how to calculate heat absorbed in a reaction

Running a process in reverse produces heat flow of the same magnitude but of opposite sign as running the forward process. The change in enthalpy that occurs when a specified amount of solute dissolves in a given quantity of solvent. If the enthalpy change listed for the reaction is positive, then that reaction absorbs heat as it proceeds the reaction is endothermic . In both cases, the magnitude of the enthalpy change is the same; only the sign is different. At constant pressure, heat flow equals enthalpy change: If the enthalpy change listed for a reaction is negative, then that reaction releases heat as it proceeds the reaction is exothermic ( exo- = out). To find the heat absorbed by the solution, you can use the equation hsoln = q n. Zumdahl, Steven S., and Susan A. Zumdahl. An exothermic one releases heat to the surroundings. The change in enthalpy shows the trade-offs made in these two processes. The equation tells us that \(1 \: \text{mol}\) of methane combines with \(2 \: \text{mol}\) of oxygen to produce \(1 \: \text{mol}\) of carbon dioxide and \(2 \: \text{mol}\) of water. \[\ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ} \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right)\nonumber \]. You can use the information in the last two sections along with one simple formula to calculate the heat absorption in a specific situation. \[\ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \rightarrow \ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ}\nonumber \]. If you encounter Kelvin as a unit for temperature (symbol K), for changes in temperature this is exactly the same as Celsius, so you dont really need to do anything. When chemists are interested in heat flow during a reaction (and when the reaction is run at constant pressure), they may list an enthalpy change\r\n\r\n\r\n\r\nto the right of the reaction equation. Heat is a measure of molecular energy; the total amount of heat depends upon the number of molecules, dictated by the mass of the object. This means that when the system of gas particles expands at constant temperature, the ability of the system to expand was due to the heat energy acquired, i.e. Formula of Heat of Solution. The enthalpy of a system is determined by the energies needed to break chemical bonds and the energies needed to form chemical bonds. Optionally, check the standard enthalpy of formation table (for your chosen compounds) we listed at the very bottom. Work done by an expanding gas is called pressure-volume work, (or just \(PV\) work). In the process, \(890.4 \: \text{kJ}\) is released and so it is written as a product of the reaction. You can then email or print this heat absorbed or released calculation as required for later use. If the pressure in the vessel containing the reacting system is kept at a constant value, the measured heat of reaction also represents the change in the thermodynamic quantity called enthalpy, or . This change of thermal energy in the thermodynamic system is known as change of enthalpy or delta h written as H in chemistry and calculated using the formula H = cmT. The heat capacity of the calorimeter or of the reaction mixture may be used to calculate the amount of heat released or absorbed by the Using Calorimetry to Calculate Enthalpies of Reaction Molar enthalpy = DH/n. Hence the total internal energy change is zero. Here are the molar enthalpies for such changes:\r\n

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  Molar enthalpy of fusion:

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  Molar enthalpy of vaporization:

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\r\nThe same sorts of rules apply to enthalpy changes listed for chemical changes and physical changes. The heat flow for a reaction at constant pressure, q p, is called enthalpy, H. Heat Capacity of an object can be calculated by dividing the amount of heat energy supplied (E) by the corresponding change in temperature (T). The process is shown visually in Figure \(\PageIndex{2B}\). refers to the enthalpy change for one mole equivalent of the reaction. However, the water provides most of the heat for the reaction. Energy released should be a positive number. You must also know its specific heat, or the amount of energy required to raise one gram of the substance 1 degree Celsius. Example 1. An equation which shows both mass and heat relationships between products and reactants is called a thermochemical equation. If you seal the end of a syringe and push on the plunger, is that process isothermal? The masses of 4He and 12C are 4. Heat is another form of energy transfer, but its one that takes place when two objects are at different temperatures to each other. How to calculate specific heat Determine whether you want to warm up the sample (give it some thermal energy) or cool it down (take some thermal energy away). The mass of \(\ce{SO_2}\) is converted to moles. First, recognize that the given enthalpy change is for the reverse of the electrolysis reaction, so you must reverse its sign from 572 kJ to 572 kJ. All Your Chemistry Needs. = 30% (one significant figure). Still, isn't our enthalpy calculator a quicker way than all of this tedious computation? to the right of the reaction equation. The change in enthalpy of a reaction is a measure of the differences in enthalpy of the reactants and products. Fortunately, since enthalpy is a state function, all we have to know is the initial and final states of the reaction. Dummies has always stood for taking on complex concepts and making them easy to understand. To find enthalpy change: Use the enthalpy of product NaCl ( -411.15 kJ ). \end{matrix} \label{5.4.7} \), \( \begin{matrix} For example, stirring a cup of coffee does work in the liquid inside it, and you do work on an object when you pick it up or throw it. Example 7.7 Problem To calculate an energy change for a reaction: add together the bond energies for all the bonds in the reactants - this is the 'energy in' If the volume increases at constant pressure (\(V > 0\)), the work done by the system is negative, indicating that a system has lost energy by performing work on its surroundings. Example #4: A student wishes to determine the heat capacity of a coffee-cup calorimeter. Based on the stoichiometry of the equation, you can also say that 802 kJ of heat is released for every 2 mol of water produced. The reaction is exothermic and thus the sign of the enthalpy change is negative. Here's another practice problem on enthalpy stoichiometry (also known as thermochemical equations), this time we have a combustion reaction. The quantity of heat for a process is represented by the letter \(q\). For example, let's look at the reaction Na+ + Cl- NaCl. Therefore We have the formula, Therefore, Q = 1672 J Physics Formulas Customize your course in 30 seconds 4. Then, the reversible work that gave rise to that expansion is found using the ideal gas law for the pressure: wrev = 2V 1 V 1 nRT V dV = nRT ln(2V 1 V 1) = nRT ln2 = 1.00 mols 8.314472 J/mol K 298.15 K ln2 = 1718.28 J So, the heat flowing in to perform that expansion would be qrev = wrev = +1718.28 J Answer link If so, the reaction is endothermic and the enthalpy change is positive. Step 1: List the known quantities and plan the problem. all the heat flowing in goes into pressure-volume work and does not change the temperature. The law of conservation of energy states that in any physical or chemical process, energy is neither created nor destroyed. The heat capacity of the calorimeter or of the reaction mixture may be used to calculate the amount of heat released or absorbed by the . In this video we will learn how to calculate the internal energy of a chemical reaction (DeltaE) when the number of moles of a gas on both sides of the chemi. Second, recall that heats of reaction are proportional to the amount of substance reacting (2 mol of H₂O in this case), so the calculation is\r\n\r\n","blurb":"","authors":[{"authorId":9161,"name":"Peter J. Mikulecky","slug":"peter-j-mikulecky","description":"

Christopher Hren is a high school chemistry teacher and former track and football coach. It is a state function, depending only on the equilibrium state of a system. stoichiometric coefficient. Therefore, the term 'exothermic' means that the system loses or gives up energy. If you select the former: If you want to calculate the enthalpy change from the enthalpy formula: With Omni you can explore other interesting concepts of thermodynamics linked to enthalpy: try our entropy calculator and our Gibbs free energy calculator! 2 H 2(g) + O 2(g . Exercise \(\PageIndex{1}\): Thermite Reaction. Because the surroundings are gaining heat from the system, the temperature of the surroundings increases. But an element formed from itself means no heat change, so its enthalpy of formation will be zero. Ice absorbs heat when it melts (electrostatic interactions are broken), so liquid water must release heat when it freezes (electrostatic interactions are formed): \( \begin{matrix} When chemists are interested in heat flow during a reaction (and when the reaction is run at constant pressure), they may list an enthalpy change\r\n\r\n\r\n\r\nto the right of the reaction equation. After covering slides 17-21 from the Unit 9 Thermochemistry PowerPoint, the student will be able to practice calculating heat of reactions by using the standard heat of formation table. A thermochemical equation is a chemical equation that includes the enthalpy change of the reaction. Transcribed image text: Calculate the enthalpy of the reaction Hess's law states that "the heat released or absorbed in a chemical process is the same 2NO(g)+ O2( g) 2NO2( g) whether the process takes place in one or in several steps." It is important to recall the following given the following reactions and enthalpies of formation: rules . For an isothermal process, S = __________? acid and a base. When heat is absorbed, the change is said to be endothermic, and the numerical value of the heat is given a positive sign (q > 0). Although laymen often use the terms "heat" and "temperature" interchangeably, these terms describe different measurements. Download full answer. Our equation is: Heat Capacity = E / T. [1] "Calculating the Final Temperature of a Reaction From Specific . The reaction is highly exothermic. Subscribe 24K views 8 years ago Thermochemistry This video shows you how to calculate the heat absorbed or released by a system using its mass, specific heat capacity, and change in. Subtract its initial temperature from its final temperature. The formula for the heat of reaction is H reaction =n-m Heat of formation of reactants= (1mol of Mg) (0)+ (2mol of HCl) (-167.2kJ/mol) Heat of formation of reactants=-334.4kJ Since the heat of formation of Mg in the standard state is zero. Use this equation: q = (specific heat) x m x t; Where q is heat flow, m is mass in grams, and t is the temperature change. Endothermic reactions absorb energy from the surroundings as the reaction occurs. The sign of \(q\) for an exothermic process is negative because the system is losing heat. . Certain parts of the world, such as southern California and Saudi Arabia, are short of freshwater for drinking. Now, consider another path of the reaction. The subscript \(p\) is used here to emphasize that this equation is true only for a process that occurs at constant pressure. In practical terms for a laboratory chemist, the system is the particular chemicals being reacted, while the surroundings is the immediate vicinity within the room. The second law of thermodynamics dictates that heat only flows from hotter objects to colder ones, not the other way around. Let's practice our newly obtained knowledge using the above standard enthalpy of formation table. The enthalpy change that acompanies the melting (fusion) of 1 mol of a substance. Step 2: Write the equation for the standard heat of formation. If you put cold water in a pan, and turn on the stove, the flames heat the pan and the hot pan heats the water. The system is the specific portion of matter in a given space that is being studied during an experiment or an observation. Example 1: Calculate the heat change that occurs with ethanol combustion 7.3: Heats of Reactions and Calorimetry Calorimetry is a science where you try to find the heat transfer during a chemical reaction, phase transition, or temperature change. The enthalpy calculator has two modes. Thermochemistry Worksheet 2 (Enthalpy Changes) by. Try the plant spacing calculator. BBC GCSE Bitesize: Specific Heat Capacity, The Physics Classroom: Measuring the Quantity of Heat, Georgia State University Hyper Physics: First Law of Thermodynamics, Georgia State University Hyper Physics: Specific Heat. The overall amount of heat q = q 1 + q 1 = 11,724 J or 11.7 kJ with three significant digits. The most straightforward answer is to use the standard enthalpy of formation table! We are given H for the processthat is, the amount of energy needed to melt 1 mol (or 18.015 g) of iceso we need to calculate the number of moles of ice in the iceberg and multiply that number by H (+6.01 kJ/mol): \[ \begin{align*} moles \; H_{2}O & = 1.00\times 10^{6} \; \cancel{\text{metric ton }} \ce{H2O} \left ( \dfrac{1000 \; \cancel{kg}}{1 \; \cancel{\text{metric ton}}} \right ) \left ( \dfrac{1000 \; \cancel{g}}{1 \; \cancel{kg}} \right ) \left ( \dfrac{1 \; mol \; H_{2}O}{18.015 \; \cancel{g \; H_{2}O}} \right ) \\[5pt] & = 5.55\times 10^{10} \; mol \,\ce{H2O} \end{align*} \], B The energy needed to melt the iceberg is thus, \[ \left ( \dfrac{6.01 \; kJ}{\cancel{mol \; H_{2}O}} \right )\left ( 5.55 \times 10^{10} \; \cancel{mol \; H_{2}O} \right )= 3.34 \times 10^{11} \; kJ \nonumber \]. Whether you need help solving quadratic equations, inspiration for the upcoming science fair or the latest update on a major storm, Sciencing is here to help. One possible solution to the problem is to tow icebergs from Antarctica and then melt them as needed. When fuels burn they release heat energy and light energy to the surroundings in exothermic reactions known as combustion reactions. Example \(\PageIndex{1}\): Melting Icebergs. Solution. 002603 u and 12 u respectively. When we study energy changes in chemical reactions, the most important quantity is usually the enthalpy of reaction (\(H_{rxn}\)), the change in enthalpy that occurs during a reaction (such as the dissolution of a piece of copper in nitric acid). Please note that the amount of heat energy before and after the chemical change remains the same. Energy changes in chemical reactions are usually measured as changes in enthalpy. A calorimeter is an insulated container, and . Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at https://status.libretexts.org. Where. The thermochemical reaction is shown below. For example, it may be quoted in joules / gram degrees C, calories / gram degrees C or joules / mol degrees C. A calorie is an alternate unit of energy (1 calorie = 4.184 joules), grams are 1/1000 of a kilogram, and a mole (shortened to mol) is a unit used in chemistry. You can calculate the enthalpy change in a basic way using the enthalpy of products and reactants: H=Hproducts - Hreactants. The heat of reaction is the enthalpy change for a chemical reaction. The thermochemical reaction can also be written in this way: \[\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) \: \: \: \: \: \Delta H = -890.4 \: \text{kJ}\nonumber \]. The first law of thermodynamics states that the change in internal energy of a substance is the sum of the heat transferred to it and the work done on it (or the heat transferred to it minus the work done by it). How to calculate the enthalpy of a reaction? -571.7 kJ. A reaction that takes place in the opposite direction has the same numerical enthalpy value, but the opposite sign. PDF. The enthalpy change listed for the reaction confirms this expectation: For each mole of methane that combusts, 802 kJ of heat is released. General Chemistry: Principles & Modern Applications. To find enthalpy change: All pure elements in their standard state (e.g., oxygen gas, carbon in all forms, etc.) If a chemical reaction is carried out inside a calorimeter, the heat evolved or absorbed by the reaction can be determined. Here are the molar enthalpies for such changes:\r\n

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  Molar enthalpy of fusion:

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  Molar enthalpy of vaporization:

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\r\nThe same sorts of rules apply to enthalpy changes listed for chemical changes and physical changes. This allows you to learn about Thermodynamics and test your knowledge of Physics by answering the test questions on Thermodynamics. Specific heat = 0.004184 kJ/g C. Solved Examples. To find enthalpy: The aperture area calculator helps you to compute the aperture area of a lens. To calculate the heat absorbed we need to know how many moles of C there are. . The chemical equation for this reaction is as follows: \[ \ce{Cu(s) + 4HNO3(aq) \rightarrow Cu(NO3)2(aq) + 2H_2O(l) + 2NO2(g)} \label{5.4.1}\]. The LibreTexts libraries arePowered by NICE CXone Expertand are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. If the system gains a certain amount of energy, that energy is supplied by the surroundings. What causes energy changes in chemical reactions? The more interesting quantity is the change of enthalpy the total energy that was exchanged within a system. Let's assume the formation of water, H2O, from hydrogen gas, H2, and oxygen gas, O2. It is the thermodynamic unit of measurement used to determine the total amount of energy produced or released per mole in a reaction. The main issue with this idea is the cost of dragging the iceberg to the desired place. S surr = -H/T. Figure out . The mass of sulfur dioxide is slightly less than \(1 \: \text{mol}\). Planning out your garden? Learn to use standard heats of formation to calculate standard heats of reaction INTRODUCTION Chemical and physical changes usually involve the absorption or liberation of heat, given the symbol q. $1.50. Measure the mass of the empty container and the container filled with a solution, such as salt water. For this reason, the enthalpy change for a reaction is usually given in kilojoules per mole of a particular reactant or product. Simplify the equation. The \(89.6 \: \text{kJ}\) is slightly less than half of 198. Georgia State University: HyperPhysics -- Specific Heat. This is a quantity given the symbol c and measured in joules / kg degree Celsius. it is entirely consumed first, and the reaction ends after that point), and from there, utilize the following equation for heat flow at a constant pressure: \mathbf(Delta"H"_"rxn" = (q_"rxn")/"mols limiting reagent" = (q_"rxn")/(n . When an endothermic reaction occurs, the heat required is absorbed from the thermal energy of the solution, which decreases its temperature (Figure 1). If 4 mol of Al and 2 mol of Fe2O3 react, the change in enthalpy is 2 (851.5 kJ) = 1703 kJ. When methane gas is combusted, heat is released, making the reaction exothermic. The equation is: Here, Q means heat (what you want to know), m means mass, c means the specific heat capacity and T is the change in temperature. One way to report the heat absorbed or released would be to compile a massive set of reference tables that list the enthalpy changes for all possible chemical reactions, which would require an incredible amount of . If the products contain more heat than the reactants, they must have absorbed heat from the surroundings; so if H > 0, then H is the amount of heat absorbed by an endothermic reaction. Enthalpy is an extensive property (like mass). -H is heat of reaction. Read on to learn how to calculate enthalpy and its definition. The answer is the absorbed heat measured in joules. Whether it's to pass that big test, qualify for that big promotion or even master that cooking technique; people who rely on dummies, rely on it to learn the critical skills and relevant information necessary for success. ), Given: energy per mole of ice and mass of iceberg, Asked for: energy required to melt iceberg. As a result, the heat of a chemical reaction may be defined as the heat released into the environment or absorbed . Calculating an Object's Heat Capacity. The key to solving the problem of calculating heat absorption is the concept of specific heat capacity. Record the difference as the temperature change. The change in enthalpy that occurs during a combustion reaction. The negative sign associated with \(PV\) work done indicates that the system loses energy when the volume increases. The magnitude of H for a reaction is proportional to the amounts of the substances that react. The heat of reaction or neutralization, q neut, is the negative of the heat gained by the calorimeter which includes the 100.0 g of water. Step 1: Balance the given chemical equation. Exothermic reactions have negative enthalpy values (-H). Unless otherwise specified, all reactions in this material are assumed to take place at constant pressure. Because the heat is absorbed by the system, the \(177.8 \: \text{kJ}\) is written as a reactant. Petrucci, et al. Heats of reaction are typically measured in kilojoules. The energy released or absorbed during a chemical reaction can be calculated using the stoichiometric coefficients (mole ratio) from the balanced chemical equation and the value of the enthalpy change for the reaction (H): energy =. Enthalpy is an extensive property, determined in part by the amount of material we work with. As an example, imagine increasing the temperature of 2 kg of water from 10 degrees C to 50 degrees C. The change in temperature is T = (50 10) degrees C = 40 degrees C. From the last section, the specific heat capacity of water is 4,181 J / kg degree C, so the equation gives: Q = 2 kg 4181 J / kg degree C 40 degrees C. So it takes about 334.5 thousand joules (kJ) of heat to raise the temperature of 2 kg of water by 40 degrees C. Sometimes specific heat capacities are given in different units. Calculate heat absorption using the formula: Q = mc T Q means the heat absorbed, m is the mass of the substance absorbing heat, c is the specific heat capacity and T is the change in temperature. Enthalpy measures the total energy of a thermodynamic system either in the form of heat or volume multiplied by pressure. Calculate heat absorption using the formula: Q means the heat absorbed, m is the mass of the substance absorbing heat, c is the specific heat capacity and T is the change in temperature. - q neutralization = q cal The heat of neutralization is the heat evolved (released) when 1 mole of water is produced by the reaction of an acid and base. H = heat change. The enthalpy change that accompanies the vaporization of 1 mol of a substance. Calculate the amount of energy released or absorbed (q) q = m c g t. Mostly heat transfer takes place between the reacting system as one medium and surrounding as the other in chemical reactions. If more energy is produced in bond formation than that needed for bond breaking, the reaction is exothermic and the enthalpy is negative. We can also describe H for the reaction as 425.8 kJ/mol of Al: because 2 mol of Al are consumed in the balanced chemical equation, we divide 851.5 kJ by 2. To find the heat absorbed by the solution, you can use the equation q = m c T Here q is the heat gained by the water m is the mass of the water c is the specific heat of water T is the change in temperature, defined as the difference between the final temperature and the initial temperature of the sample How do you calculate heat absorbed by a calorimeter? We have stated that the change in energy (\(U\)) is equal to the sum of the heat produced and the work performed. Figure \(\PageIndex{1}\): An Example of Work Performed by a Reaction Carried Out at Constant Pressure. A Because enthalpy is an extensive property, the amount of energy required to melt ice depends on the amount of ice present. Then, the change in enthalpy is actually: For more particular problems, we can define the standard enthalpy of formation of a compound, denoted as HfH_\mathrm{f}\degreeHf. Subtract the mass of the empty container from the mass of the full container to determine the mass of the solution. n = number of moles of reactant. Refer again to the combustion reaction of methane. For example, let's look at the reaction Na+ + Cl- NaCl. Step 2: Calculate moles of solute (n) n = m M. Step 3: Calculate mount of energy (heat) released or absorbed per mole of solute (Hsoln) Hsoln = q n. Which factors are needed to determine the amount of heat absorbed? For a chemical reaction, the enthalpy of reaction (\(H_{rxn}\)) is the difference in enthalpy between products and reactants; the units of \(H_{rxn}\) are kilojoules per mole. Heat Absorbed During a Reaction (Example) 13,871 views Jan 22, 2014 43 Dislike Share Save LearnChemE 151K subscribers Organized by textbook: https://learncheme.com/ Calculate the amount of. Reversing a reaction or a process changes the sign of H. He is the coauthor of Biochemistry For Dummies and Organic Chemistry II For Dummies. The heat gained by the calorimeter, q Enthalpy Heat of formation Hess's law and reaction enthalpy change Worked example: Using Hess's law to calculate enthalpy of reaction Bond enthalpy and enthalpy of reaction Bond enthalpies Science > Chemistry library > Thermodynamics > Enthalpy 2023 Khan Academy Terms of use Privacy Policy Cookie Notice Heat of formation Google Classroom About H = +44 kJ. Find the solution's specific heat on a chart or use the specific heat of water, which is 4.186 joules per gram Celsius. Get the Most useful Homework explanation. The chemical equation of the reaction is: $$\ce {NaOH (s) +H+ (aq) + Cl- (aq) -> Na+ (aq) +Cl- (aq) + H2O (l)}$$ This is the ONLY information I can use and I cannot search up anything online. Plugging in the values given in the problem . The total amount of heat absorbed or evolved is measured in Joule (J). \end{matrix} \label{5.4.8} \). Each Thermodynamics tutorial includes detailed Thermodynamics formula and example of how to calculate and resolve specific Thermodynamics questions and problems. Here's a summary of the rules that apply to both:\r\n

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  The heat absorbed or released by a process is proportional to the moles of substance that undergo that process. For example, 2 mol of combusting methane release twice as much heat as 1 mol of combusting methane.

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  Running a process in reverse produces heat flow of the same magnitude but of opposite sign as running the forward process. For example, freezing 1 mol of water releases the same amount of heat that is absorbed when 1 mol of water melts.

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\r\nTry an example: here is a balanced chemical equation for the oxidation of hydrogen gas to form liquid water, along with the corresponding enthalpy change:\r\n\r\n\r\n\r\nHow much electrical energy must be expended to perform electrolysis of 3.76 mol of liquid water, converting that water into hydrogen gas and oxygen gas?\r\n\r\nFirst, recognize that the given enthalpy change is for the reverse of the electrolysis reaction, so you must reverse its sign from 572 kJ to 572 kJ. (a) If heat flows from a system to its surroundings, the enthalpy of the system decreases, Hrxn is negative, and the reaction is exothermic; it is energetically downhill.

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