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Chemical reactions

Physical changes do not involve the production of new substances and are easily reversible. Chemical changes involve

the production of new substances as a result of a chemical reaction between two or more substances. Some indicators

of a chemical reaction (or chemical change) are:

Reactants are permanently converted into new products that have different physical properties.

Chemical reactions are difficult to reverse Changes to the enthalpy of the system occurs, i .e. heat is released or absorbed

Energy changes in chemical reactions

The enthalpy of the system represents the stored chemical potential energy and depends on the chemical bonding

present in the substance. Exothermic reactions are associated with the decrease in the enthalpy of a system, while

endothermic reactions are associated with an increase in the enthalpy of a system.

For exothermic reactions (reactions that release energy), the change in enthalpy (H) has a negative value

For endothermic reactions (reactions that absorb energy), the change in enthalpy (H) has a positive value

Bond-breaking and bond-making

Chemical reactions such as exothermic or endothermic reactions require either the breaking of bonds or the

formation of new bonds, which explain the resulting changes in enthalpy to the system.

From the Law of Conservation of Energy, the quantity of energy released when one bond is formed is the same

amount of energy as when absorbed when this bond is broken (i.e. the energy input required).

If the energy absorbed to break bonds is greater than the energy released when new bonds are formed to makeproducts, then the reaction will be endothermic.

If the energy absorbed to break bonds is less than the energy released when new bonds are formed to make

products, then the reaction will be exothermic.

The bondenergyis the amount of energy that is associated with breaking that particular bond. The stronger the

chemical bonding is in a compound, the higher the bond energy. For this reason triple bonds between carbon

atoms require more energy to break than double bonds between carbon atoms.

Heat of combustion

The combustion reactions especially those of hydrocarbons are exothermic releasing large quantities of energy fromthe system. This is because the enthalpy of the products, i.e. water and carbon dioxide is much less than those in the

reactants.

For example the combustion of methane: CH4 (g)+ 2O2 (g) CO2 (g) +2H2O(l)+ 890kJ energy

There is a decrease in the enthalpy of the system of 890kJ for every mole of methane that burns.

This large decrease in enthalpy indicates that much less energy is needed to break the bonds in the reactants than

the energy that is released when the products are formed.

The molarheatofcombustion(H) of a substance is the heat released when one mole of the substance undergoes

complete combustion with oxygen at standard atmospheric pressure (100 kPa). Since it is defined in terms of heat

released it has by definition a positive value. The minus sign in H ensures that the value is always positive because

all combustion reactions are exothermic and release energy.

When new substances are formed, i.e. bonds are made, energy is released. When bonds are broken, for example in

decomposition, energy is absorbed from the surroundings in order to break the bond.

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Determining the rate of a reaction

In general, reaction rate can be determined by noting the rate of disappearance of reactants or rate of appearance of

products. Some reactions occur quickly, while others are slower.

By examining many different reactions, chemists have been able to produce some guidelines that are useful for

predicting reaction rates:

If a reaction does not involve bonding rearrangements, it is likely to be rapid at room temperature. For example,

the reaction between Ag+(aq)and Cl(aq)occurs as a result of simple collisions between these two types of ions. No

complex bond-breaking and bond-forming processes are required.

If a reaction involves the breaking of bonds, it is slow at room temperature. For example, the reaction between

methane and oxygen involves the breaking of bonds in both the methane and oxygen.

These processes will only take place if the collisions between the molecules occur with sufficient energy and havesuitable orientation.

Combustion and reaction rates

Slow combustion

Slowcombustionis the reaction usually between metals and oxygen. When a very active metal such as sodium, the

reaction occurs in a few seconds, while in the case of iron, the reaction rate is slower. These reactions are relatively

slow; releasing considerable amounts of heat, but because of their slow reaction rate and the oxide layer formed is

very thin, the rise in temperature is not significant. Some examples of slow combustion reactions include:

Rusting is the reaction occurs between iron and oxygen to form rust, which is iron oxide.

Burning of wood, coal and coke is another slow reaction, due to the small surface area of the fuel used.

Spontaneous combustion

Spontaneouscombustionis when the fuel starts burning for no apparent reason, when the temperature is higher

than the ignition temperature. This is because the reactants already have sufficient energy to overcome the activation

energy barrier, and just need to be mixed to ensure a reaction. Some examples of spontaneous combustion reactions

include:

White phosphorus (P4) is an allotrope of phosphorus that must be stored underwater, otherwise it will combust

spontaneously when present in air, at low temperatures.

Brown coal deposits are known to combust spontaneously when exposed to air, due to the temperature change

when it starts oxidisation.

Explosive combustion

Explosive combustionreactions are extremely fast reactions that occurs when gases expanded at an extremely fast

rate, and is caused by a chemical between a fuel and air. The difference between spontaneous combustion and

explosive combustion is that the former requires the mixing of the fuel and the air in the right ratio (to ensure

complete combustion).Some examples of explosive combustion reactions include:

Hydrogen and oxygen react to form water, when hydrogen gas in air is sparked. This explosive combustion is used

in rocket engines to produce the necessary thrust to propel the rocket forward.

The rate of a reaction, at any particular time, is defined as the change in concentration of a substance per unit

time, and therefore is measured in mol L1

s1

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Hydrogen and chlorine are stable unless in the presence of light, where a chain reaction resulting an explosive

combustion reaction and the form hydrochloric acid.

Dust particles can form explosive mixtures in air, since they have high surface area.

Factors affecting the rate of reaction

Concentration of reactants

The rate of reaction can be altered by altering the concentration of the

reactants. As the concentration of the reactant increases so do this rate of

the reaction. This can be determined experimentally, for example the

reaction between hydrochloric acid and magnesium.

Surface area of reactants

The surface area of the solid or liquid has an effect on the reaction rate.

The relationship is the larger the surface area, the faster the reaction. For example: a log of wood that is smaller pieces

burns faster than larger pieces of wood. This is also true with particular matter whose surface area is large, despite

being small in size, has the potential with sparking, combust explosively.

Temperature

The temperature also has an effect on the reaction rate, as temperature increases the reaction becomes faster. This is

notable also when cooking, that cooking time is quicker, when done at higher temperatures.

Catalysts

The addition of a catalysthas the effect of dramatically increasing the rate of the reaction. A catalyst is a substance or

mixture that increases the rate a chemical reaction without being permanently consumed in the reaction.

In industry, a wide variety of processes utilise catalysts to make industrial reactions faster and therefore more

economically viable. Some of the uses include the purification of metals, and also in the formation of plastics, rubbers

and synthetic fibres. An example of a catalyst

that is used is magnesium oxide MgO2, which

when added to hydrogen peroxide H2O2

increases the rate of its decomposition into

water and oxygen gas.

Collision theory

The collisiontheoryassumes that if particles

are to react, they must first undergo an

appropriate collision. The collision theory

requires that for a collision between reactant

particles to lead a chemical reaction, the

following conditions must be fulfilled:

The molecules must collide with enough energy to cause the bonds of the reactant molecules to break

The molecules must collide with an orientation that is favourable for bond-breaking and bond-forming

For a reaction to occur between reactant molecules, they must collide with a certain amount of energy, called theactivation energy. Unless this quantity of energy is reached, the colliding molecules simply bounce back of the

molecules and disperse.

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In addition to this, subsequent collisions require to have a favourable orientation that is the colliding substances must

do so at the appropriate angles and in the right position to allow the breaking of bonds (in the reactants) and the

formation of bonds in the products.

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Applying collision theory

The different activation energies for different reactions correspond to the ease in which the bond-breaking and

bond-making process can occur.

Concentration

The result of increasing the concentration is an increase in the number of collisions that occur, and therefore the

reaction rate will increase. For gases, increasing the pressure results in an increase of collisions, and therefore thereaction rate will increase.

Surface area

In a solid or liquid, increasing the surface area means that more of the reactant is able to be collided with. This as a

result, means an increased rate of reaction. Solids often powdered as this increases surface area and therefore reaction

rate. Liquids are often used as sprays or subject to vigorous agitation in order a larger surface area is available.

Temperature

Moving particles possess kinetic energy. As these particles are heated, they become excited and move with a fastervibrations. This results in an increase of kinetic energy of the particles. The increased velocity results in a greater rate of

reaction, as there are more frequent collisions. However this change in reaction rate is small due to velocity increase

alone.

However the main contributor to an increased rate of reaction is that more reactant molecules have sufficient energy

to overcome the activation energy barrier and therefore react to form products.

Catalysts

Catalysts lower the activation energy by allowing the reactants to partially react with them, and assisting in the

bond-breaking process. The catalyst means that less collision energy is required, so increased likelihood of successful

collisions and results in a faster reaction rate.

The kinetic energy of a particle is proportional to its temperature: EK T

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