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Causes of Change

This guide explains the significance of enthalpy (H) as the total energy of a system, highlighting its measurement through changes (ΔH). It covers molar enthalpy change when heating or cooling substances, specific calculations involving molar heat capacities, and sign conventions indicating whether a process is endothermic or exothermic. Examples include heating iron and cooling aluminum, providing insights into thermodynamic principles and how to predict energy changes in chemical reactions. Enhance your understanding of these critical thermodynamic concepts with practical calculations.

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Causes of Change

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  1. Causes of Change Using Enthalpy

  2. Because enthalpy is the total energy of a system, it is a very important quantity. • However, like we know, it can only be measured through a changed. • There is no way to determine the true value of H, but ΔH can be measured as a change occurs. Molar Enthalpy Change

  3. The enthalpy change for one mole of a pure substance is called the __________. • Consider the following (heating iron): Fe(s, 300K)  Fe(s, 1100K) ΔH = 20.1 kJ/mol • This indicates that when 1 mol of solid iron is heated from ___K to ___K, its molar enthalpy increases by 20,100 Joules. Molar Enthalpy Change

  4. The iron does not change state OR goes through a chemical reaction. • This means the change in enthalpy represents only a change in the kinetic energy of the iron atoms. • When a pure substance is heated or cooled, the amount of heat involved is the same as the change in enthalpy (ΔH) ΔH = q Molar Heat Capacity Governs the Changes

  5. With this said, the molar enthalpy change is related to the molar heat capacity by the following equation: Molar enthalpy change = CΔT =(molar heat cap)(change in T) This equation DOES NOT apply to chemical reactions or changes in state. Molar Heat Capacity Governs the Changes

  6. Calculating Molar Enthalpy Change for Heating • How much does the molar enthalpy change when ice warms from -5.4ºC to -0.2ºC? The molar heat capacity of water is 37.4 J/K*mol. Practice! What are the units?? 0.19 kJ/mol.

  7. Calculating Molar Enthalpy Change for Cooling • Calculate the molar enthalpy change when an aluminum can that has a temperature of 19.2ºC is cooled to a temperature of 4.00ºC. The molar heat capacity for Al is 24.2 J/K*mol. Practice Some More!! -368 J/mol

  8. Lead has a molar heat capacity of 26.4 J/K*mol. What molar enthalpy change occurs when lead is cooled from 302ºC to 275ºC? -713 J/mol

  9. Calculate the molar enthalpy change when Hg is cooled 10 K. The molar heat capacity of mercury is 27.8 J/K*mol. -300 J/mol

  10. Notice the molar enthalpy change for the first problem was positive, meaning that the heating of the sample requires energy • The heating of a sample is an endothermic process. • The second problem had a negative molar enthalpy change, which means the heating gives off energy (gets hotter) • This implies that the process is ____________. Signs of Molar Enthalpy Change

  11. You can use the signs of the enthalpy change to determine whether a process is exo- or endothermic. • Processes that have positive enthalpy changes are endothermic. • Processes that have negative enthalpy changes are exothermic. Exothermic and Endothermic

  12. Enthalpy changes can be found for a system of substances as well • Consider the following: H2(g) + Br2(g) 2HBr(g) + heat • This indicates that the enthalpy of 2HBr is LESS than the enthalpy of H2 and Br2, so the enthalpy change is ________ (pos or neg) • This negative enthalpy change indicates a _____________ process. Enthalpy of a System

  13. Enthalpy is the first of three thermodynamic properties that we will cover. • Thermodynamics – the science that examines various processes and the energy changes that accompany the processes. • Allows chemists to predict whether a chemical reaction can occur and what kind of energy it will have. Thermodynamics

  14. Think about the heating of iron again: Fe(s, 300K)  Fe(s, 1100K) ΔH = 20.1 kJ/mol • Similarily, a phase change or a chemical reaction can be written using the same style. • Example: H2(g, 298K) + Br2(g, 298K)  2HBr(g, 298K) ΔH = -72.8 kJ Writing Equations of H Changes

  15. Notice that the enthalpy change for this reaction and other chemical reactions are written using the symbol ΔH. • Think about the last reaction: Is it an exothermic or an endothermic process?? ___________________ Writing Equations of H Changes

  16. Also look over your notes and slideshows for a quiz involving all information and calculations.

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