Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 55 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

Most chemical reactions occur at atmospheric pressure. Under conditions of constant pressure, the heat change associated with the reaction, &#916;Q, equals the change in enthalpy, &#916;H, also called the enthalpy or heat of reaction.

Enthalpy of the reaction is the difference between the enthalpies of the products and the reactants. When the enthalpy of the products is greater than the enthalpy of the reactants, &#916;H is positive. Such reactions absorb heat and are endothermic.

On the contrary, if the enthalpy of the reactants is greater than the enthalpy of the products, &#916;H is negative. Such reactions release heat and are exothermic.

For any chemical reaction, the magnitude of the accompanying enthalpy change depends on the stoichiometric amounts of reactants and products, as indicated by the coefficients of the balanced equation.

A balanced chemical equation that includes phase labels and enthalpy of reaction, &#916;H, is called a thermochemical equation.

Consider the combustion of methane &#8211; a primary source of fuel. Burning methane releases heat to the surroundings. The exothermic nature of the reaction is indicated by the negative enthalpy change in the thermochemical equation.

The equation for combustion shows that when one mole of methane gas reacts with 2 moles of oxygen gas to yield 1 mole of carbon dioxide gas and 2 moles of liquid water, 890.8 kilojoules of heat are released to the surroundings. 

The molar ratio between the reactants or products and the heat of reaction can be used as conversion factors to calculate the heat exchanged during the reaction.

If a gas cylinder contains 25.5 kilograms of methane, and all of the methane in the cylinder undergoes combustion, what amount of heat will be produced? In general, the conceptual plan is to convert the mass to moles and then moles to the heat of reaction.

To begin, 25.5 kilograms is multiplied by 1000 to find the mass in grams. Then, the mass of methane is divided by its molar mass &#8211; 16.0 grams per mol, to yield 1594 moles of methane.

Finally, using the conversion factor between moles of methane and heat of reaction, 1594 moles of methane releases 14.2 times ten to the 6 kilojoules of heat - which is the heat of the reaction. The answer is negative since the reaction is exothermic as heat evolves in the reaction.

thermochemical equations

For a chemical reaction (the system) carried out at constant pressure &#8211; with the only work done caused by expansion or contraction &#8211; the enthalpy of reaction (also called the heat of reaction, &#916;Hrxn) is equal to the heat exchanged with the surroundings (qp).

<img alt="Thermodynamics equation: &#916;H_rxn = q_p; enthalpy change at constant pressure." src="/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

The change in enthalpy is an extensive property, and it depends on the amounts of the reactants participating in the reaction (or the number of moles of reactants). The change in enthalpy is specific to the reaction, and the physical states of the reactant and product species are important. An exothermic reaction is characterized by a &#8722;&#916;Hrxn value, while an endothermic reaction has a +&#916;Hrxn value. 

Because the amount of heat released or absorbed by a reaction corresponds to the amount of each substance consumed or produced by the reaction, it is convenient to use a thermochemical equation to represent the changes in both matter and energy. In a thermochemical equation, the change in enthalpy of a reaction is shown as &#916;Hrxn, and it is generally provided following the equation for the reaction. The magnitude of &#916;Hrxn indicates the amount of heat associated with the reaction shown in the chemical equation. The sign of &#916;Hrxn indicates if the reaction is exothermic or endothermic, as written. In the following equation, 1 mole of hydrogen gas and 1/2 mole of oxygen gas (at some temperature and pressure) react to form 1 mole of liquid water (at the same temperature and pressure).

<img alt="Chemical reaction equation, H&#60;sub&#62;2&#60;/sub&#62;(g) + 1/2 O&#60;sub&#62;2&#60;/sub&#62;(g) &#8594; H&#60;sub&#62;2&#60;/sub&#62;O(l), synthesis process." src="/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

This equation indicates that 286 kJ of heat is released to the surroundings. In other words, 286 kJ of heat is released (reaction is exothermic) for every mole of hydrogen that is consumed or for every mole of water that is produced. Therefore, the enthalpy of reaction is a conversion factor that can be used to calculate the amount of heat that is released or absorbed during reactions involving specific amounts of reactants and products.

<img alt="Chemical equation diagram for hydrogen, oxygen, and water enthalpy; includes -286 kJ/mol values." src="/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

If the coefficients of the chemical equation are multiplied by some factor (i.e., if the amount of a substance is changed), the change in enthalpy must be multiplied by that same factor.

(two-fold increase in amounts)

<img alt="Chemical equation, enthalpy calculation: \(2\text{H}_2(g) + \text{O}_2(g) \to 2\text{H}_2\text{O}(l)\), \(\Delta H = -572\) kJ." src="/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

(two-fold decrease in amounts)

<img alt="Chemical thermodynamics equation, &#916;Hrxn = -143 kJ, depicting reaction enthalpy change." src="/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

To illustrate that the enthalpy change of a reaction depends on the physical states of the reactants and products, consider the formation of gaseous water (or water vapor). When 1 mole of hydrogen gas and &#189; mole of oxygen gas react to form 1 mole of gaseous water, only 242 kJ of heat are released, as opposed to 286 kJ of heat, which is released when liquid water forms.

<img alt="Static equilibrium equation, H&#8322;(g) + 1/2 O&#8322;(g) &#8594; H&#8322;O(g), &#916;H_rxn=-242 kJ, chemical reaction diagram." src="/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

For a chemical reaction (the system) carried out at constant pressure &ndash; with the only work done caused by expansion or contraction &ndash; the ...

Education

JoVE Core

Thermochemistry

KEY TERMS AND CONCEPTS

heat changes in chemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property.

The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a positive one. For that reason, a thermochemical equation is useful because it shows both the chemical change and the energy change together.

In a thermochemical equation, ΔHrxn is written after the chemical equation. Its magnitude tells how much heat is linked to the reaction as written. Its sign shows whether the reaction is exothermic or endothermic in that form.

For example, 1 mole of hydrogen gas and 1/2 mole of oxygen gas can react to form 1 mole of liquid water. This reaction releases 286 kJ of heat to the surroundings. That means 286 kJ is released for every mole of hydrogen used or every mole of water made, so ΔHrxn can act like a conversion factor.

If the coefficients in the chemical equation are multiplied, the enthalpy change must be multiplied by the same factor. A two-fold increase in the amounts gives a two-fold increase in ΔHrxn, and a two-fold decrease gives a two-fold decrease. The physical state matters too. When the same reaction makes gaseous water instead of liquid water, only 242 kJ of heat are released.

Heat Changes in Chemical Equations

energy basics

How Energy Changes in Chemical Reactions

Chemical reactions change matter and energy at the same time. A match lighting is a simple example. These energy changes are part of everyday life and appear in food, fuels, and industry. Food provides energy through metabolic reactions. The macronutrients in food break down and help keep the body functioning. Fuels such as gasoline, natural gas, and coal are burned to produce energy for transportation, heating, and electricity generation. Industrial chemical reactions also use large amounts...

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internal energy

Internal Energy, Heat, and Work

Internal energy is the total of all possible kinds of energy in a substance. It is often written as U, and sometimes as E. For a system that changes energy, the important value is the change in internal energy, or ΔU. ΔU is the difference between the final internal energy and the initial internal energy. The exact values of U final and U initial are usually not known, but the first law of thermodynamics only requires ΔU. A positive ΔU means the system gained energy from the surroundings. A...

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Pressure-Volume Work in Chemistry

Pressure-volume work shows how gas volume changes can transfer energy in chemistry. As a system changes, its internal energy may change too. Energy can move between the system and the surroundings through heat and work. Internal energy, heat, and work are related by an energy equation. Heat depends on a measured temperature change. Work depends on a measured change in volume called pressure-volume work. Work can be defined as a force acting through a distance. Pressure-volume work, also...

enthalpy

Enthalpy and Heat at Constant Pressure

Enthalpy describes heat changes in chemical and physical processes. Chemists use the symbol H for enthalpy. It is defined as a system’s internal energy, E, plus the product of pressure, P, and volume, V. Enthalpy is a state function, so only changes in enthalpy can be measured, not the enthalpy of a substance by itself. For many reactions, the pressure stays constant. Under those conditions, the enthalpy change, ΔH, can be written in a way that includes pressure-volume work. The term PΔV...

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property. The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a...

constant pressure calorimetry

Coffee Cup Calorimeter Heat Changes

Coffee cup calorimetry measures heat changes in reactions and physical changes using an insulated calorimeter. The calorimeter is calibrated and designed to limit heat exchange with the outside environment. That makes it possible to measure the heat taken in or released by a substance more accurately, including the energy content of foods. In calorimetry, the system is the substance or substances undergoing change. The surroundings are everything else inside the calorimeter, including the...

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standard enthalpy of formation

Using Standard Enthalpy Values in Chemistry

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enthalpies of reaction

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

key terms and concepts

Learn how ΔHrxn shows heat released or absorbed in reactions, why equation coefficients matter, and how physical state changes the enthalpy value.

Heat Changes in Chemical Equations | JoVE K12

chemistry

JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

Matter and its Interactions

PS.Chm.4.1

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

ohios-learning-standards-and-model-curriculum

Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Tennessee Academic Standards

CHEM1.PS3.3

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

CH.7.c

CHEM.13

CHEM.13.A

CHEM.13.C

Florida Department of Education

SC.912.P.10

SC.912.P.10.7

Georgia Standards of Excellence

SC3.

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Heat Changes in Chemical Equations

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