HOW FAST DOES A REACTION GO

An important distinction must be made between reactions, such as that between copper and hydrochloric acid, that will not occur under any circumstances and others that want to occur, but that are just too slow to be perceptible at moderate temperatures or in the absence of a catalyst. The latter type of reaction often does take place under the proper conditions or with a catalyst. Rates of reactions are quite important in chemistry. 

Chemical reactions may involve several kinds of processes. Reactions may consist of putting elements together to make compounds, or taking compounds apart to produce the component elements. Reactions may occur between compounds, between 

A combination reaction is one in which two reactants bond together to form a single product. An example of such a reaction is provided by the burning of elemental phosphorus as one of the steps in the manufacture of phosphoric acid, a widely used industrial chemical and fertilizer ingredient The reaction is 

A decomposition reaction is the opposite of a combination reaction. An example of a decomposition reaction in which a compound decomposes to form the elements in it is provided by the manufacture of carbon black. This material is a finely divided form of pure carbon, C, and is used as a filler in rubber tire manufacture and as an ingredient in the paste used to fill electrical dry cells. It is made by heating methane (natural gas) to temperatures in the range of 1260-1425°C in a special furnace, causing the following reaction to occur  

Decomposition reactions may also involve the breakdown of a compound to another compound and an element, or to two or more compounds. As an example of the latter, consider the reaction for the manufacture of calcium oxide, CaO, commonly called quicklime  

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In previous chapters, we considered questions like How much energy does a reaction liberate or consume and In which direction will a reaction proceed We then asked questions like To what extent will a reaction proceed in that direction, before it stops and even Why do reactions occur at all In this chapter, we look at a different question How fast does a reaction proceed Straightaway, we make assumptions. Firstly, we need to know whether the reaction under study can occur there is no point in looking at how fast it is not going if a reaction is not thermodynamically feasible So we first assume the reaction can and does occur. 

IN SUBSEQUENT CHAPTERS, WE WILL LOOK MORE CLOSELY AT THE RELATIONSHIPS AND LAWS THAT GOVERN CHEMICAL REACTIONS. HoW CAN WE PREDICT WHETHER OR NOT A REACTION WILL TAKE PLACE OnCE STARTED, HOW FAST DOES THE REACTION PROCEED HoW FAR WILL THE REACTION GO BEFORE IT STOPS ThE LAWS OF THERMODYNAMICS (DISCUSSED IN Chapter 18) help us answer the first question. Chemical... 

How can we predict whether or not a reaction will take place Once started, how fast does the reaction proceed How far will the reaction go before it stops The laws of thermodynamics (to be discussed in Chapter 18) help us answer the first question. Chemical kinetics, the subject of this chapter, provides answers to the question about the speed of a reaction. The last question is one of many answered by the study of chemical equihbrium, which we will consider in Chapters 14, 15, and 16. 

Although the basic organization of the book is unchanged from the previous edition, there is in this edition a separate chapter on Pourbaix diagrams, very useful tools that indicate the thermodynamic potential-pH domains of corrosion, passivity, and immunity to corrosion. A consideration of the relevant Pourbaix diagrams can be a useful starting point in many corrosion studies and investigations. As always in corrosion, as well as in this book, there is the dual importance of thermodynamics (In which direction does the reaction go Chapters 3 and 4) and kinetics (How fast does it go Chapter 5). 

Also, how fast or how slowly a reaction occurs has absolutely nothing to do with the position of the equilibrium for the reaction, nor does the pathway taken from reactants to products (or vice versa). The value of the activation enei or the number of reaction steps required to go from reactants to the final products also has no bearing on the position of equihbrium. The position of equilibrium is determined only by the energy difference between reactants and products, and the temperature. 

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