Name: heat transfer and specific heat
Uploaded: 2025-06-17T07:55:58.000Z
Duration: 2 min 46 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Energy

PS.Chm.7.2

Quantifying Heat

L'énergie interne est définie par l'interconversion de la chaleur et du travail. Mais comment la chaleur est-elle mesurée? Chaque substance possède de l'énergie thermique.Lorsqu'un système subit une différence de température avec son environnement, l'énergie thermique passe de la matière de température plus élevée à la matière de température plus basse. Ce transfert d'énergie thermique est également appelé transfert de chaleur et est mesuré en Joules. La température, d'autre part, décrit une propriété de l'énergie thermique et est mesurée en Celsius ou Kelvin.Ainsi, la température est indépendante de la substance, contrairement à la chaleur, qui dépend du nombre et du type de molécules dans un système. La chaleur peut être mesurée par une technique appelée la calorimétrie, qui utilise la relation entre la chaleur et un changement de température d'un système. Lorsqu'un morceau de fer à 114 degrés Celsius est placé dans l'eau à température ambiante, la chaleur s'écoule du fer à l'eau jusqu'à ce qu'ils soient à la même température.Une fois que le flux net de chaleur, ils sont en équilibre thermique. Dans un système isolé, l'eau gagne exactement autant de chaleur que le fer perd. Mais la température finale est seulement un degré au-dessus de la température de départ de l'eau, tandis que le fer a refroidi de 93 degrés Celsius.La quantité de chaleur transférée est proportionnelle au changement de température delta T.La constante de proportionnalité C ou la"capacité calorifique"est la quantité de chaleur nécessaire pour augmenter la température d'un système d'un kelvin, ou d'un degré Celsius. La capacité calorifique dépend du type de la substance:un système avec une grande capacité calorifique comme l'eau doit absorber plus de chaleur pour augmenter sa température qu'un système avec une plus petite capacité calorifique, comme le fer. La capacité thermique dépend également de la masse de la substance.Capacité calorifique spécifique, Cs, ou capacité calorifique molaire, Cm, décrit la quantité de chaleur nécessaire pour augmenter la température de 1 gramme, ou 1 mole d'une substance par 1 degré Celsius. Ainsi, l'unité est donnée en Joules par gramme de degré Celsius, ou en Joules par mole de degré Celsius. Ainsi, si les capacités calorifiques spécifiques et le changement de température d'un système sont connus, la valeur de la chaleur peut être calculée à partir de leur produit.

quantification de la chaleur

<img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-02.png" style="height: 40px;" />
 
 La plus grande poêle en fonte, bien qu'elle soit faite de la même substance, a besoin de 90 700 J d'énergie (q) pour que sa température augmente de 50,0 °C (&DeltaT). La plus grande poêle a une capacité thermique (proportionnellement) plus grande car la plus grande quantité de matériau nécessite une quantité d'énergie (proportionnellement) plus grande pour générer le même changement de température : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-03.png" style="height: 40px;" />
 
 La capacité thermique massique (c) d’une substance, qu'on appelle couramment “ chaleur spécifique ”, est la quantité de chaleur nécessaire pour augmenter la température de 1 gramme d’une substance de 1 degré Celsius (ou 1 Kelvin) :  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-04.png" style="height: 40px;" />
 
 La capacité thermique massique dépend uniquement du type de substance absorbant ou dégageant de la chaleur. C'est une propriété intensive, donc elle ne dépend pas de la quantité de la substance. Par exemple, la petite poêle en fonte a une masse de 808 g. La chaleur spécifique du fer (le matériau utilisé pour fabriquer la casserole) est donc : 
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-05.png" style="height: 40px;" />
 
 La grande poêle à frire a une masse de 4040 g. À l'aide des données pour cette poêle, nous pouvons également calculer la chaleur spécifique du fer :  
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-06.png" style="height: 40px;" />
 
 Bien que la grande poêle soit plus massive que la petite poêle, puisqu'elles sont toutes les deux fabriquées avec le même matériau, elles génèrent toutes les deux la même valeur pour la chaleur spécifique (pour le matériau de construction, le fer). Notez que la chaleur spécifique est mesurée en unités d'énergie par température par masse et qu'elle est une propriété intensive, dérivée d'un rapport de deux propriétés extensives (chaleur et masse). La capacité thermique molaire, qui est également une propriété intensive, est la capacité thermique par mole d'une substance particulière, et elle a les unités J/mol⋅°C. 
 
 L'eau a une chaleur spécifique relativement élevée (environ 4,2 J/g⋅°C pour le liquide et 2,09 J/g⋅°C pour le solide) ; la plupart des métaux ont des chaleurs spécifiques beaucoup plus faibles (habituellement moins de 1 J/g⋅°C). Connaissant la masse (m) d'une substance et sa chaleur spécifique (c), on peut déterminer la quantité de chaleur, q entrant ou sortant de la substance en mesurant le changement de température (ΔT) avant et après que la chaleur soit gagnée ou perdue :  
 
 <img alt="Eq7" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-07v3.png" style="height: 20px;" />
 
 <img alt="Eq8" src="https://www.jovecore.com/CDNSource/lessons/794/794_Equations-08v4.png" style="height: 20px;" />
 
 Si une substance reçoit de l'énergie thermique, sa température augmente, sa température finale est supérieure à sa température initiale, Tfinal − Tinitial a une valeur positive et la valeur de q est positive. Si une substance perd de l'énergie thermique, sa température diminue, la température finale est inférieure à la température initiale, Tfinal − Tinitial a une valeur négative et la valeur de q est négative. 
 
 Notez que la relation entre la chaleur, la chaleur spécifique, la masse et le changement de température peut être utilisée pour déterminer l'une de ces quantités (pas seulement la chaleur) si les trois autres sont connues ou peuvent être déduites. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/5-1-energy-basics">OpenStax Chemistry 2e, Section 5.1 : Notions d’énergie</a> et <a href="https://openstax.org/books/chemistry-2e/pages/5-3-enthalpy">OpenStax Chemistry 2e, Section 5.3 : Enthalpie</a>.

Quantification de la chaleur

Énergie thermique 

Au microscope, l’énergie thermique est l’énergie cinétique associée au mouvement aléatoire des atomes et des molécules. La température est un ...

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KEY TERMS AND CONCEPTS

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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 slowly, the thermal energy is lower and the temperature drops. If no chemical reaction or phase change, such as melting or vaporizing, occurs, adding thermal energy raises temperature and removing it lowers temperature.

Heat, written as q, is the transfer of thermal energy between two bodies at different temperatures. Heat flow increases the thermal energy of one body and decreases the thermal energy of the other. It moves spontaneously from hot to cold and continues until both substances reach the same temperature. Changes in heat are tracked by changes in temperature.

The SI unit for heat, work, and energy is the joule. One joule is the energy used when a force of 1 newton moves an object 1 meter. It is also equal to 1 kg m^2/s^2, or 1 newton-meter. A kilojoule is 1000 joules, and one calorie is defined as 4.184 joules.

Heat capacity, C, tells how much heat a body absorbs or releases when its temperature changes by 1 degree C, or 1 kelvin. It depends on both the type of substance and the amount present, so it is an extensive property. A large cast iron frying pan has a heat capacity five times greater than a small one because it has five times more mass. More mass means more atoms, so more energy is needed to make all of them vibrate faster.

The small cast iron frying pan in the example takes 18,140 J of energy to rise by 50.0 °C. The larger pan needs 90,700 J to rise by the same amount. Even though both pans are made of the same material, the larger one needs proportionally more energy because it contains more matter.

Specific heat capacity, often called specific heat, is the heat needed to raise 1 gram of a substance by 1 degree Celsius, or 1 kelvin. It depends only on the kind of substance, so it is an intensive property. Using the mass and temperature change of the small pan, the specific heat of iron can be calculated. The large pan gives the same result, even though its mass is greater.

Specific heat is measured in units of energy per temperature per mass. Because it is a ratio of heat and mass, it is intensive even though those two quantities are extensive. The molar heat capacity is another intensive property. It is the heat capacity per mole of a substance and has units of J/mol·°C.

Water has a relatively high specific heat, about 4.2 J/g·°C for liquid water and 2.09 J/g·°C for solid water. Most metals have much lower specific heats, usually less than 1 J/g·°C. If the mass of a substance, its specific heat, and its temperature change are known, the amount of heat entering or leaving it can be found. A positive q means thermal energy is gained and temperature rises. A negative q means thermal energy is lost and temperature falls.

Heat Transfer and Specific Heat

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

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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 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

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

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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 heat moves between objects, why temperature changes, and how heat capacity and specific heat help measure thermal energy in materials.

Heat Transfer and Specific Heat | JoVE K12

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Modes of Heat Transfer

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PS.Chm.7.1

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First Law of Thermodynamics

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Enthalpy

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Standard Enthalpy of Formation

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PS.Chm.7.3

Understand the relationship among pressure, temperature, volume, and phase.

Heat Transfer and Specific Heat

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