MadSci Network: Chemistry

Re: In solid and liquid reaction, does more solid increase reaction rate.

Date: Thu Apr 19 23:51:22 2001
Posted By: Chris Kaiser, Process Engineer, Anvil Corporation
Area of science: Chemistry
ID: 985838217.Ch

Short answer: it depends on the reaction. But you probably want a little more detail than that.

The study of reaction rates is known as kinetics, and it can be a very complicated field. In the chemical manufacturing industry, it can also be very important. The rate of reaction (and therefore the rate of chemical production, the necessary reactant flow rates, and the amount of heat input to or removal from the reaction vessel) can be dependent on almost any variable you can think of.

All reaction rates are dependent on temperature. You have probably heard the terms exothermic and endothermic used to describe chemical reactions - they refer to reactions which create heat or absorb heat, respectively. Exothermic reactions - the ones which generate heat - need to have that heat removed. Therefore, they occur most quickly at lower temperatures. Endothermic reactions need heat input to proceed, so their reaction rates are higher at higher temperatures.

In addition to the above, some reactions (mostly those which occur in the gas phase) have rates that are dependent on pressure. Some reactions use a catalyst, which is a chemical component which participates in the reaction (like a reactant) but is not consumed by the reaction (unlike the reactants). Catalysts increase the rate of a reaction, and for reactions which use catalysts, the reaction rate is dependent upon the concentration of catalyst in the reaction vessel.

Now we get to your question: how is the rate of reaction affected by the concentration of the reactants? Unlike the general statements above regarding the effect of temperature, pressure, and catalyst concentration on reaction rate, it is impossible to make a general statement about the relationship between reaction rates and reactant concentration. Generally speaking, the reaction rate does depend on the concentration of each of the reactants. Also generally speaking, as the concentration of a reactant increases, all other things being held constant, the reaction rate increases. However, doubling the concentration of one reactant does not always double the reaction rate. Some reaction rates are dependent on the square or even the cube of the concentration of one of the reactants. Some reaction rates are completely independent of the concentration of one of the reactants. It is impossible to know how the rate of your reaction depends on the concentration of the liquid reactant or the solid reactant without knowing the identity of the two reactants.

Despite the above, you shouldn't lose hope. This is a perfect opportunity for some well-designed experiments. Try measuring your reaction rates (not sure exactly how to do this without knowing what reaction you're dealing with) while varying the concentration of one reactant and holding the other concentration constant. Repeat the experiments varying the other concentration and holding the first one constant. To obtain accurate data, you will also need to hold the temperature reasonably constant. Based on the results of these experiments, you can develop an empirical rate equation. This will allow you to predict the rate of reaction for any combination of reactant concentrations.

I hope this helps answer your question.

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