Heat & Enthalpy Changes
Objectives:• 1. Define the terms enthalpy and enthalpy changes.• 2. Explain what information is given by the standard
enthalpy change of a reaction.• 3. Explain how a calorimeter is used to determine
the quantity of heat transferred in a chemical reaction.
Key Terms:• Enthalpy, standard enthalpy change, calorimetry
Calorimetry
• Calorimetry is the measurement of the heat gained or lost in a chemical reaction.– A calorimeter is a device that isolates the outside environment.
• Any loss or gain of energy in the products results in a change in temperature only in the calorimeter.
• The rise (or drop) of temperature in the calorimeter is dependent on the amount of heat released or gained and the heat capacity of the surroundings– Bomb calorimeter
» q = (masswater)(Cwater)(DT)
Enthalpy• The difference in the energies related to products and
reactants along is called enthalpy. – Enthalpy is represented by the capital letter H
• DH = -qenvironment
DH = Hproducts – Hreactants
» if exothermic then enthalpy is negative (- DH)» if endothermic then enthalpy is positive (+DH)
– Standard Enthalpy change (DHo) occurs at constant temperature (25oC) and pressure (1atm)• Represents a single mole• Is calculated using the most stable allotrope of an element
Interpreting enthalpy equations
• C(s) + H2O(l) + 113kJ g CO(g) + H2(g)
DHo = + 113kJ
• In this reaction 113kJ of heat energy is absorbed in the endothermic reaction. – Since the products have more energy than the reactants
the products are said to have undergone positive enthalpy (a positive gain in chemical potential energy).
• endothermic reactions result in positive enthalpy (+H)
Interpreting enthalpy equations
• C3H8(g) + 5O2(g) g 3CO2(g) + H2O(g) + 2043kJ
DHo = - 2043kJ
• In this reaction 2043kJ of heat energy is released in the exothermic reaction. – Since the products have less energy than the reactants
the products are said to have undergone negative enthalpy (a negative gain in chemical potential energy).
– exothermic reactions result in negative enthalpy (-H)
Solving Enthalpy
Consider this question.• C3H8(g) + 5O2(g) g 3CO2(g) + H2O(g) + 2043kJ DHo = -
2043kJ– If 2.50 moles of oxygen are reacted with 1.00 mol of
C3H8, how much heat is produced in the reaction?– What is the enthalpy?
• (2.5/5) x 2043kJ = qrxn – DH = -q
» Since you start out with 1/2 the quantity of O2 required for the reaction, the heat produced is also halved. (1021.5kJ)
» DH = -1020kJRatio of coefficients
Using Calorimetry to solve enthalpy
• Examine the reaction of sodium hydroxide water• NaOH(s) g Na+
(aq) + OH-(aq)
– 0.050 moles of sodium hydroxide reacts in a calorimeter containing 75.0g of water. The temperature of the water is raised from 19.8oC to 26.7oC. What is the enthalpy of the rxn?• q = (masswater)(Cwater)(DT)
• qNaOH = (75g )x(4.184J)/goC)x(26.7oC-19.8oC) = +2170J» Denthalpy (DH)= -Denergy (-Dq)» Enthalpy (remember that the sign must change for enthalpy)
• DH = -2.170kJ – Since the water in the calorimeter represents the environment, the reaction is
exothermic
• DHo = (1 mol NaOH / 0.050 mol NaOH) x -2.17kJ = -4.3kJ
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