REACTION KINETICS: The Discovery and Applications of Sonochemistry.
INTRODUCTION
Unlike industrial chemists, who mostly want to accelerate chemical reactions, curators of the world’s great libraries are desperate to stop reactions. If they fail, they will be the keepers of a growing collection of crumbling paper.
Millions of books, documents, drawings and photographs are already falling to pieces. Many rare and valuable publications are close to total disintegration as their paper becomes brittle and starts to crumble. This applies not just to publications of many centuries ago, but to those as recent as the mid-19th century. What is the cause, and is it preventable? Untreated paper is too porous to print on. Ink would seep into the pores to produce fuzzy print. So it has to be ‘sized’, a process that fills in the pores with a compound that leaves the paper with a smooth surface. Since 1850, the bulk of book paper has been sized with aluminium sulphate – and this is the culprit.
Aluminium ions in the paper react with moisture to produce hydrogen ions. These hydrogen ions break down the cellulose molecules that the paper is composed of, and so it becomes brittle and eventually crumbles. Neutralising the acidity caused by hydrogen ions will halt this slow decomposition, and there are ways of doing this that do not damage the books. However, the cost can be prohibitive sometimes over 1000 pounds per book.
Paper manufacturers are helping to prevent this problem in future by producing more ‘acid-free’ paper treated with sizing agents that do not produce hydrogen ions. Papers treated with these should have a life expectancy of well over 200 years – nearly ten times that of the crumbling paper.
The rate of the reaction that destroys these millions of publications is certainly slow, but already too many important and irreplaceable books have been damaged. Understanding reaction kinetics has helped to save some of the world’s most treasured volumes, but for others it is too late.
WHY BOTHER WITH REACTION KINETICS?
Reaction kinetics is the study of rates of reaction. In this segment, I’ll discuss the factors that affect the rate of a chemical reaction. I will look into reaction rates in more details and consider how collision theory could explain why some factors, such as concentration and temperature, could accelerate or reduce rates of reaction. A reaction is described as spontaneous if it tends to occur. A very important spontaneous reaction is that between petrol and air in a car engine. Petrol and air do not react in a petrol tank at 25 °C because the reaction rate is so slow that it cannot be measured. But petrol explodes in air when the energy from a spark is added, as in the internal combustion engine. Being spontaneous does not mean that a reaction is necessarily fast.
For a reaction to occur spontaneously, there must be an overall increase in entropy. This is the Second law of thermodynamics. This information can be used to predict whether a reaction is spontaneous, but it cannot predict how fast a reaction will be.
The reaction between nitrogen and hydrogen is another spontaneous one, yet nothing seems to happen at room temperature. However, under the conditions of the Haber process, ammonia is produced – one of the world’s most important industrial chemical reactions. Industrial chemists need to know just how fast they can make a reaction proceed. A reaction that is too slow unlikely to be a commercial proposition.
With a knowledge of the kinetics of a reaction, you can also understand how the reaction takes place and which species are involved in each step. The series of steps involved in a reaction is called the reaction mechanism. When the reaction mechanism is known, then ways to alter the rate of reaction by changing the conditions can be determined. This information is, of course, essential to industrial chemists seeking cheaper ways to bring about higher yields. Also important is that information about the reaction mechanism of a drug in the body can tell doctors how long the drug will remain effective before another dose is required. This often helps pharmaceutical companies to produce drugs with fewer side effects from unwanted reactions
RATES OF REACTION AND THEIR MEASUREMENT
This is a definition for the rate of a reaction:
The rate of a reaction is the change in the concentration of product formed per unit of time, or the change in the concentration of reactant used per unit of time.
The reaction rate for a reactant that is consumed is negative. The concentration of a reactant changes during the course of a typical chemical reaction. Note that concentration is denoted by the use of square brackets. So, the concentration of a reactant is shown as [Reactant]. However, in any given reaction, the decrease in the concentration of the reactant is not necessarily the same as the increase in concentration of the product. For example, if 2 moles of reactant produce 1 mole of product, then the initial concentration of the reactant would be twice that of the final concentration of the product.
FINDING THE RATE OF REACTION AT A PARTICULAR TIME
If the rate of reaction were constant, and is shown on a graph, the graph would be a straight line. Clearly, it is changing with time and will eventually fall to zero when the reaction is finished. The graph will show the change of concentration during the course of the reaction and is therefore known as a rate curve. The rate of reaction at any instant in time is given by the gradient. The gradient at any point on a curve is found by drawing the tangent to the curve at that point and taking its gradient. The tangent at time zero is called the initial rate, which occurs when the reactants are first mixed. The gradient of this tangent is the steepest of any taken along the rate curve, which means that the reaction is fastest at the start. The reaction rate at any time can be calculated by dividing the amount of product in moles by the time taken in seconds.
Progress (Rate) Curve
en:User:Poccil, public domain.
MEASURING REACTION RATES
When the rate of a reaction is being studied, it is crucial to know what the reactants are, what the products are and what state these substances are in. This is conveniently found in the stoichiometric equation. This is a balanced chemical equation which states the amount of each reactant that reacts and the amount of each product formed. For example, the stoichiometric equation for the formation of nitrogen dioxide, an atmospheric pollutant, from nitrogen monoxide and oxygen in a car exhaust is:
2NO(g) + O2(g) →2NO2(g)
Although this does not detail the steps by which the reaction occurs, it clearly states that only one product is formed and that only two reactants are involved. It also states that three volumes of reactants produce two volumes of product.
Provided we know the stoichiometric equation, we can decide on how to measure the concentration changes that occur as the reaction proceeds. In the above case there is a reduction in volume, which we could use to measure changes in the rate of reaction with time. If the reaction is carried out at a constant volume in a pressure vessel, then change in pressure could be measured.
Anything that changes during a reaction and can be measured may be used to determine a reaction rate, provided that it is proportional in some way to the concentration of a particular reactant or product.
Measuring change in volume of gas produced
The reaction between dilute hydrochloric acid and magnesium ribbon produces hydrogen according to this stoichiometric equation:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
The volume of hydrogen produced could be used to follow (monitor) changes in the rate of this reaction.
Changes in colour
A colorimeter can be used to measure the change in colour of a reaction. This instrument measures the amount of electromagnetic radiation absorbed by substances in the visible part of the spectrum. In the reaction between zinc and aqueous copper(II) sulphate, for example, the blue coloration of copper(II) sulphate disappears as the colourless zinc sulphate solution is formed.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
blue solution → colourless solution
Changes in electrical conductivity
Many reactions involve a change of conductivity because the number of ions in the reaction mixture changes during the reaction. In the following reaction, the number of ions decreases as the products are formed and the conductivity of the reaction can be followed using a conductivity cell:
BrO3-(aq) + 6I-(aq) + 6H+ (aq) → 3I2(s) + 3H2O(l) + Br-(aq)
Chemical analysis
All the techniques so far described follow a reaction continuously and do not interfere with the progress of the reaction. Chemical analysis, however, involves taking samples of the reaction mixture at regular intervals of time. The reaction in the sample is stopped as soon as it is withdrawn. This can be accomplished by rapid cooling, by removing one of the reactants or the catalyst, or by diluting the reaction mixture. The process of stopping a reaction (or slowing it down to a rate of almost zero) is called quenching.
Methyl ethanoate, CH3COOCH3, an ester, is an important industrial solvent. It can be hydrolyzed using aqueous sodium hydroxide:
CH3COOCH3(l) + NaOH(aq) → CH3COO-Na+(aq) + CH3OH(aq)
The rate of reaction can be followed by monitoring the concentration of sodium hydroxide as it is used up during the course of the reaction.
Several identical reaction solutions are set to react. The temperature is kept constant, since changes in temperature affect the reaction rate. They are quenched by dilution with ice-cold water, at different time intervals from the start of the reaction. The concentration of sodium hydroxide that remains in each reaction solution is determined by titrating it with an acid, such as dilute hydrochloric acid.
SONOCHEMISTRY
In the 1920s it was first discovered that ultrasound – sound with a frequency above 18 kHz – produces chemical effects. The study of sonochemistry, as it is called, did not really take off until the 1980s, when reliable and inexpensive ultrasound generators became readily available. Now, there is a host of interesting and important applications of ultrasound in chemistry.
Propagation of a plane compression wave
Christophe Dang Ngoc Chan (cdang) - Own work, CC BY-SA 3.0
The figure above shows the propagation of a sound wave through a liquid. As the wave travels through the liquid, it creates alternate regions of high pressure (compression) and low pressure (rarefaction). These travelling pressure changes are exceptionally rapid when the liquid is excited by ultrasound.
When a liquid is excited by ultrasound, the rapid changes in pressure (see the figure above) produce an effect known as cavitation. As the rarefactions travel through the liquid, they pull its molecules apart to produce tiny cavities or bubbles. The compressions cause the bubbles to collapse, which releases tremendous amounts of energy. It has been estimated that the temperature near the collapse may be about 7000 K, which is the temperature at the surface of the Sun. Even higher temperatures – up to 2 × 106 K – may be generated as cavitation bubbles implode. The pressure created could make a gas as dense as a metal. However, the rate of cooling is astonishing at 1010 K per second, so overall the liquid does not become hot.
These localized energy hot spots are used to increase the rates of chemical reactions. They also produce highly reactive radicals. For example, the water molecule can be torn apart to produce H• and OH• radicals. Radicals have unpaired electrons, which makes them highly reactive. So these radicals can combine to produce hydrogen gas and hydrogen peroxide. The OH• radical is also a potent oxidizing agent, which can react with other chemicals placed in the water.
Early on, sonochemistry led to the production of catalysts that have particles so minute that they are called nanostructured catalysts. (A nanometre is 10-9 metres.) The surface area of these catalyst particles is huge. Nowadays, special combinations of metals in catalysts produced with the aid of ultrasound are making chemical processes more efficient, and it is likely that alternatives to platinum-based catalysts will be found.
Tailoring polymer molecules with ultrasound to enhance particular properties is another exciting prospect. The polymer chains are dissolved in a solvent, in which they are subjected to the awesome energy of cavitation bubbles. The chains break into smaller structures that, under the action of the bubbles, recombine to form different monomers in blocks along the chains.
Fetal Ultrasound
BruceBlaus, Own work. CC BY-SA 4.0
There is so much unexplored potential in sonochemistry. Another possibility is that some organochlorine pollutants in water supplies could be broken down by ultrasound into harmless products. In a contrasting application, tiny haemoglobin spheres have been synthesized and may make it possible to produce artificial blood.
Thanks for reading.
REFERENCES
Sonochemical reaction and synthesis
Sonochemistry: Ultrasound in Organic Chemistry
https://www.thoughtco.com/definition-of-chemical-kinetics-604907