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