An exothermic reaction occurs once the temperature of a device increases due to the evolution of heat. This warm is released right into the surroundings, causing an overall an adverse quantity because that the heat of reaction (\(q_rxn 0\)).

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Exothermic and endothermic reactions cause energy level differences and therefore distinctions in enthalpy (\(ΔH\)), the sum of every potential and also kinetic energies. ΔH is figured out by the system, not the surrounding atmosphere in a reaction. A device that releases heat to the surroundings, one exothermic reaction, has a negative ΔH by convention, since the enthalpy that the commodities is lower than the enthalpy that the reaction of the system.

\< \ceC(s) + O2(g) -> CO2 (g) \tagΔH = –393.5 kJ \>

\<\ce H2 (g) + 1/2 O2 (g) -> H2O(l) \tagΔH = –285.8 kJ \>

The enthalpies of this reactions are less than zero, and are therefore exothermic reactions. A system of reactants that absorbs heat from the next site in an endothermic reaction has actually a positive \(ΔH\), because the enthalpy the the commodities is higher than the enthalpy that the reactants of the system.

\< \ceN2(g) + O2(g) -> 2NO(g) \tagΔH = +180.5 kJ > 0\>

\< \ce C(s) + 2S(s) -> CS2(l) \tagΔH = +92.0 kJ > 0\>

Because the enthalpies of these reactions room greater than zero, they space endothermic reactions.

The Equilibrium Constant

The equilibrium constant (\(K_c\)) specifies the relationship amongst the concentration of yellowcomic.comical substances connected in a reaction at equilibrium. The Le Chatelier"s principle states that if a stress, together as transforming temperature, pressure, or concentration, is inflicted on an equilibrium reaction, the reaction will change to regain the equilibrium. For exothermic and endothermic reactions, this included stress is a adjust in temperature. The equilibrium continuous shows how much the reaction will progression at a specific temperature by identify the ratio of commodities to reactions making use of equilibrium concentrations.

The equilibrium expression for the adhering to equation


is given below:

\< K_c = \dfrac^c^d^a^b \labelEquation:Kc\>


Kc is the equilibrium constant (for concentrations) , , , room concentrations a, b, c, and also d space the stoichiometric coefficients the the well balanced equation Exothermic reaction Endothermic reactions
If Kc to reduce with rise in temperature, the reaction shifts to the left. If Kc boosts with rise in temperature, the reaction to move to the right.
If Kc increases with a decreases in temperature, the reaction to move to the right. If Kc decreases through a decrease in temperature, the reaction to shifts to the left.

If the assets dominate in a reaction, the value for K is greater than 1. The bigger the K value, the much more the reaction will have tendency toward the right and thus come completion.

If K=1, no the reactants nor the assets are favored. Keep in mind that this is no the exact same as both being favored. If the reactants overcome in a reaction, climate K

Example \(\PageIndex1\) : The Haber Process

Suppose the the adhering to reaction is at equilibrium and also that the concentration the N2 is 2 M, the concentration that H2 is 4 M, and the concentration of NH3 is 3 M. What is the worth of Kc?

\<\ce N2 + 3H2 2NH3 \nonumber\>

The coefficients and also the concentrations room plugged into the \(K_c\) expression (Equation \refEquation:Kc) to calculation its value.

\<\beginalign* K_c &= \dfrac^2^1^3 \\<4pt> &= \dfrac<3>^2<2>^1<4>^3 \\<4pt> &= \dfrac9128 \\<4pt> &= 0.07 \endalign*\>

Exercise \(\PageIndex1\)

Determine \(K_c\) because that the adhering to yellowcomic.comical reaction in ~ equilibrium if the molar concentration of the molecules are:

0.20 M \(\ceH2\), 0.10 M \(\ceNO\), 0.20 M \(\ceH2O\), and also 0.10M\(\ceN2\).

\<\ce2H2 (g) + 2NO (g) 2H2O (g) + N2 (g) \nonumber\>


Using the \(K_c\) expression (Equation \refEquation:Kc) and plugging in the concentration worths of each molecule:

\< \beginalign* K_c &= \dfrac^c^d^a^b \\<4pt> &=\dfrac<\ceH2O>^2<\ceN2>^1<\ceH2>^2<\ceNO>^2 \\<4pt> &=\dfrac0.20^2\, 0.10.20^2 \, 0.10 ^2 \\<4pt> &= 10\endalign*\>

Exercise \(\PageIndex2\)

For the ahead equation, does the equilibrium donate the products or the reactants?


Because\(K_c= 10 > 1\), the reaction favors the products.

Exercise \(\PageIndex3\)

In the following reaction, the temperature is increased and the \(K_ c\) worth decreases from 0.75 come 0.55. Is this an exothermic or endothermic reaction?

\<\ceN_2 (g) + 3H_2 2NH_3 (g) \nonumber\>


Because the K worth decreases with an increase in temperature, the reaction is one exothermic reaction.

Exercise \(\PageIndex4\)

In the complying with reaction, in i m sorry direction will certainly the equilibrium transition if there is an increase in temperature and also the enthalpy that reaction is given such the \(ΔH\) is -92.5 kJ?

\<\cePCl3(g) + Cl2(g) PCl_5(g) \nonumber\>


In the initial reaction, the energy given turn off is negative and hence the reaction is exothermic. However, an increase in temperature permits the device to absorb energy and thus favor an endothermic reaction; the equilibrium will shift to the left.

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Contributors and also Attributions

Alyson Salmon, Nikita Patel (UCD), Deepak Nallur (UCD)

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