Heat of Reaction — Definition & Overview

What is the Heat of Reaction?

When chemicals react to keep the reaction going, all substances need energy and a constant temperature. The heat of reaction is the amount of heat added or removed from a reaction which maintains the temperature of the reaction. If heat is released during a reaction, it is called exothermic. If heat is needed for the reaction to progress it is called endothermic.

Relationship with Enthalpy

When chemical reactions are carried out in a laboratory setting, they typically are carried out at atmospheric pressure. If chemical reactions are under a constant pressure the heat of reaction can be called enthalpy ($H$). Since the amount of heat in a reaction can change between reactants and products, the enthalpy change ($\Delta H$) is used in thermodynamic calculations which describes the energy of reactions.

If $\Delta H$ is negative, the reaction is exothermic and a positive $\Delta H$ represents an endothermic reaction.

Thermodynamic Equations

When considering heat of reaction, it is important to know these equations and their meaning:

The heat transferred ($q$) when the value is positive, the reaction gains energy and when it is negative, it loses energy. This is value looks at the total energy of a reaction system ($\Delta E$), pressure ($P$), and change in volume ($\Delta V$): $$q\ =\ \Delta E\ +\ P \Delta V$$

When a reaction is carried out at a constant pressure ($q_p$) the heat transferred can be described as the change of enthalpy in a system ($\Delta H$): $$q\ =\ \Delta E\ +\ P \Delta V\\ at\ constant\ pressure\ E\ +\ PV\ =\ H\\ q_p\ =\ \Delta H$$

(Video) Chemistry 10.5 Heat of Reaction

by IsaacsTEACH

In this video, Isaac explains the heat or enthalpy of a reaction. He shows you how to write a thermochemical equation. He also shows you how we denote the heat of reaction for different varieties of a reaction. After watching this video, you'll understand the heat of reaction!

At its heart, chemistry is all about energy. How do we initiate a chemical reaction? We need the atoms and molecules to collide with the proper energy. We call it activation energy. The activation energy allows the atoms to overcome the electrostatic interactions. So, we break and reform chemical bonds.

As bonds are broken and reformed, energy is absorbed or released. We almost always observe the energy transfer as a change in the temperature. That's one way of recognizing a chemical reaction! Remember, temperature is the state variable for heat. So, on the molecular level, we are monitoring the movement of energy. If the temperature increases, our reaction is exothermic. Energy got transferred from the system to the surroundings. If the temperature decreases, the chemical reaction is endothermic. Energy moved from the surroundings into the system.

So, we conceptually understand what's happening. But how do we convert between temperature change and energy? For that, we use the heat of reaction.

What is Heat of Reaction?

So far, we've seen two components of energy change. The first was pressure-volume work. Our familiar example is a gas lifting a piston. The gas applied a force to the piston against gravity. So, the system did work on the surroundings. The other part was heat. That's the energy transferred between objects with different temperatures. The summation of heat and work equaled the internal energy. The change in internal energy measured the system's total energy change.

What does this have to do with chemical reactions? During a chemical reaction, energy is released or absorbed. A temperature change corresponds to heat. That's how we determine if a reaction is exothermic or endothermic. But how do we deal with the work term? We use constant pressure. We defined the energy change at constant pressure as the enthalpy. Our definition allowed us to cancel the work terms. So, enthalpy equals heat. But do chemical reactions happen at constant pressure? Many do! Often, you'll mix your reactant in a beaker and watch the reaction occur. The reaction occurs in the open air. So, it takes place at atmospheric pressure!

So, what is the heat of reaction? It's the energy released or absorbed during a chemical reaction. To simplify our calculations, we try to use constant pressure reactions. That allows us to calculate the heat of reaction via enthalpy.

Writing Heats of Reaction

To write the heat of reaction, we use a thermochemical equation. They're just a regular chemical equation, except we add the enthalpy change. So, all the rules for balancing chemical equations still apply. Let's look at a few examples. The enthalpy change has units of kilojoules per mole.

Let's start with the combustion of methane.

$CH_{4} + 2O_{2} \rightarrow CO_{2} + 2H_{2}O \, \Delta H=-890.4 \frac{KJ}{mol}$

Notice that the enthalpy change is negative. That's because our reaction is exothermic.

Now let's look at the synthesis of sodium chloride.

$Na+\frac{1}{2}Cl_{2}\rightarrow NaCl \, \Delta H=-573.6 \frac{KJ}{mol}$

Here we have the formation of an ionic bond. Notice that it releases energy. The electrostatic interactions between the sodium and chloride ions lower the potential energy. So, energy is released. We observe an increase in stability.

Calculating Heats of Reaction

So, how do we calculate the heat of reaction values? That's the essence of a lot of chemical thermodynamics. It's such an important concept that we have many different methodologies. We'll explore many of them in future lessons. I'll list them below so you can explore them as needed. But, I would start with calculating enthalpy change.

• Calculating Standard Enthalpies of Formation

• Hess's Law of Heat Summation

• Bond Energies

• Calculating enthalpies for phase changes.

• Heat Capacity, Specific Heats, Calculating Specific Heats, and Heat Capacity of Water
• Heating and Cooling Curves
• Phase change diagram of water

Experimentally Determining Enthalpy Change

Remember, chemistry is an experimental science. So, we also have to measure enthalpy changes experimentally. To do so, we use a calorimeter. The most basic calorimeter is a constant pressure system. But we can also do constant volume calorimetry with bomb calorimeters. We use constant volume calorimetry for reactions that produce gasses. The most common example is combustion reactions. Since the volume change is zero, we eliminate pressure-volume work. Again, we can determine the heat of reaction.