The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions like atoms colliding are involved, then the direction will always be in the direction of increased entropy.
This does not contradict the second law, however, since such a reaction must have a sufficiently large negative change in enthalpy heat energy.
The increase in temperature of the reaction surroundings results in a sufficiently large increase in entropy, such that the overall change in entropy is positive. Spontaneity does not imply that the reaction proceeds with great speed. For example, the decay of diamonds into graphite is a spontaneous process that occurs very slowly, taking millions of years.
The rate of a reaction is independent of its spontaneity, and instead depends on the chemical kinetics of the reaction. Every reactant in a spontaneous process has a tendency to form the corresponding product. This tendency is related to stability. An endergonic reaction also called a nonspontaneous reaction or an unfavorable reaction is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed.
The total amount of energy is a loss it takes more energy to start the reaction than what is gotten out of it so the total energy is a negative net result. These reactions usually do not require energy to proceed, and therefore occur spontaneously. In a chemical reaction, breaking and forming bonds between atoms is a form of energy. However, some exergonic reactions do not occur spontaneously and require a small input of energy to start the reaction.
This input of energy is called activation energy. Once the activation energy requirement is fulfilled by an outside source, the reaction proceeds to break bonds and form new bonds and energy is released as the reaction takes place.
Wikipedia has related information at Gibbs free energy. An important concept in the study of metabolism and energy is that of chemical equilibrium. Most chemical reactions are reversible. They can proceed in both directions, often transferring energy into their environment in one direction, and transferring energy in from the environment in the other direction. The same is true for the chemical reactions involved in cell metabolism, such as the breaking down and building up of proteins into and from individual amino acids, respectively.
Reactants within a closed system will undergo chemical reactions in both directions until a state of equilibrium is reached. This state of equilibrium is one of the lowest possible free energy and a state of maximal entropy. Equilibrium in a chemical reaction, is the state in which both reactants and products are present in concentrations which have no further tendency to change with time.
Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. Equilibrium means that the relative concentrations of reactants and products is not changing in time BUT it does NOT mean that there is no interconversion between substrates and products - it just means that when reactant is converted to product that product is converted to reactant at an equal rate. Either a rebalancing of substrate or product concentrations by adding or removing substrate or product or a positive change in free energy, typically by the transfer of energy from outside the reaction, is required to move a reaction out of a state of equilibrium.
In a living cell, most chemical reactions do not reach a state of equilibrium - this would require that they reach their lowest free energy state. Energy is therefore required to keep biological reactions out of their equilibrium state. In this way, living organisms are in a constant energy-requiring, uphill battle against equilibrium and entropy.
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