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

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

De meeste chemische reacties vinden plaats bij atmosferische druk. Onder omstandigheden van constante druk is de warmteverandering geassocieerd met de reactie delta Q, gelijk aan de verandering in enthalpie delta H, ook wel de enthalpie of reactiewarmte genoemd. Enthalpie van de reactie is het verschil tussen de enthalpie van de producten en de reactanten.Wanneer de enthalpie van de producten groter is dan de enthalpie van de reactanten, is delta H positief. Dergelijke reacties absorberen warmte en zijn endotherm. Als de enthalpie van de reactanten daarentegen groter is dan de enthalpie van de producten, is delta H negatief.Dergelijke reacties geven warmte af en zijn exotherm. Voor elke chemische reactie hangt de grootte van de bijbehorende enthalpie-verandering af van de stoichiometrische hoeveelheden reactanten en producten, zoals aangegeven door de coëfficiënten van de uitgebalanceerde vergelijking. Een uitgebalanceerde chemische vergelijking die faselabels en reactie-enthalpie, delta H, omvat, wordt een thermochemische vergelijking genoemd.Denk aan de verbranding van methaan een primaire brandstofbron. Bij verbranding van methaan komt warmte vrij aan de omgeving. De exotherme aard van de reactie wordt aangegeven door de negatieve enthalpie-verandering in de thermochemische vergelijking.De vergelijking voor verbranding laat zien dat wanneer een mol methaangas reageert met 2 mol zuurstofgas om 1 mol koolstofdioxidegas en 2 mol vloeibaar water op te leveren, 890, 8 kilojoule warmte worden vrijgegeven aan de omgeving. De molaire verhouding tussen de reactanten of producten en de reactiewarmte kan worden gebruikt als conversiefactoren om de warmte-uitwisseling tijdens de reactie te berekenen. Als een gasfles 25, 5 kilogram methaan bevat en al het methaan in de cilinder verbrandt, welke hoeveelheid warmte wordt er dan geproduceerd?In het algemeen is het conceptuele plan om de massa om te zetten naar molen en vervolgens molen naar de reactiewarmte. Om te beginnen wordt 25, 5 kilogram vermenigvuldigd met 1000 om de massa in grammen te vinden. Vervolgens wordt de massa van methaan gedeeld door de molaire massa 16, 0 gram per mol, om 1594 mol methaan op te leveren.Ten slotte, gebruikmakend van de conversiefactor tussen molen methaan en reactiewarmte, geeft 1594 molen methaan 14, 2 keer tien vrij voor de 6 kilojoule warmte de warmte van de reactie. Het antwoord is negatief, omdat de reactie exotherm is aangezien er warmte vrijkomt in de reactie.

thermochemische vergelijkingen

Voor een chemische reactie (het systeem) die wordt uitgevoerd bij constante druk – waarbij de enige verrichte arbeid wordt veroorzaakt door uitzetting of samentrekking – is de reactie-enthalpie (ook wel de reactiewarmte genoemd, ΔHrxn) gelijk aan de warmte die wordt uitgewisseld met de omgeving (qp ). <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;"> De verandering in enthalpie is een uitgebreide eigenschap en hangt af van de hoeveelheden reactanten die aan de reactie deelnemen (of het aantal mol reactanten). De verandering in enthalpie is specifiek voor de reactie en de fysieke toestand van de reactant en de productsoort is belangrijk. Een exotherme reactie wordt gekenmerkt door een −ΔHrxn-waarde, terwijl een endotherme reactie een +ΔHrxn-waarde heeft. Omdat de hoeveelheid warmte die vrijkomt of wordt geabsorbeerd door een reactie overeenkomt met de hoeveelheid van elke stof die door de reactie wordt verbruikt of geproduceerd, is het handig om een thermochemische vergelijking te gebruiken om de veranderingen in zowel materie als energie weer te geven. In een thermochemische vergelijking wordt de verandering in enthalpie van een reactie weergegeven als ΔHrxn, en deze wordt doorgaans weergegeven volgens de vergelijking voor de reactie. De grootte van ΔHrxn geeft de hoeveelheid warmte aan die gepaard gaat met de reactie die wordt weergegeven in de chemische vergelijking. Het teken van ΔHrxn geeft aan of de reactie exotherm of endotherm is, zoals geschreven. In de volgende vergelijking reageren 1 mol waterstofgas en 1/2 mol zuurstofgas (bij een bepaalde temperatuur en druk) om 1 mol vloeibaar water te vormen (bij dezelfde temperatuur en druk). <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;"> Deze vergelijking geeft aan dat 286 kJ warmte wordt afgegeven aan de omgeving. Met andere woorden: voor elke mol waterstof die wordt verbruikt of voor elke mol water die wordt geproduceerd, komt 286 kJ warmte vrij (de reactie is exotherm). Daarom is de reactie-enthalpie een conversiefactor die kan worden gebruikt om de hoeveelheid warmte te berekenen die vrijkomt of wordt geabsorbeerd tijdens reacties waarbij specifieke hoeveelheden reactanten en producten betrokken zijn. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;"> Als de coëfficiënten van de chemische vergelijking met een bepaalde factor worden vermenigvuldigd (dat wil zeggen als de hoeveelheid van een stof wordt veranderd), moet de verandering in enthalpie met diezelfde factor worden vermenigvuldigd. (tweevoudige toename van de hoeveelheden) <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;"> (tweevoudige afname van de hoeveelheden) <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;"> Om te illustreren dat de enthalpieverandering van een reactie afhangt van de fysieke toestand van de reactanten en producten, beschouwen we de vorming van gasvormig water (of waterdamp). Wanneer 1 mol waterstofgas en ½ mol zuurstofgas reageren en 1 mol gasvormig water vormen, komt er slechts 242 kJ warmte vrij, in tegenstelling tot 286 kJ warmte die vrijkomt bij de vorming van vloeibaar water. <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;">

Thermochemische vergelijkingen

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

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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...

first law of thermodynamics

Energy Conservation and Internal Energy

Energy conservation describes how energy changes form during chemical and physical changes. Energy can be converted from one form into another, but the energy present before a change still exists in some form after the change is complete. This is the law of conservation of energy. It says energy can neither be created nor destroyed. The law of conservation of matter also applies during chemical change. In most chemical reactions, the total amount of matter does not show a detectable change.

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...

quantifying heat

Heat Transfer and Specific Heat

Heat transfer and specific heat explain how thermal energy moves and how materials respond to it. Thermal energy is the kinetic energy linked to the random motion of atoms and molecules. Temperature is a measure of how hot or cold a substance is, and it depends on how much thermal energy those particles have. When atoms and molecules move or vibrate faster, their average kinetic energy is higher. That means the object has more thermal energy and a higher temperature. When they move more...

quantifying work

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...

thermochemical 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...

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...

constant volume calorimetry

Bomb Calorimeter Heat Measurement

Bomb calorimetry measures the heat released by reactions that produce a lot of energy, especially combustion reactions. A coffee cup calorimeter works at constant pressure and is useful for reactions in solution. A bomb calorimeter works at constant volume and is used for reactions that make large amounts of heat and gaseous products. The word "bomb" refers to the strong, explosive force some reactions can have. In a bomb calorimeter, the volume does not change, so 94V = 0. That means the...

hess's law

Calculating Reaction Heat with Hess's Law

Calculating reaction heat with Hess's law lets chemists find enthalpy changes without measuring every reaction directly. Some reactions are hard or impossible to test in the lab. Even when a reaction can be measured, it is often easier to calculate the heat change from other known enthalpy values. Hess's law says that if a process can be written as the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the steps. This works...

standard enthalpy of formation

Using Standard Enthalpy Values in Chemistry

Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature. Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in...

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

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Heat Changes in Chemical Equations

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