Reaction Rates Can Often Be Increased by the Addition of a Catalyst

6 Reaction Rates Can Often Be Increased by the Addition of a Catalyst 

We saw in Section 14.5 that the reaction rate for the decomposition of hydrogen per-oxideJepends on thq onc traton erf iodide ions even though I does not appear in the ov ( aQ)n. insyaH I the reaction. A catalyst is a sub- 

The catalyst may react to form an intermediate, but it is regenerated in a subsequent step of the reaction. In the laboratory preparation of molecular oxygen, for example, a sample of potassium chlorate is decomposed by heating, as follows 

However, this thermal decomposition process is very slow in the absence of a catalyst. The rate of decomposition can be increased dramatically by adding a small amount of manganese dioxide (Mn02), a black powdery substance that acts as a catalyst. All the Mn02 can be recovered at the end of the reaction, just as all the 1 ions remain following the decomposition of H2O2. 

A catalyst speeds up a reaction by providing a set of elementary steps with more favorable kinetics than those that exist in its absence. From Equation 14.19, we know that the rate constant k (and hence the rate) of a reaction depends on the frequency factor A and the activation energy —the larger the A or the smaller the the greater the rate. In many cases, a catalyst increases the rate by lowering the activation energy for the reaction. 

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To ensure a strong bond between liner and insulation as well as propellant to liner, it is necessary that liner as well as propellant cure well at the interfaces. This means that in many cases the rubber insulation must undergo some treatment to remove substances which may interfere with the liner cure. Such substances are usually low molecular weight compounds and can often be removed by heating—e.g., water, which would otherwise react with isocyanate in a polyurethane liner. In addition the insulation and/or the cured liner surface may be washcoated with a cure catalyst which will increase the reaction rate of alcoholic hydroxyl groups over the rate of reaction of water with isocyanate to such an extent that the latter reaction can no longer compete with the cure reaction. 

Promoters are often needed in addition to the actual catalyst itself. In general promoters increase the rate of a desired reaction this can occur because of a general increase in reaction rates or because of an increase in selectivity towards one product by comparison to others. Most promoters tend to be specific to a particular reaction and catalyst but some promoters can accelerate different reactions. For example potassium (usually added as K2O) promotes the silver catalyzed ethylene oxidation to ethylene oxide (Section 2.4), and also promotes carbon monoxide hydrogenation over Fe catalysts (Section 4.8). 

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