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The transition state in a reaction is the point at which the original bonds have stretched to their limit. Transition states are only in existence for extremely brief (10^-15 s) periods of time. The energy required to reach the transition state is equal to the activation for that reaction. Multi-stage reactions involve a number of transition points, here the activation energy is equal to the one requiring the most energy. After this time either the molecules move apart again with original bonds reforming, or the bonds break and new products form. This is possible because both possibilities result in the release of energy (shown on the enthalpy profile diagram, Fig. 1, as both positions lie below the transition state). A substance that modifies the transition state to lower the activation energy is termed a catalyst; a biological catalyst is termed an enzyme.

Fig 1. The enthalpy profile of a reaction between two hydrogen peroxideProperties General Name Hydrogen peroxide Chemical formula H O Appearance Colourless liquid Physical Formula weight 34. 0 amu Melting point 272. 4 °C) Boiling point 423 K (150 °C) Density 1. 4 ×103 kg/ m3 Solubility miscible Thermochemistry ΔH0 -136 molecules to form water and oxygen. The Transition State is the point at which the original bonds have been stretched as far as they can without breaking (this is illustrated by the structural diagram below the graph.) The energy required to reach the transition state is equal to the activation energy of the reaction.

At low temperatureTemperature is the physical property of a system which underlies the common notions of "hot" and "cold"; the material with the higher temperature is said to be hotter. General description The formal properties of temperature are studied in thermodynamics.s for a particular reaction most, but not all, molecules will not have enough energy to react. However there will nearly always be a certain number with enough energy at any temperature, because temperature is a measure of the average energy of the system - individual molecules can have more or less energy than the average. Increasing the temperature increases the proportion of molecules with more energy than the activation energy, and consequently the rate of reaction increases. Typically the activation energy is given as the energy in kilojoulesThe joule (symbol J also called newton metre or coulomb volt is the SI unit of energy and work. The unit is pronounced to rhyme with "tool", and is named in honour of the physicist James Prescott Joule (1818-1889). 1 joule 1 N · 1 m 1 newton · 1 metre 1 k needed for one moleThe mole (symbol: mol) is one of the seven SI base units and is commonly used in chemistry. It measures the amount of substance of a system and is defined as the amount of substance that contains as many elementary entities as there are atoms in exactly 0 of reactants to react.

See also Energy -- Chemical reaction