Examples of reaction quotient in the following topics:
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- The reaction quotient is a measure of the relative amounts of reactants and products during a chemical reaction at a given point in time.
- The reaction quotient, Q, is a measure of the relative amounts of reactants and products during a chemical reaction at a given point in time.
- Three properties can be derived from this definition of the reaction quotient:
- As the reaction proceeds, assuming that there is no energy barrier, the species' concentrations, and hence the reaction quotient, change.
- Calculate the reaction quotient, Q, and use it to predict whether a reaction will proceed in the forward or reverse direction
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- Equilibrium constants and reaction quotients can be used to predict whether a reaction will favor the products or the reactants.
- If a reaction is not at equilibrium, you can use the reaction quotient, Q, to see where the reaction is in the pathway:
- If you know the equilibrium constant for a reaction, and you know all the concentrations, you can predict in what direction the reaction will proceed.
- Therefore, the reverse reaction is favored.
- Evaluate whether a chemical reaction has reached equilibrium from the reaction coefficient (Q) and the equilibrium constant (K), and use the latter to predict whether the reaction will favor the reactants or products
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- The thermodynamics of redox reactions can be determined using their standard reduction potentials and the Nernst equation.
- Q is the reaction quotient $\frac{C^cD^d}{A^aB^b}$.
- It can be further simplified if the reaction has reached equilibrium, as in that case Q is the equilibrium constant K:
- The relationship between the Gibbs free energy change and the standard reaction potential is:
- Translate between the equilibrium constant/reaction quotient, the standard reduction potential, and the Gibbs free energy change for a given redox reaction
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- In the late 19th century, Josiah Willard Gibbs formulated a theory to predict whether a chemical reaction would be spontaneous based on free energy:
- Here, ΔG is the change in Gibbs free energy, T is absolute temperature, R is the gas constant, and Q is the reaction quotient.
- In chemistry, a reaction quotient is a function of the activities or concentrations of the chemical species involved in a chemical reaction.
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- The equilibrium constants for reactions that contain substances that are all in the same phase, and reactions that contain substances in different phases, need to be calculated differently.
- The former are called homogenous reactions, and the later are called heterogeneous reactions.
- The equilibrium constant K for a given reaction is defined as the ratio of the products of a reaction to the reactants, measured at equilibrium.
- In a general reaction
- The reaction quotient measured at equilibrium is the equilibrium constant K.
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- The standard cell potential for the reaction is then +0.34 V - (-0.76 V) = +1.10 V.
- In this equation, E is the cell potential, Eo is the standard cell potential (i.e., measured under standard conditions), F is Faraday's constant, R is the universal gas constant, T is the temperature in degrees Kelvin, Q is the reaction quotient (which has the same algebraic from as the equilibrium constant expression, except it applies to any time during the reaction's progress), and n is the number of moles of electrons that are transferred in the balanced chemical equation of the redox process.
- The cell potential is zero at equilibrium (E=0), and Q (the reaction quotient) can now be designated as the equilibrium constant K.
- Calculate the equilibrium constant K, from the following reaction studied at a temperature of 298K:
- Schematic of a galvanic cell for the reaction between Zn and Cu.
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- Assuming this reaction is an elementary step, we can write the rate laws for both the forward and reverse reactions:
- However, we know that the forward and reverse reaction rates are equal in equilibrium:
- Notice that the left side of the equation is the quotient of two constants, which is simply another constant.
- Predicting the Direction of a Reaction From the Value of Keq
- A Keq >>1 is indicative that the forward reaction is highly favored over the reverse reaction, and at equilibrium, the concentrations of the products are much greater than those of the reactants.
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- If the units are ignored, the quotients do not numerically equal 1, but 1/12 or 12.
- Since the two quotients are equal to 1, multiplying or dividing by the quotients is the same as multiplying or dividing by 1.
- You can also use these quotients to convert from inches to feet or from feet to inches.
- If there is confusion regarding which quotient to use in the conversion, just make sure the units cancel out correctly.
- The units behave just like numbers in products and quotients—they can be multiplied and divided.
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- Changes in temperature can affect the equilibrium state of a reversible chemical reaction.
- Reactions can be classified by their enthalpies of reaction.
- A diagram of the reaction coordinate for an exothermic reaction is shown in .
- Exothermic reactions will be shifted toward the reactants.
- Endothermic reactions, on the other hand, will be shifted towards product formation as heat is removed from the reaction's surrounding environment.
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- The reverse process is called a retro ene reaction.
- This is the same bond bookkeeping change exhibited by electrocyclic reactions, but no rings are formed or broken in an ene reaction unless it is intramolecular.
- The following examples illustrate some typical ene reactions, with equation 3 being an intramolecular ene reaction.
- Reaction 4 is drawn as a retro ene reaction, although this has not been demonstrated to be general for all reactions of allylic alcohols with thionyl chloride.
- A similar acid-catalyzed reaction of simple aldehydes with alkenes to give allylic alcohols, 1,3-diols or 1,3-dioxanes is known as the Prins reaction.