Heat of solution lab report answers

Determining the Enthalpy of a Chemical Reaction

Introduction

Thermochemistry is the study of energy and its transformations. As chemical bonds break and

form in a chemical reaction, energy in the form of heat (qrxn) is either released or absorbed by the

reaction (the system). At constant pressure (coffee cup calorimetry), the heat released or absorbed

is equal to the enthalpy change (ΔHrxn) of the reaction. The heat exchanged between the system

and surroundings can be measured if the reaction is performed in a container, known as a

calorimeter which insulates the reaction from its surroundings. If heat is released, the reaction is

exothermic and ΔH will be negative. If heat is absorbed, the reaction is endothermic and ΔH will

be positive.

In this experiment, you will determine enthalpy changes (ΔHrxn) of chemical reactions by coffee

cup calorimetry. It is difficult to directly measure heat exchange between reactants and products

(the system) directly. Therefore, we measure the heat change that occurs in the surroundings by

monitoring temperature changes at constant pressure. If we conduct a reaction between two

substances in aqueous solution, then the heat gained or lost by the solution can be calculated with

the following equation:

qsoln = Cp x m x ∆T

qsoln represents the heat that is gained or lost by the solution; Cp is the specific heat of water (4.18

J / g ºC); m is the mass of water, and ∆T is the temperature change of the solution.

The Styrofoam cups used to carry out the reactions are good insulators; however, they still absorb

some of the heat exchange involved in the reaction. Therefore, the heat of the calorimeter (qcal)

must also be taken into consideration to accurately calculate qrxn. Therefore, the heat gained/ lost

by the solution and the calorimeter is equal to the heat lost/ gained by the reaction:

(qcal + qsoln) = - qrxn qrxn = ΔHrxn (at constant pressure)

In this experiment, you will determine the calorimeter constant (Ccal) of two Styrofoam cup

calorimeters. You will also measure ΔHrxn for a series of reactions and use Hess’s Law to compare

calculated ΔHrxn values to experimental values.

This is a 2 week lab. Part A will be completed during week 1 and Part B will be completed

during week 2.

Week 1

Part A: You will be given two Styrofoam cups and some aluminum foil to make two calorimeters.

You will need to determine the heat capacity of your calorimeter (Ccal). By definition, Ccal is

defined as the amount of heat required to raise the temperature of the calorimeter 1 oC. To

determine Ccal you will measure the temperature change associated with mixing warm water with

room temperature water. The heat lost by the warm water is equal to the heat gained by both the

room temperature water and the calorimeter apparatus. The heat gained by the room temperature

water and the calorimeter is equal to the heat lost by the warm water. ΔT will be (Tfinal – Tinitial).

Therefore,

qlost = (specific heat of water) x (grams of warm water) x (ΔTlost)

qgain = (specific heat of water) x (grams of room temperature water) x (ΔTgain)

The heat gained by the calorimeter, qcal, is the difference between the heat lost by the warm water

and the heat gained by the room temperature water. Since qlost is negative, qcal is

qcal = - (qlost + qgain)

Therefore, to calculate the Ccal, the heat gained by the calorimeter (qcal) is divided by the change

in the temperature of the calorimeter. Since the room temperature water was in the calorimeter to

start and the temperature of the calorimeter increased, the change in temperature of the calorimeter

will be equal to ΔTgain.

Ccal = qcal / ΔTgain

You will need to take the heat capacity of the calorimeter into consideration for each of your

enthalpy calculations in this experiment.