Thermochemistry

Thermochemistry

The study of heat energy changes associated with physical and chemical processes.

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

) is a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume. Most chemical reactions in engineering occur at constant atmospheric pressure, so we focus on the change in enthalpy ( $\Delta H$ ), which is exactly equal to the heat transferred (

q_p

Exothermic vs. Endothermic

Exothermic Reactions ( $\Delta H < 0$ ): Heat is released from the system to the surroundings.
- Examples: Combustion of fuels, hydration of cement, condensation of steam.
- Engineering Impact: Can lead to dangerous temperature rises if not dissipated. In thick concrete dams, cooling pipes are embedded to remove this heat.
Endothermic Reactions ( $\Delta H > 0$ ): Heat is absorbed by the system from the surroundings.
- Examples: Melting ice, evaporation of water, thermal decomposition of limestone ( $CaCO_3$ ) in a kiln.
- Engineering Impact: Requires continuous energy input (fuel consumption).

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

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 (Joules, J)	-
$m$	Mass (grams, g)	-
$c$	Specific heat capacity (J/(g·°C))	-
$\Delta T$	Change in temperature (T_final - T_initial)	-

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

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

Variables

Symbol	Description	Unit
$\Delta H^\circ_{rxn}$	Standard enthalpy change of the reaction	-
$\Delta H_f^\circ$	Standard enthalpy of formation (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.

$\Delta H_f^\circ$ for an element in its standard state (e.g., $O_2(g)$ , $C(\text{graphite})$ ) is defined as zero.
A large negative $\Delta H_f^\circ$ indicates a highly stable compound.

Thermodynamics vs Thermochemistry

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.

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Enthalpy (ΔH\Delta HΔH)

Exothermic vs. Endothermic

Calorimetry Simulator (Quenching Steel)

Steel Properties (Hot)

Water Properties (Cold)

Equilibrium Results

Calorimetry and Heat Capacity

Specific Heat Formula

Specific Heat Values

Hess's Law

Standard Enthalpy of Reaction

Standard Enthalpy of Formation (ΔHf∘\Delta H_f^\circΔHf∘​)

Thermodynamics vs Thermochemistry

First Law of Thermodynamics

Enthalpy ( $\Delta H$ )

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