Thermochemistry Lecture 4 May 9th, 2024

3.5 Enthalpy

  • it is not always convenient to work under constant volume conditions

  • the simplest set of conditions to prepare is often constant pressure (just leave the lid off the the container)

  • in this case, w is not zero

  • constant pressure does not mean that w is easy to measure or predict.

  • how do we determine delta rU in this case? We don’t…

  • Define a new state functions for constant pressure I

    • Rather than trying to track, measure, calculate, or predict w for constant pressure conditions, we define a new state function:

    • Enthalpy (H): The sum of the internal energy of a system and the product of its pressure and volume:

      • H = U + PV

    • is the enthalpy a state function (yes. but why?)

    • why is enthalpy useful?

  • Define a new state functions for constant pressure II

    • Under conditions of constant pressure, changes in enthalpy can be described as

      • delta rH = delta rU + P(delta V)

    • However, delta rU = q + w and P(delta V) = -w, so

      • delta rH = q + w - w = Qp

        • where the subscript P in Qp indicates constant pressure conditions

  • Define a new state functions for constant pressure III

    • We have combined existing state functions (U, P, and V) to get a new one (H)

    • this new state function is identical to an easily measured quantity Qp

    • Once we learn how to measure delta rH for reactions, we will also learn how to use tabulated data to predict delta rH for reactions without new measurements

  • Endothermic and Exothermic Reactions I

    • Endothermic Reactions: A chemical reaction that absorbs heat from its surroundings at constant pressure (delta rH > 0)

    • Exothermic Reactions: A chemical reaction that releases heat to its surroundings at constant pressure (delta rH < 0)

  • Enthalpy

    • Remember enthalpy (H) is useful because enthalpy changes are easily related to measurements made at constant pressure

    • this means that we do not need to use devices that are tightly sealed and able to withstand high pressure.

  • The Coffee-cup Calorimeter I

    • the image below illustrates a coffee-cup calorimeter

    • such devices measure temperature changes due to reactions at constant pressure conditions

    • these devices are particularly well suited to reactions in solutions

  • The Coffee-cup Calorimeter II

    • Coffee-cup calorimeters are often used to investigate reactions in solutions

    • in most cases, sufficient solution is used so that the heat capacity is not significantly changed by the addition of a reactant

    • in such cases, we make use of two equations,

      • Qsolution = Msolution x Cs,solution x delta T , and

      • Qr = -Qsolution

  • Enthalpy

    • there are several important properties of delta rH that we need to explore

    • the points outlined on the next few slides will be very useful in working through complex calculations

  • Energy, Enthalpy and Stoichiometry I

    • We used the symbols delta rU and delta rH (with subscript r) to desrcibe the changes in internal energy or enthalpy for a reaction

    • these two properties are specific for whatever reaction we are investigating

    • we need to keep these reactions balanced in order to work with it

  • The effect of changing amounts I

    • if we know delta rH for a balanced chemical equation, then scaling (multiplying) the quantities of all reactants and products by the same amount results in delta rH being scaled by the same amount

  • The effect of changing direction I

    • If a reaction is somehow made to happen in reverse, then delta rH must change signs (be negative to the unreversed reaction)

  • Hess’ Law (Doing a Reaction in steps) I

    • if a reaction can determined as a sequence of steps, the delta rH for the overall reaction is the sum of delta rH’s of the steps

    • this idea is sometimes described as Hess’ Law

  • Why Hess’ Law works I

    • Remember, delta rH is a state function

    • As long as we use the same amount of material with the same starting and ending conditions, delta rH will be the same for the two different experiments

  • Enthalpy

    • the result of other peoples measurements can be used to predict the results of new reactions

    • this is what tables of standard enthalpies of formation (delta rH) are for