Chemical kinetics

3.1 Chemical reactions – Basic concepts

A chemical reaction is a process in which material changes from a beginning mass to a resulting substance. The significance of a chemical reaction is that new material or materials are produces along with the disappearance of the mass that changed to make the new. This does not mean that new elements have been made. In order to make new elements, the nuclear contents must change. There are magnitudes of difference in the amounts of energy in ordinary chemical reactions compared to nuclear reactions; the rearrangement of the nuclei of atoms to change to new elements is enormous compared to the smaller energies of chemical changes. A chemical equation is a best method to describe what goes on in a chemical reaction.

3.1.1 Examples of chemical changes

Chemical reactions, also called chemical changes, are not limited to happening in a chemistry lab. Here are some examples of chemical reactions with the corresponding chemical equations:

An iron bar rusts

The iron reacts with oxygen in the air to make rust.

Methane burns

Methane combines with oxygen in the air to make carbon dioxide and water vapor.

As a general rule, biochemical process makes poor examples of basic chemical reactions because the actual reaction is carried on within living things and under enzyme control.

3.1.2 Examples of physical changes

Here are some examples of changes that are not chemical reactions. In each case, physical processes may reclaim the original material or materials.

• Water boils out of a kettle or condenses on a cold glass
• An aluminum pot is put on a burner and gets hot
• Dry ice goes from a solid to a gaseous form of carbon dioxide (sublimation)
• Gold melts or solidifies
• Sand is mixed in with salt
• A piece of chalk is ground to dust
• Glass breaks
• An iron rod gets magnetized
• A lump of sugar dissolves in water

3.2 Classification of chemical reactions

Chemists have identified millions of different compounds and an equal number of chemical reactions to form them. When scientists are confronted with an overwhelming number of things, they tend to classify them into groups, in order to make them easier to study and understand. One popular classification scheme for chemical reactions breaks them up into five major categories or types.

3.2.1 Synthesis (Also called direct combination)

A synthesis reaction involves two or more substances combining to make a more complex substance. The reactants may be elements or compounds, and the product will always be a compound. The general formula for this type of reaction can be shown as:

A + B ---> AB

or

Element or compound + element or compound --> compound

Some examples of synthesis reactions are shown below;

2H2(g) + O2(g) ----> 2H2O(g)

C(s) + O2(g) ----> CO2(g)

CaO(s) + H2O(l) ----> Ca(OH)2(s)

3.2.2 Decomposition

In a decomposition reaction, one substance is broken down into two or more, simpler substances. This type of reaction is the opposite of a synthesis reaction, as shown by the general formula below;

AB ----> A + B

or

Compound ---> element or compound + element or compound

Some examples of decomposition reactions are shown below;

C12H22O11(s) ----> 12C(s) + 11H2O(g)

Pb(OH)2(cr) ----> PbO(cr) + H2O(g)

2Ag2O(cr) ----> 4Ag(cr) + O2(g)

3.2.3 Single displacement (Also called single replacement)

In this type of reaction, a neutral element becomes an ion as it replaces another ion in a compound. The general form of this equation can be written as;

In the case of a positive ion being replaced:

A + BC ----> B + AC

or

In the case of a negative ion being replaced:

A + BC ----> C + BA

In either case we have;

element + compound ----> element + compound

Some examples of single displacement reactions are shown below:

Zn(s) + H2SO4(aq) ----> ZnSO4(aq) + H2(g)

2Al(s) + 3CuCl2(aq) ---> 2AlCl3(aq) + 3Cu(s)

Cl2(g) + KBr(aq) ----> KCl(aq) + Br2(l)

3.2.4 Double displacement (Also called double replacement)

Like dancing couples, the compounds in this type of reaction exchange partners. The basic form for this type of reaction is shown below;

AB + CD ----> CB + AD

or

Compound + Compound ----> Compound + Compound

Some examples of double displacement reactions are shown below;

AgNO3(aq) + NaCl(aq) ----> AgCl(s) + NaNO3(aq)

ZnBr2(aq) + 2AgNO3(aq) ----> Zn(NO3)2(aq) + 2AgBr(cr)

H2SO4(aq) + 2NaOH(aq) ----> Na2SO4(aq) + 2H2O(l)

3.2.5 Combustion

When organic compounds like propane are burned, they react with the oxygen in the air to form carbon dioxide and water. The reason why these combustion reactions will stop when all available oxygen is used up is because oxygen is one of the reactants. The basic form of the combustion reaction is shown below:

hydrocarbon + oxygen ----> carbon dioxide and water

Some examples of combustion reactions are:

CH4(g) + 2O2(g) ----> 2H2O(g) + CO2(g)

2C2H6(g) + 7O2(g) ----> 6H20(g) + 4CO2(g)

C3H8(g) + 5O2(g) ----> 4H2O(g) + 3CO2(g)

3.3 Chemical reaction profile

When the reaction proceeds the reactants decrease in concentration while the products increase in concentration. Consider the following two reactions:

A + B --> C

A + C --> D

Where C is the desired product. Let’s assume that this reaction takes place over a solid catalyst.

Figure 3.1
Schematic representation of a reaction process

The reaction concentration profile may resemble:

Figure 3.2
A typical reaction profile

3.4 Classification of reactors

In general, chemical reactors have been broadly classified in two ways:

• According to the type of operation
• According to design features

The former classification is mainly for homogeneous reactions and divides the reactors into batch, continuous, or semi-continuous types. Brief descriptions of these types are as follows.

3.4.1 Batch reactor

This type takes in all the reactants at the beginning and processes them according to predetermined course of reaction during which no material is fed into or removed from the reactor. Usually it is in a form of tank with or without agitation and is used primarily in a small-scale production. Most of the basic kinetic data for reactor design are obtained from the type.

Figure 3.3
Batch reactor

The Sequencing Batch Reactor (SBR) is a batch process for treating wastewater. This process is capable of achieving biological P removal, nitrification, de-nitrification and BOD5 removal in one reactor.

Sequencing batch reactors are suitable for plants that need flexibility and control or that have limited space available. One bioreactor serves as a multipurpose reactor. Cycles that occur in the SBR process are anaerobic, anoxic, react, settling, decant and idle cycles.

The reactor is also used for settling the mixed liquor and decanting the treated wastewater, thus eliminating the need for a secondary clarifier. The liquid level in the reactor and the cycle time can be varied, allowing for the flexibility required to achieve the various processes. A typical operation consists of three or four cycles per day.

Figure 3.4
Sequencing Batch Reactor (SBR)

3.4.2 Continuous stirred tank reactor

This reactor consists of a well -stirred tank containing the enzyme, which is normally immobilised. The substrate stream is continuously pumped into the reactor at the same time as the product stream is removed. If the reactor is behaving in an ideal manner, there is total back mixing and the product stream is identical with the liquid phase within the reactor and invariant with respect to time. Some molecules of substrate may be removed rapidly from the reactor, whereas others may remain for substantial periods. The distribution of residence times for molecules in the substrate stream is shown in figure 3.5.

The CSTR is an easily constructed, versatile and cheap reactor, which allows simple catalyst charging and replacement. Its well -mixed nature permits straightforward control over the temperature and pH of the reaction and the supply or removal of gases. CSTRs tend to be rather large as the: need to be efficiently mixed. Their volumes are usually about five to ten time the volume of the contained immobilised enzyme. This, however, has the advantage that there is very little resistance to the flow of the substrate stream, which may contain colloidal or insoluble substrates, so long as the insoluble particles are not able to sweep the immobilised enzyme from the reactor. The mechanical nature of the stirring limits the supports for the immobilised enzymes to materials, which do not easily disintegrate to give ‘fines’ which may enter the product stream. However, fairly small particle (down to about 10 μm diameter) may be used, if they are sufficiently dense to stay within the reactor. This minimises problems due to diffusion resistance.

Figure 3.5
CSTR

Continuous Stirred Tank Reactors (CSTR) for gases (e.g. ozone, carbon dioxide) under defined climate conditions.

Advantages

A continuous process operates at a steady state in which all the variables go to stable value. How fast this steady state is reached depends on the residence time distribution behaviour of the process and this varies from a single mean residence time for a process with ideal plug flow and multiple mean residence times for a fully back mixed reactor like for instance a CSTR. After the steady state is reached, the heat production becomes constant making accurate temperature control much better.

Because of the time independent conversion-space relationship it is possible to fine-tune the exact location and flow rates of additional feeds. In a traditional semi-batch process these additions are generally made on a time based profile because there are not sufficient robust on-line sensors to allow feedback control. For the continuous process, the slower off-line measurements can be used to adjust for instance temperature or feed flow rates. This way, the product properties can be kept constant. This contrasts sharply with a semi-batch process where minor disturbances in the early stages of the reaction (caused by for instance poor temperature control) result in a different conversion-time history and in different product properties because the additions are made at the “wrong” time.

Disadvantages

Emulsion polymerisation can be very sensitive to residence time distribution, especially if particle nucleation is involved. It has been shown that a reactor system with too much back mixing (the ultimate case being a single CSTR) can result in non-steady behaviour. In that case the particle number and the conversion can start to oscillate resulting in a variation in product properties. Even if these oscillations do not occur, the resulting number of particles (and because of this also the volumetric production rate) will be considerably lower.

Another negative effect of residence time distribution is that if a grade change is required also some “twilight” product will be produced which has some intermediate properties and, depending on what change is imposed, has to be considered as waste or low quality product. This limits the flexibility of reactor systems with considerable back-mixing to applications where large amounts of a single grade are required and where the different grades differ only slightly. In all other cases, only a continuous reactor system with near plug flow behaviour will be suitable.

3.4.3 Plug flow reactors (PFR)

The most important characteristic of a PBR is that material flows through the reactor as a plug; they are also called plug flow reactors (PFR). Ideally, the entire substrate stream flows at the same velocity, parallel to the reactor axis with no back -mixing. All material present at any given reactor cross -section has had an identical residence time. The longitudinal position within the PBR is, therefore, proportional to the time spent within the reactor; all products emerging with the same residence time and all substrate molecules having an equal opportunity for reaction

Figure 3.6
Plug Flow Reactors (PFR)

Figure 3.7
Cross-section of plug flow reactor

3.4.4 Fluidised bed reactors (FBR)

These reactors generally behave in a manner intermediate between CSTRs and PBRs. They consist of a bed of immobilised enzyme which is fluidised by the rapid upwards flow of the substrate stream alone or in combination with a gas or secondary liquid stream, either of which may be inert or contain material relevant to the reaction. A gas stream is usually preferred, as it does not dilute the product stream. There is a minimum fluidisation velocity needed to achieve bed expansion, which depends upon the size, shape, porosity and density of the particles and the density and viscosity of the liquid. This minimum fluidisation velocity is generally fairly low (about 0.2 - 1.0 cm s-1) as most immobilised-enzyme particles have densities close to that of the bulk liquid. In this case the relative bed expansion is proportional to the superficial gas velocity and inversely proportional to the square root of the reactor diameter.

Fluidising the bed requires a large power input but, once fluidised, there is little further energetic input needed to increase the flow rate of the substrate stream through the reactor figure 3.8. At high flow rates and low reactor diameters almost ideal plug-flow characteristics may be achieved. However, the kinetic performance of the FBR normally lies between that of the PBR and the CSTR, as the small fluid linear velocities allowed by most biocatalyst particles causes a degree of back mixing that is often substantial, although never total.

Figure 3.8
Fluidized bed reactor

FBRs are chosen where a high conversion is needed but the substrate stream is colloidal or the reaction produces a substantial pH change or heat output. They are particularly useful if the reaction involves the utilisation or release of gaseous material.

Here are some other applications of FBR technology:

• Fertilizers from coal
• Oil Decontamination of sand
• Industrial and municipal waste treatment
• Radioactive waste solidification
•  

3.5 Catalysts

A catalyst is a substance, which changes the rate of a chemical reaction (usually speeding it up), but when the reaction is finished, the catalyst is chemically the same as it was at the beginning. This means that none of it is used up in the reaction

Catalysts are quite substrate-specific, which means that they are only good at changing the rate of one type of chemical reaction and not much good with any others. Catalysts are very important in industry, where it would be uneconomic (or even impossible) to carry out certain chemical reactions.

Examples of industrial processes, which use catalysts that contain transition metals, are:

• Making margarine from vegetable oils (Nickel catalyst)
• Making sulphuric acid (Vanadium catalyst)
• Making ammonia (Iron catalyst)
• Making nitric acid (Platinum catalyst)
• Making sulphur dioxide (Platinum catalyst)
• Making polymers (plastics) (Titanium catalyst)

3.5.1 Classification of catalyst

Homogeneous catalyst

A catalyst in the same phase (usually liquid or gas solution) as the reactants and products is called homogeneous catalyst.

These precious metal compounds and salts are typically used as homogeneous catalysts. The active metal component includes

Palladium

• Good Selectivity
• Suitable Micro porous Structure
• High Strength
• Long Life (over Two Years)

Figure 3.9
Palladium

Platinum

Platinum is used as a catalytic agent in processing of nitric acid, fertilizers, synthetic fibers, and a variety of other materials. In catalytic processes, the catalyst material is not consumed and can be recycled for future use. This makes chemical demand for platinum quite volatile. Platinum is essential in many of these processes and there are few satisfactory substitutes.

Figure 3.10
Platinum

Rhodium

Rhodium catalyst gives extremely high activity in hydrogenation of an aromatic compound. It hydrogenates many compounds at room temperature and atmospheric pressure. Normally, a palladium metal catalyst is used for hydrogenation of olefins, but rhodium catalyst gives even higher activity than palladium metal catalyst in this reaction.

Iridium

The iridium catalyst proved to be more stable under a wide range of conditions, and more soluble so that it has no tendency to precipitate out of solution. This means that the catalyst can be continuously recycled within the plant. New catalyst does not need to be added. As well as this iridium is considerably cheaper.

The rarest of the PGMs, iridium is second only to osmium as the densest element and is the most corrosive resistant known. It is white with a yellowish hue.

Although brittle, it is extremely hard (over 4 times that of platinum itself) and with its high melting point, temperature stability and corrosion resistance is used in high-temperature equipment such as the crucibles used to grow crystals for laser technology.

Its biological compatibility is what we owe most to iridium as this enables it to be used in a range of medical and surgical applications. Iridium can be found in health technology combating cancer, Parkinson’s disease, heart conditions and even deafness and blindness.

Its durability prolongs the life of electronic components and products, which exploit iridium’s conductivity and stability.

A shiny, oxidation-resistant metal, iridium also adds to the brilliance and durability of jewels. It also has industrial applications such as the production of chlorine and caustic soda.

Osmium

Osmium is the densest substance known and the hardest of all Platinum Group Metals (PGMs.). It is 10 times harder than platinum itself.

It is these extraordinary qualities that see osmium used in a range of applications in which frictional wear must be avoided, including fountain pen nibs, styluses, and instrument pivots. Especially when alloyed to other PGMs.

Its conductivity means it can be used as a more effective and durable alternative to gold as plating in electronic products.

Like the other PGMs it is an extremely efficient oxidation catalyst and contributes to the environment through use in fuel cells. This quality is also uniquely applied in forensic science for staining fingerprints and DNA (as osmium tetroxide).

Ruthenium

Ruthenium’s catalytic qualities make it a key element in catalysts for fuel cells. Due to its hardness and corrosion resistance, ruthenium is used to coat electrodes in the chloralkali process, which produces chlorine and caustic soda for a wide range of industrial and domestic applications.

In the future, the use of ruthenium in alloys for aircraft turbine blades will help reduce the CO2 impact of air travel on the environment. If current prototypes are successful, their high melting points and high temperature stability will allow for higher temperatures and, therefore, a more efficient burning of aircraft fuel.

Heterogeneous catalysts

Heterogeneous catalysts are sometimes called surface catalysts because they position the reactant molecules on their very surface. Many metals serve as heterogeneous catalysts in which the reactant molecules have an interface between themselves and the catalyst surface. In the reaction known as Hydrogenation, double bonds between carbons accept two hydrogen atoms and use the Pi electrons between the two carbons in order to attach these hydrogen atoms to the carbon atom. The di-atomic Hydrogen molecule attaches itself to the surface of a metal catalyst such as Platinum, Nickel, or Paladium. The double bonded organic molecule does the same. The single bond between the Hydrogen atoms is broken, and so is the Pi bond between the two carbons within the organic molecule broken. The Hydrogen atoms then form a single bond with its single electron and one of the two Pi electrons that previously constituted the Pi bond between the two carbon atoms. Once the Hydrogen has been attached the product molecule disengages from the surface only to have fresh reactant molecules take its place upon the surface of the metal. Heterogeneous catalysts are, as a rule, not as efficient as homogeneous catalysts.

Figure 3.11
Heterogeneous catalyst

Elemental maps showing the distribution of chromium (red), nickel (blue), and copper (green) in heterogeneous catalyst pellet.

Auto catalyst

An auto catalyst is a cylinder of circular or elliptical cross section made from ceramic or metal formed into a fine honeycomb and coated with a solution of chemicals and platinum group metals. It is mounted inside a stainless steel canister (the whole assembly is called a catalytic converter) and is installed in the exhaust line of the vehicle between the engine and the silencer (muffler).

Vehicle exhaust contains a number of harmful elements, which can be controlled by the platinum group metals in auto catalysts.

The major exhaust pollutants are:

• Carbon monoxide, which is a poisonous gas
• Oxides of nitrogen, which contribute to acid rain, low level ozone and smog formation and which exacerbate breathing problems
• Hydrocarbons, which are involved in the formation of smog and have an unpleasant smell
• Particulate, which contains known cancer-causing compounds

Auto catalysts convert over 90 per cent of hydrocarbons, carbon monoxide and oxides of nitrogen from gasoline engines into less harmful carbon dioxide, nitrogen and water vapour. Auto catalysts also reduce the pollutants in diesel exhaust by converting 90 per cent of hydrocarbons and carbon monoxide and 30 to 40 per cent of particulate into carbon dioxide and water vapor.

3.6 Promoters

Promoters are not catalysts by themselves but increase the effectiveness of a catalyst. For example, alumina Al2O3, is added to finely divided iron to increase the ability of the iron to catalyze the formation of ammonia from a mixture of nitrogen and hydrogen. A poison reduces the effectiveness of a catalyst. For example, lead compounds poison the ability of platinum as a catalyst. Thus, leaded gasoline shall not be used for automobiles equipped with catalytic converters.

3.6.1 Enzymes – the biological catalysts

When a catalyst is doing its job in a living thing, this kind of catalyst is called an Enzyme. Many of the reactions in catabolism are favorable. This means that these reactions will occur spontaneously even outside of a living organism. The problem is, they are way too slow to be of any use in a biological system. If cells did not have ways of speeding up catabolism, life would be nearly impossible. Enzymes accelerate almost all biological reactions.

Enzymes (biological catalysts) are used in the baking, brewing and dairy industries.

Beverages

Beer and lager are alcoholic drinks made in the following way:

• A source of sugar (barley) is added to water and allowed to grow (barley is a plant!). Enzymes in the mixture start to make sugars
• The mixture is then flavoured with hops
• After a bit of a technical process, this mixture has yeast (a living fungus and there are lots of them) added to it
• The yeast ‘eats’ the sugar and gives out alcohol as a waste product

Baking

Bread is a food made in the following way:

Flour, water, salt, yeast and other ingredients (to give flavour) are mixed together to give ‘dough’. The dough is left to ‘rise’. Both of the reactions described above start to happen. The carbon dioxide gas bubbles get trapped in the dough and it starts to get a bit bigger (a bit like a balloon). Alcohol is also made in the dough. Then the dough is ‘kneaded’ to get rid of any big gaps. It may be allowed to rise again, followed by kneading again. The dough is shaped and ‘baked’ at over 100°C. During the baking, the yeast is killed (remember, it starts as a living fungus), the alcohol evaporates away (no getting drunk on bread then!) and sugars remain, giving extra flavour to the bread once it has cooled. Fermentation takes place from when the flour and yeast meet, right up to the temperature at which the yeast dies (about 46°C).

When milk ‘goes off’, this means certain bacteria from around the milk start to multiply, using the milk as a food. The waste materials given out by the bacteria are often nasty and, what’s more, if we drink the milk, the bacteria may multiply in us! Even some harmless bacteria will ‘sour’ milk.

Yoghurt

Milk contains sugar. The bacterium Lactobacillus bulgaricus is added to the milk and its enzymes ferment the sugar (at about 43°C for 4 to 5 hours) into lactic acid. It is this acid the gives yoghurt its pleasant acidic taste. It is also this acid, which stops harmful bacteria from multiplying. The yoghurt is now more like a watery paste.

Fruit and their juices can be added to give a range of flavoured yoghurts.

Most yoghurt has been pasteurized (heated to the kill the bacteria), but this is not done to ‘live’ yoghurts, which still have the bacteria doing their job.

Another microbe called Streptococcus thermophilus is also present in live yoghurt giving it its creamy flavour. Some yeast may also be present.

Cheese

Traditionally, an enzyme called rennin was taken form the stomach juices of calves. When this enzyme is added to milk, it starts to ‘clot’ (form a paste). If the watery part of the mixture (the ‘whey’) is removed, the part, which is left behind, is the ‘curd’, the raw material of cheese. Salt is then added.

Some specially designed Lactobacillus bacteria are also used instead to make the curd.

There are many cheeses. The curd can be allowed to age slightly to give cream cheeses or it can be squeezed and left much longer to slightly decompose giving Cheddar and Cheshire cheeses. If the curd is left long enough, decomposition goes so far that the cheese starts to turn back into a liquid.

3.6.2 Transition metals as catalysts

The first period of transition metals are represented by these metals:

Sc Ti V Cr Mn Fe Co Ni Cu and Zn

Typical common features among them are the presences of d electrons, and in many of them, and their unfilled d orbital. As a result, transition metals form compounds of variable oxidation states. Thus, these metals are electron banks that lend out electrons at appropriate time, and store them for chemical species at other times.

3.7 Efficiency criteria of a chemical process

In addition to the general economic criteria, there are factors to reflect the efficiency of a chemical process.

 

Fractional Conversion

It is the fraction of a reactant that has undergone a chemical change at a particular stage of the reaction process.

Yield

The yield of a product is the ratio of the quantity of the product actually obtained to its maximum obtainable quantity.

Selectivity

Total or integral selectivity, is the ratio between the amount of reactant used up in a desired reaction and the total amount of the same reactant used up during the overall reaction.

Throughput and Production Rates

Throughput is the quantity of the product obtained per unit time. The maximum throughput of a chemical plant is normally referred to as its design capacity.

Production rate is defined as the throughput per unit of some quantity characterizing the standard geometry of the equipment, such as its volume, cross-sectional area, etc.