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In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation. An exothermic reaction occurs when the temperature of a system increases due to the evolution of heat. This heat is released into the surroundings, resulting in an overall negative quantity for the heat of reaction (\(q_{rxn} < 0\)). An endothermic reaction occurs when the temperature of an isolated system decreases while the surroundings of a non-isolated system gains heat. Endothermic reactions result in an overall positive heat of reaction (\(q_{rxn} > 0\)). Exothermic and endothermic reactions cause energy level differences and therefore differences in enthalpy (\(ΔH\)), the sum of all potential and kinetic energies. ΔH is determined by the system, not the surrounding environment in a reaction. A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system. \[ \ce{C(s) + O2(g) -> CO2 (g)} \tag{ΔH = –393.5 kJ} \] \[\ce{ H2 (g) + 1/2 O2 (g) -> H2O(l)} \tag{ΔH = –285.8 kJ} \] The enthalpies of these reactions are less than zero, and are therefore exothermic reactions. A system of reactants that absorbs heat from the surroundings in an endothermic reaction has a positive \(ΔH\), because the enthalpy of the products is higher than the enthalpy of the reactants of the system. \[ \ce{N2(g) + O2(g) -> 2NO(g)} \tag{ΔH = +180.5 kJ > 0}\] \[ \ce{ C(s) + 2S(s) -> CS2(l)} \tag{ΔH = +92.0 kJ > 0}\] Because the enthalpies of these reactions are greater than zero, they are endothermic reactions. The equilibrium constant (\(K_c\)) defines the relationship among the concentrations of chemical substances involved in a reaction at equilibrium. The Le Chatelier's principle states that if a stress, such as changing temperature, pressure, or concentration, is inflicted on an equilibrium reaction, the reaction will shift to restore the equilibrium. For exothermic and endothermic reactions, this added stress is a change in temperature. The equilibrium constant shows how far the reaction will progress at a specific temperature by determining the ratio of products to reactions using equilibrium concentrations. The equilibrium expression for the following equation \[aA + bB \rightleftharpoons cC + dD \] is given below: \[ K_c = \dfrac{[C]^c[D]^d}{[A]^a[B]^b} \label{Equation:Kc}\] where If the products dominate in a reaction, the value for K is greater than 1. The larger the K value, the more the reaction will tend toward the right and thus to completion.
Example \(\PageIndex{1}\) : The Haber Process Suppose that the following reaction is at equilibrium and that the concentration of N2 is 2 M, the concentration of H2 is 4 M, and the concentration of NH3 is 3 M. What is the value of Kc? \[\ce{ N2 + 3H2 <=> 2NH3} \nonumber\] The coefficients and the concentrations are plugged into the \(K_c\) expression (Equation \ref{Equation:Kc}) to calculate its value. \[\begin{align*} K_c &= \dfrac{[NH_3]^2}{[N_2]^1[H_2]^3} \\[4pt] &= \dfrac{[3]^2}{[2]^1[4]^3} \\[4pt] &= \dfrac{9}{128} \\[4pt] &= 0.07 \end{align*}\]
Exercise \(\PageIndex{1}\) Determine \(K_c\) for the following chemical reaction at equilibrium if the molar concentrations of the molecules are:
\[\ce{2H2 (g) + 2NO (g) <=> 2H2O (g) + N2 (g)} \nonumber\] AnswerUsing the \(K_c\) expression (Equation \ref{Equation:Kc}) and plugging in the concentration values of each molecule: \[ \begin{align*} K_c &= \dfrac{[C]^c[D]^d}{[A]^a[B]^b} \\[4pt] &= \dfrac{[\ce{H2O}]^2[\ce{N2}]^1}{[\ce{H2}]^2[\ce{NO}]^2} \\[4pt] &= \dfrac{0.20^2\, 0.1}{0.20^2 \, 0.10 ^2 }\\[4pt] &= 10 \end{align*}\]
Exercise \(\PageIndex{2}\) For the previous equation, does the equilibrium favor the products or the reactants? AnswerBecause \(K_c = 10 > 1\), the reaction favors the products.
Exercise \(\PageIndex{3}\) In the following reaction, the temperature is increased and the \(K_ c\) value decreases from 0.75 to 0.55. Is this an exothermic or endothermic reaction? \[\ce{N_2 (g) + 3H_2 <=>2NH_3 (g) } \nonumber\] AnswerBecause the K value decreases with an increase in temperature, the reaction is an exothermic reaction.
Exercise \(\PageIndex{4}\) In the following reaction, in which direction will the equilibrium shift if there is an increase in temperature and the enthalpy of reaction is given such that \(ΔH\) is -92.5 kJ? \[\ce{PCl3(g) + Cl2(g) <=> PCl_5(g)} \nonumber\] AnswerIn the initial reaction, the energy given off is negative and thus the reaction is exothermic. However, an increase in temperature allows the system to absorb energy and thus favor an endothermic reaction; the equilibrium will shift to the left. Contributors and Attributions
Thermometer with markings in degrees Celsius and in kelvins General informationUnit systemSIUnit oftemperatureSymbolKNamed afterWilliam Thomson, 1st Baron KelvinConversions x K in ...... corresponds to ... Celsius (x − 273.15) °C Fahrenheit (1.8 x − 459.67) °F Rankine 1.8 x °RaThe kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius.[1][2][3][4] It is named after the Belfast-born and University of Glasgow based engineer and physicist William Thomson, 1st Baron Kelvin (1824–1907). The Kelvin scale is an absolute thermodynamic temperature scale, meaning it uses absolute zero as its null (zero) point.[2][5] Historically, the Kelvin scale was developed by shifting the starting point of the much older Celsius scale down from the melting point of water to absolute zero, and its increments still closely approximate the historic definition of a degree Celsius, but since 2019 the scale has been defined by fixing the Boltzmann constant k to be exactly 1.380649×10−23 J⋅K−1.[1] Hence, one kelvin is equal to a change in the thermodynamic temperature T that results in a change of thermal energy kT by 1.380649×10−23 J. The temperature in degree Celsius is now defined as the temperature in kelvins minus 273.15,[2] meaning that a change or difference in temperature has the same value when expressed in degrees Celsius as in kelvins, and that 0 °C is equal to 273.15 K. The kelvin is the primary unit of temperature for engineering and the physical sciences, while in most countries Celsius remains the dominant scale outside of these fields. In the United States, outside of the physical sciences the Fahrenheit scale predominates, with the kelvin or Rankine scale employed for absolute temperature. Those are defined using the kelvin[6][7]. The kelvin is never referred to nor written as a degree. The word kelvin is not capitalised, but is pluralised as appropriate. The unit symbol K is capitalized. For example, "It is 50 degrees Fahrenheit outside" vs "It is 10 degrees Celsius outside" vs "It is 283 kelvins outside".[8] HistoryPrecursorsDuring the 18th century, multiple temperature scales were developed,[9] notably Fahrenheit and centigrade (later Celsius). These scales predated much of the modern science of thermodynamics, including atomic theory and the kinetic theory of gases which underpin the concept of absolute zero. Instead, they chose defining points within the range of human experience that could be reproduced easily and with reasonable accuracy, but lacked any deep significance in thermal physics. In the case of the Celsius scale (and the long since defunct Newton scale and Réaumur scale) the melting point of water served as such a starting point, with Celsius being defined, from the 1740s up until the 1940s, by calibrating a thermometer such that:
This definition assumes pure water at a specific pressure chosen to approximate the natural air pressure at sea level. Thus an increment of 1 °C equals 1/100 of the temperature difference between the melting and boiling points. This temperature interval would go on to become the template for the kelvin.[citation needed] Lord KelvinIn 1848, William Thomson, who was later ennobled as Lord Kelvin, published a paper On an Absolute Thermometric Scale.[10][11][12] Using the soon-to-be-defunct caloric theory, he proposed an "absolute" scale based on the following parameters:
"The arbitrary points which coincide on the two scales are 0° and 100°"
"The characteristic property of the scale which I now propose is, that all degrees have the same value; that is, that a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T − 1)°, would give out the same mechanical effect, whatever be the number T. This may justly be termed an absolute scale, since its characteristic is quite independent of the physical properties of any specific substance." As Carnot's theorem is understood in modern thermodynamics to simply describe the maximum efficiency with which thermal energy can be converted to mechanical energy and the predicted maximum efficiency is a function of the ratio between the absolute temperatures of the heat source and heat sink:
It follows that increments of equal numbers of degrees on this scale must always represent equal proportional increases in absolute temperature. The numerical value of an absolute temperature, T, on the 1848 scale is related to the absolute temperature of the melting point of water, Tmpw, and the absolute temperature of the boiling point of water, Tbpw, by:
On this scale, an increase of 222 degrees always means an approximate doubling of absolute temperature regardless of the starting temperature. In a footnote Thomson calculated that "infinite cold" (absolute zero, which would have a numerical value of negative infinity on this scale) was equivalent to −273 °C using the air thermometers of the time. This value of "−273" was the negative reciprocal of 0.00366—the accepted coefficient of thermal expansion of an ideal gas per degree Celsius relative to the ice point, giving a remarkable consistency to the currently accepted value.[citation needed] Within a decade, Thomson had abandoned caloric theory and superseded the 1848 scale with a new one[11][13] based on the 2 features that would characterise all future versions of the kelvin scale:
In 1892 Thomson was awarded the noble title 1st Baron Kelvin of Largs, or more succinctly Lord Kelvin. This name was a reference to the River Kelvin which flows through the grounds of Glasgow University. In the early decades of the 20th century, the Kelvin scale was often called the "absolute Celsius" scale, indicating Celsius degrees counted from absolute zero rather than the freezing point of water, and using the same symbol for regular Celsius degrees, °C.[14] Triple point standardIn 1873 William Thomson's older brother James coined the term triple point[15] to describe the combination of temperature and pressure at which the solid, liquid, and gas phases of a substance were capable of coexisting in thermodynamic equilibrium. While any two phases could coexist along a range of temperature-pressure combinations (e.g. the boiling point of water can be affected quite dramatically by raising or lowering the pressure), the triple point condition for a given substance can occur only at 1 pressure and only at 1 temperature. By the 1940s, the triple point of water had been experimentally measured to be about 0.6% of standard atmospheric pressure and very close to 0.01 °C per the historical definition of Celsius then in use. In 1948, the Celsius scale was recalibrated by assigning the triple point temperature of water the value of 0.01 °C exactly and allowing the melting point at standard atmospheric pressure to have an empirically determined value (and the actual melting point at ambient pressure to have a fluctuating value) close to 0 °C. This was justified on the grounds that the triple point was judged to give a more accurately reproducible reference temperature than the melting point.[16] In 1954, with absolute zero having been experimentally determined to be about −273.15 °C per the definition of °C then in use, Resolution 3 of the 10th General Conference on Weights and Measures (CGPM) introduced a new internationally standardised Kelvin scale which defined the triple point as exactly: 273.15 + 0.01 = 273.16 "degrees Kelvin"[17][18] In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K.[8][19] The 13th CGPM also held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."[4][20][21] After the 1983 redefinition of the metre, this left the kelvin, the second, and the kilogram as the only SI units not defined with reference to any other unit. In 2005, noting that the triple point could be influenced by the isotopic ratio of the hydrogen and oxygen making up a water sample and that this was "now one of the major sources of the observed variability between different realizations of the water triple point", the International Committee for Weights and Measures (CIPM), a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the kelvin would refer to water having the isotopic composition specified for Vienna Standard Mean Ocean Water.[4][22][23] 2019 redefinitionIn 2005, the CIPM began a programme to redefine the kelvin (along with the other SI units) using a more experimentally rigorous method. In particular, the committee proposed redefining the kelvin such that the Boltzmann constant takes the exact value 1.3806505×10−23 J/K.[24] The committee had hoped that the program would be completed in time for its adoption by the CGPM at its 2011 meeting, but at the 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger program.[25] The redefinition was further postponed in 2014, pending more accurate measurements of the Boltzmann constant in terms of the current definition,[26] but was finally adopted at the 26th CGPM in late 2018, with a value of k = 1.380649×10−23 J⋅K−1.[27][24][1][2][4][28] For scientific purposes, the main advantage is that this allows measurements at very low and very high temperatures to be made more accurately, as the techniques used depend on the Boltzmann constant. It also has the philosophical advantage of being independent of any particular substance. The unit J/K is equal to kg⋅m2⋅s−2⋅K−1, where the kilogram, metre and second are defined in terms of the Planck constant, the speed of light, and the duration of the caesium-133 ground-state hyperfine transition respectively.[2] Thus, this definition depends only on universal constants, and not on any physical artifacts as practiced previously. The challenge was to avoid degrading the accuracy of measurements close to the triple point. For practical purposes, the redefinition was unnoticed; water still freezes at 273.15 K (0 °C),[2][29] and the triple point of water continues to be a commonly used laboratory reference temperature. The difference is that, before the redefinition, the triple point of water was exact and the Boltzmann constant had a measured value of 1.38064903(51)×10−23 J/K, with a relative standard uncertainty of 3.7×10−7.[30] Afterward, the Boltzmann constant is exact and the uncertainty is transferred to the triple point of water, which is now 273.1600(1) K. The new definition officially came into force on 20 May 2019, the 144th anniversary of the Metre Convention.[28][1][2][4] Practical usesColour temperatureThe kelvin is often used as a measure of the colour temperature of light sources. Colour temperature is based upon the principle that a black body radiator emits light with a frequency distribution characteristic of its temperature. Black bodies at temperatures below about 4000 K appear reddish, whereas those above about 7500 K appear bluish. Colour temperature is important in the fields of image projection and photography, where a colour temperature of approximately 5600 K is required to match "daylight" film emulsions. In astronomy, the stellar classification of stars and their place on the Hertzsprung–Russell diagram are based, in part, upon their surface temperature, known as effective temperature. The photosphere of the Sun, for instance, has an effective temperature of 5778 K. Digital cameras and photographic software often use colour temperature in K in edit and setup menus. The simple guide is that higher colour temperature produces an image with enhanced white and blue hues. The reduction in colour temperature produces an image more dominated by reddish, "warmer" colours. Kelvin as a unit of noise temperatureFor electronics, the kelvin is used as an indicator of how noisy a circuit is in relation to an ultimate noise floor, i.e. the noise temperature. The so-called Johnson–Nyquist noise of discrete resistors and capacitors is a type of thermal noise derived from the Boltzmann constant and can be used to determine the noise temperature of a circuit using the Friis formulas for noise. Derived units and SI multiplesThe only SI derived unit with a special name derived from the kelvin is the degree Celsius. Like other SI units, the kelvin can also be modified by adding a metric prefix that multiplies it by a power of 10:
Unicode characterThe symbol is encoded in Unicode at code point U+212A K KELVIN SIGN. However, this is a compatibility character provided for compatibility with legacy encodings. The Unicode standard recommends using U+004B K LATIN CAPITAL LETTER K instead; that is, a normal capital K. "Three letterlike symbols have been given canonical equivalence to regular letters: U+2126 Ω OHM SIGN, U+212A K KELVIN SIGN, and U+212B Å ANGSTROM SIGN. In all three instances, the regular letter should be used."[31] See also
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Look up kelvin in Wiktionary, the free dictionary. Page 2
The 18th century lasted from January 1, 1701 (MDCCI) to December 31, 1800 (MDCCC). During the 18th century, elements of Enlightenment thinking culminated in the American, French, and Haitian Revolutions. During the century, slave trading and human trafficking expanded across the shores of the Atlantic, while declining in Russia,[1] China,[2] and Korea. Revolutions began to challenge the legitimacy of monarchical and aristocratic power structures, including the structures and beliefs that supported slavery. The Industrial Revolution began during mid-century, leading to radical changes in human society and the environment. Western historians have occasionally defined the 18th century otherwise for the purposes of their work. For example, the "short" 18th century may be defined as 1715–1789, denoting the period of time between the death of Louis XIV of France and the start of the French Revolution, with an emphasis on directly interconnected events.[3][4] To historians who expand the century to include larger historical movements, the "long" 18th century[5] may run from the Glorious Revolution of 1688 to the Battle of Waterloo in 1815[6] or even later.[7] The period is also known as the "century of lights" or the "century of reason". In continental Europe, philosophers dreamed of a brighter age. For some, this dream turned into a reality with the French Revolution of 1789, though this was later compromised by the excesses of the Reign of Terror. At first, many monarchies of Europe embraced Enlightenment ideals, but in the wake of the French Revolution they feared loss of power and formed broad coalitions to oppose the French Republic in the French Revolutionary Wars. The 18th century also marked the end of the Polish–Lithuanian Commonwealth as an independent state. Its semi-democratic government system was not robust enough to rival the neighboring states of the Prussia, Russia, and Austria, which partitioned the Polish–Lithuanian Commonwealth between themselves, changing the landscape of Central Europe and politics for the next hundred years. The Ottoman Empire experienced an unprecedented period of peace and economic expansion, taking part in no European wars from 1740 to 1768. As a consequence, the empire was not exposed to Europe's military improvements of the Seven Years' War. The Ottoman Empire military may have fallen behind and suffered several defeats against Russia in the second half of the century. In Southwest and Central Asia, Nader Shah led successful military campaigns and major invasions, which indirectly led to the founding of the Durrani Empire. The European colonization of the Americas and other parts of the world intensified and associated mass migrations of people grew in size as part of the Age of Sail. European colonization intensified in present-day Indonesia, where the Dutch East India Company established increasing levels of control over the Mataram Sultanate. Mainland Southeast Asia would be embroiled in the Konbaung-Ayutthaya Wars and the Tây Sơn rebellion, while in East Asia, the century marked the High Qing era and the continual seclusion policies of the Tokugawa shogunate. Various conflicts throughout the century, including the War of the Spanish Succession and the French and Indian War saw Great Britain triumphing over its European rivals to become the preeminent colonial power in Europe. However, Britain lost its colonies in North America after the American Revolutionary War, which went on to form the United States, initiating the decolonization of the Americas. The European colonization of Australia and New Zealand began during the late half of the century. In the Indian subcontinent, the death of Mughal emperor Aurangzeb marked the end of medieval India and the beginning of an increasing level of European influence and control in the region, which coincided with a period of rapid Maratha expansion. By the middle of the century, the British East India Company began to conquer the eastern parts of India, a process which accelerated after their victory over the Nawab of Bengal and their French allies at the Battle of Plassey.[8] By the end of the century, Company rule in India had come to cover more regions within South Asia, the British would also expand to the south, participating in the Anglo-Mysore Wars against the Kingdom of Mysore, governed by Tipu Sultan and his father Hyder Ali.[9][10] Events1701–1750
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