Enzymology

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RATE CONSTANTS AND THE ARRHENIUS EQUATION

This page looks at the way that rate constants vary with temperature and activation energy as shown by the Arrhenius equation. [pic]
Note:  If you aren't sure what a rate constant is, you should read the page about orders of reaction before you go on. This present page is at the hard end of the rates of reaction work on this site. If you aren't reasonably confident about the basic rates of reaction work, explore the rates of reaction menu first. [pic]

The Arrhenius equation
Rate constants and rate equations
You will remember that the rate equation for a reaction between two substances A and B looks like this: [pic]
[pic]
Note:  If you don't remember this, you must read the page about orders of reaction before you go on. Use the BACK button on your browser to return to this page. [pic]

The rate equation shows the effect of changing the concentrations of the reactants on the rate of the reaction. What about all the other things (like temperature and catalysts, for example) which also change rates of reaction? Where do these fit into this equation? These are all included in the so-called rate constant - which is only actually constant if all you are changing is the concentration of the reactants. If you change the temperature or the catalyst, for example, the rate constant changes. This is shown mathematically in the Arrhenius equation.

The Arrhenius equation
[pic]
What the various symbols mean
Starting with the easy ones . . .
Temperature, T
To fit into the equation, this has to be meaured in kelvin. The gas constant, R
This is a constant which comes from an equation, pV=nRT, which relates the pressure, volume and temperature of a particular number of moles of gas. It turns up in all sorts of unlikely places! Activation energy, EA

This is the minimum energy needed for the reaction to occur. To fit this into the equation, it has to be expressed in joules per mole - not in kJ mol-1.
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