the thermochemical equation for the neutralization reaction. 

For example, you can combine thermochemical equations (1) and (2) below to find the enthalpy change for the decomposition of hydrogen peroxide, equation (3). 

Determine the enthalpy change of this reaction, given the following thermochemical equations. 

What is the relationship between the initial quantity of reactants for a reaction and the enthalpy change for a reaction Use a thermochemical equation in your answer. 

This entire expression is a thermochemical equation, a chemical equation together with the corresponding enthalpy change. The stoichiometric coefficients in a thermochemical equation are interpreted as the number of moles that react to give the reported change in enthalpy. In this case, the stoichiometric coefficient of CH4 is 1 and that of 02 is 2, so the... 

Notice that the thermochemical equation is written for the formation of 1 mol C2H5OH(l) so a fractional coefficient is necessary for 02. We report that AHf°(C2H5OH, 1) = —277.69 kj-mol1. Note also how the substance and its state are used to label the enthalpy change, so that we know which species and which form of that species we are talking about. 

A final assumption of thermochemical equations is that when you reverse the direction of a chemical reaction, you only change the sign of the enthalpy. For example, the synthesis of... 

Sample The complete combustion of butane gas, C4H10, in oxygen gas, Oz, produces C02 and H20. It is a highly exothermic process releasing 2845 kj of heat per mole of butane. Write the balanced thermochemical equation, using all whole-number coefficients. Also, determine the enthalpy change in burning 50.0 g of butane gas. 

You will write thermochemical equations and use them to calculate changes in enthalpy. 

A thermochemical equation is a balanced chemical equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy, Mi. 

Thermochemical equations for changes of state The vaporization of water and the melting of ice can be described by the following equations. 

In situations such as this, you can calculate AH for the formation of sulfur trioxide using Hess s law of heat summation. Hess s law states that if you can add two or more thermochemical equations to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction. Hess s law enables you to calculate enthalpy changes for an enormous number of chemical reactions by imagining that each reaction occurs through a series of steps for which the enthalpy changes are known. 

Using Hess s law, the enthalpy change for a reaction can be calculated by adding two or more thermochemical equations and their enthalpy changes. 

When you reverse a thermochemical equation, why must you change the sign of AH (Chapter 16)... 

Hess s law (p. 506) States that if two or more thermochemical equations can be added to produce a final equation for a reaction, then the sum of the enthalpy changes for the individual reactions is the enthalpy change for the final reaction. 

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