![]() ![]() You may have noticed that the Δ H for a chemical reaction may be positive or negative. Write the thermochemical equation for the reaction of N 2(g) with O 2(g) to make 2NO(g), which has an enthalpy change of 181 kJ.Īnswer N 2(g) + O 2(g) → 2NO(g) Δ H = 181 kJ The thermochemical equation is PCl 3(g) + Cl 2(g) → PCl 5(g) Δ H = −88 kJ Write the thermochemical equation for the reaction of PCl 3(g) with Cl 2(g) to make PCl 5(g), which has an enthalpy change of −88 kJ. A thermochemical equation is assumed to refer to the equation in molar quantities, which means it must be interpreted in terms of moles, not individual molecules. We write the equation as 2H 2(g) + O 2(g) → 2H 2O(ℓ) Δ H = −570 kJĪ chemical equation that includes an enthalpy change is called a thermochemical equation A chemical equation that includes an enthalpy change. For example, when two moles of hydrogen react with one mole of oxygen to make two moles of water, the characteristic enthalpy change is 570 kJ. ![]() The enthalpy change for a reaction is typically written after a balanced chemical equation and on the same line. When a chemical reaction occurs, there is a characteristic change in enthalpy. We will always be interested in the change in H, rather than the absolute value of H itself. The letter H stands for “enthalpy,” a kind of energy, while the Δ implies a change in the quantity. (Δ H) as the heat of a process when pressure is held constant: Δ H ≡ q at constant pressure We define the enthalpy change The heat of a process at constant pressure denoted Δ H. What we need is a definition of energy that holds when some of these conditions are specified (somewhat similar to our definition of standard temperature and pressure in our study of gases). Our study of gases in Chapter 6 "Gases" and our definition of work in Section 7.2 "Work and Heat" indicate that conditions like pressure, volume, and temperature affect the energy content of a system. ![]() #CHEMICAL REACTION THERMODYNAMICS CALCULATOR HOW TO#Now we need to learn how to properly express these energy changes. A fundamental concept is that every chemical reaction occurs with a concurrent change in energy. Now that we have shown how energy, work, and heat are related, we are ready to consider energy changes in chemical reactions.
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