Thermochemistry

Thermochemistry

Learning Objectives

Define thermochemistry and its importance in civil engineering processes.
Understand the concept of enthalpy and differentiate between exothermic and endothermic reactions.
Calculate heat transfer using calorimetry and specific heat capacity.
Apply Hess's Law and standard enthalpies of formation to determine the overall enthalpy change of a reaction.
Relate thermochemistry to the First Law of Thermodynamics.

Thermochemistry in Engineering

Thermochemistry is the study of heat transfer during chemical reactions and physical changes. For civil engineers, this is critical for understanding the heat of hydration in mass concrete (which can cause severe thermal cracking), combustion processes, and the thermal properties of building materials for energy efficiency.

\Delta H

Enthalpy ( $H$ )

A thermodynamic quantity equivalent to the total heat content of a system. It equals the internal energy of the system plus the product of pressure and volume. For reactions at constant pressure, the change in enthalpy ( $\Delta H$ ) equals the heat transferred ( $q_p$ ).

Exothermic Reaction

A reaction where heat is released from the system to the surroundings ( $\Delta H < 0$ ). Examples include the hydration of cement and combustion of fuels.

Endothermic Reaction

A reaction where heat is absorbed by the system from the surroundings ( $\Delta H > 0$ ). Examples include melting ice or the thermal decomposition of limestone.

Interactive Simulation

Use the simulation below to explore the differences between exothermic and endothermic reactions and visualize energy changes.

Calorimetry Simulator (Quenching Steel)

Steel Properties (Hot)

Mass (g)500 g

Initial Temp (°C)200 °C

Water Properties (Cold)

Mass (g)1000 g

Initial Temp (°C)20 °C

Equilibrium Results

Final Temperature ($T_f$)20.00 °C

Heat Transferred ($q$)40500 J

-q_{steel} = q_{water}

-(m_{s}c_{s}\Delta T_{s}) = m_{w}c_{w}\Delta T_{w}

-(500)(0.450)(T_f - 200) = (1000)(4.184)(T_f - 20)

T_f = 20.00 ^\circ\text{C}

Calorimetry and Heat Capacity

Measurement of Heat Flow

Calorimetry is the experimental measurement of heat flow. The core principle is the conservation of energy: heat lost by one part of an isolated system must equal the heat gained by another part.

Specific Heat Formula

Calculates the amount of heat transferred based on mass, specific heat, and temperature change.

q = mc\Delta T

Variables

Symbol	Description	Unit
$q$	Heat transferred	J
$m$	Mass	g
$c$	Specific heat capacity	$\text{J}/(\text{g}\cdot^\circ\text{C})$
$\Delta T$	Change in temperature ( $T_{\text{final}} - T_{\text{initial}}$ )	$^\circ\text{C}$

Specific Heat Values

Water has a very high specific heat ( $4.184 \, \text{J}/(\text{g} \cdot ^\circ\text{C})$ ), making it an excellent coolant for industrial processes. Concrete has a specific heat of roughly $0.88 \, \text{J}/(\text{g} \cdot ^\circ\text{C})$ .

Hess's Law

Hess's Law Principles

In many cases, it is impossible to measure the $\Delta H$ of a reaction directly. Hess's Law states that if a reaction is carried out in a series of steps, $\Delta H$ for the overall reaction equals the sum of the enthalpy changes for the individual steps. This is because enthalpy is a state function (depends only on the current state, not the path taken).

Standard Enthalpy of Reaction

Calculates the overall enthalpy change of a reaction using standard enthalpies of formation.

\Delta H^\circ_{\text{rxn}} = \sum n \Delta H_f^\circ (\text{products}) - \sum m \Delta H_f^\circ (\text{reactants})

Variables

Symbol	Description	Unit
$\Delta H^\circ_{\text{rxn}}$	Standard enthalpy change of the reaction	-
$\Delta H_f^\circ$	Standard enthalpy of formation	$\text{kJ/mol}$
$n, m$	Stoichiometric coefficients from the balanced equation	-

Standard Enthalpy of Formation ( $\Delta H_f^\circ$ )

The enthalpy change for the reaction that forms exactly 1 mole of a pure substance from its constituent elements in their standard states. The $\Delta H_f^\circ$ for an element in its standard state is zero.

Thermodynamics vs Thermochemistry

Thermodynamics Relation

Thermochemistry is technically a branch of thermodynamics. In engineering thermodynamics, we frequently refer to the First Law of Thermodynamics, which relates internal energy change to heat ( $q$ ) and work ( $w$ ).

First Law of Thermodynamics

The change in internal energy of a closed system.

\Delta U = q + w

Variables

Symbol	Description	Unit
$\Delta U$	Change in internal energy of the system	-
$q$	Heat added to the system	-
$w$	Work done on the system	-

Key Takeaways

Enthalpy ( $\Delta H$ ) represents the heat exchanged in a reaction at constant pressure.
Exothermic reactions release heat (- $\Delta H$ ), while Endothermic reactions absorb heat (+ $\Delta H$ ).
Calorimetry ( $q = mc\Delta T$ ) is used to calculate heat transfer based on temperature changes and specific heat capacity.
Hess's Law allows calculation of $\Delta H$ using standard enthalpies of formation ( $\Delta H_f^\circ$ ), proving useful when direct measurement is impossible.
The First Law of Thermodynamics formalizes the relationship between internal energy, heat, and work.