The minimum amount of energy required to convert reactant molecules into
products is
(A)
Kinetic energy
(B)
Potential energy
Kinetic energy + Potential energy
(C)
Activation energy
Answers
Answer:
Every chemical reaction involves a change in free energy, called delta G (∆G).
To calculate ∆G, subtract the amount of energy lost to entropy (∆S) from the total energy change of the system; this total energy change in the system is called enthalpy (∆H ): ΔG=ΔH−TΔS.
Endergonic reactions require an input of energy; the ∆G for that reaction will be a positive value.
Exergonic reactions release free energy; the ∆G for that reaction will be a negative value.
Key Terms
exergonic reaction: A chemical reaction where the change in the Gibbs free energy is negative, indicating a spontaneous reaction
endergonic reaction: A chemical reaction in which the standard change in free energy is positive, and energy is absorbed
Gibbs free energy: The difference between the enthalpy of a system and the product of its entropy and absolute temperature
Free Energy
Since chemical reactions release energy when energy-storing bonds are broken, how is the energy associated with chemical reactions quantified and expressed? How can the energy released from one reaction be compared to that of another reaction?
A measurement of free energy is used to quantitate these energy transfers. Free energy is called Gibbs free energy (G) after Josiah Willard Gibbs, the scientist who developed the measurement. Recall that according to the second law of thermodynamics, all energy transfers involve the loss of some amount of energy in an unusable form such as heat, resulting in entropy. Gibbs free energy specifically refers to the energy associated with a chemical reaction that is available after accounting for entropy. In other words, Gibbs free energy is usable energy or energy that is available to do work.
Calculating ∆G
Every chemical reaction involves a change in free energy, called delta G (∆G). The change in free energy can be calculated for any system that undergoes a change, such as a chemical reaction. To calculate ∆G, subtract the amount of energy lost to entropy (denoted as ∆S) from the total energy change of the system. This total energy change in the system is called enthalpy and is denoted as ∆H. The formula for calculating ∆G is as follows, where the symbol T refers to absolute temperature in Kelvin (degrees Celsius + 273): G=ΔH−TΔS.
The standard free energy change of a chemical reaction is expressed as an amount of energy per mole of the reaction product (either in kilojoules or kilocalories, kJ/mol or kcal/mol; 1 kJ = 0.239 kcal) under standard pH, temperature, and pressure conditions. Standard pH, temperature, and pressure conditions are generally calculated at pH 7.0 in biological systems, 25 degrees Celsius, and 100 kilopascals (1 atm pressure), respectively. It is important to note that cellular conditions vary considerably from these standard conditions; therefore, standard calculated ∆G values for biological reactions will be different inside the cell.