Collision Theory by Abraham Hertanto

8/8/2019 Collision Theory by Abraham Hertanto

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1 rate of reaction1 rate of reaction

1.3 The Collision Theory


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1.31.3 The Collision TheoryThe Collision Theory

1 According to the kinetic theory ofmatter, all matter is made up of tiny,discrete particles. These particles arecontinually moving and so havekinetic energy.


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2 Based on the assumption that the particles inmatter are moving all the time and collide witheach other, the collision theory was introduced toexplain

(a) how chemical reactions occur, and (b) the factors affecting the rates of reactions.


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3. When the reactants are mixed, the reactantparticles (atoms, molecules or ions) collide witheach other. However, not all collisions will resultin a chemical reaction to form the products of thereaction. It is likely that the particles collide butare bounced back without producing any changes.The collisions that are successful in producing achemical reaction are called the effectivecollisions.


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4 Collisions of particles that are unsuccessful inproducing a chemical reaction are calledineffective collisions.


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5 The collision theory states that for achemical reaction to occur, the reacting particlesmust:

(a) collide with each other so that the breaking

and formation of chemical bonds can occur.


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5 The collision theory states that for a chemicalreaction to occur, the reacting particles must:

(a) collide with each other so that the breakingand formation of chemical bonds can occur.

(b) possess energy that is equal to or more thanthe minimum energy called the activation energy. (c) collide in the correct orientation.


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Activation EnergyActivation Energy

1 The activation energy is the minimumenergy that the reactant particles must possessat the time of collision in order for a chemicalreaction to take place.


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2 The activation energy can also beconsidered as an energy barrier thatmust he overcome by the colliding particles

in order that collision will result in theformation of product molecules.


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3 (a) If the two molecules with sufficient energy (that is,energy equal to or more than the activation energy) collidein the correct orientation, the chemical bonds in thereactant molecules will break and reaction will occur toform new bonds in the product molecules (Figure 1.35). Forexample,


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3 (b) However, if two molecules, with energy equal to ormore than the activation energy, do not collide with eachother in the correct orientation, then reaction will notoccur.


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3 (c) If two reactant molecules, withenergy less than the activation energy,collide in the correct orientation, thenreaction will also not occur. The colliding

molecules will simply rebound and moveaway from each other.


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e at ng t e requency o e ect vee at ng t e requency o e ect vecollisions with factors influencing thecollisions with factors influencing the

rate of reactionrate of reaction

According to the collision theory, two importantfactors that determine the rate of a chemicalreaction are:

(a) the frequency of effective collisions and

(b) the magnitude of the activation energy


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1 Frequency of effective collisions

For a given reaction, if the frequency of collisionsbetween reactant molecules is high, it follows thatthe frequency of effective collisions that causes a

reaction to occur will also be high. As a result, therate of reaction increases.


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2 Magnitude of activation energy Reactions that have high activation energy will occur at a

slow rate. This is because only a small fraction of themolecules possess sufficient energy to overcome theactivation energy for the reaction to occur. In contrast,

reactions that possess low activation energy will occur at afast rate. This is because most of the molecules havesufficient energy to overcome the activation energy and thisenables the reaction to occur.


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3 In general, any factor that increases therate of effective collisions will also increasethe rate of reaction.


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Effect of surface area on theEffect of surface area on thereaction ratereaction rate

1 The sodium chloride crystal as shown in Figure 1.29 (a)has a surface area of 16 cm2. When this crystal is dividedinto smaller crystals as shown in Figure 1.29 (b), the totalsurface area is increased to 24 cm2.

Surface area of the NaCl crystal =(1 x 2) x 4 + (2 x 2) x 2

=16cm2


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Total surface area


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2. The smaller the particle size, the greater thetotal surface area exposed for reaction to occur.


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3 In the reaction between hydrochloric acid andmarble chips (calcium carbonate),

CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) +CO2(g)

the rate of reaction will increase ifpowderedmarble is used instead of marble chips.


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4. The total surface area is larger in powderedmarble. With greater surface area exposed forreaction with the acid (Figure 1.30), collisionsbetween reacting molecules become more

frequent.


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5. This means that the frequency ofeffective collisions (that is, collisions withthe correct orientation and with energyequal to or greater than activation energy)

between reacting particles will increase. Asa result, the rate of reaction also increases.


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Effect of concentration on theEffect of concentration on therate of reactionrate of reaction

1 Magnesium reacts with dilutehydrochloric acid according to theequation:

Mg(s) + 2HCl(aq)

MgCl2(aq) + H2(g) When the concentration of hydrochloric

acid increases, the rate ofreaction alsoincreases.


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2 Figure 1.37 shows the arrangement ofparticles in 1 mol dm-3 hydrochloric acidand 2 mol dm-3 (more concentrated)hydrochloric acid.


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3 When the number of particles increases,the frequency of collisions alsoincreases. As a result, the frequency ofeffective collisions increases. This causesthe rate of reaction to increase.


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Effect of pressure on the rate ofEffect of pressure on the rate ofreactionreaction

1 In chemical reactions involving gases,increasing the pressure increases the rate ofreaction. Conversely, decreasing the pressuredecreases the rate of reaction. For example, the

rate of reaction between nitrogen and oxygen toproduce nitrogen monoxide can be increased byincreasing the pressure.

N2(g) + O2(g) 2NO(g)


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2 At low pressures, the gaseous molecules arespread far apart (Figure 1.32 (a)). When thepressure is increased, the volume of the gas isreduced (Figure 1.32(b)).

Number of particles same


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This means that at high pressures, (a) the number of gaseous molecules per

unit volume is increased, and (b) the gaseous molecules are packed

closer together.

Number of particles same


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3 As a result, increasing the pressure causesthe gaseous molecules to collide more frequently.Consequently, the frequency of effectivecollisions increases and the rate of reaction also

increases.


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Effect of temperature on theEffect of temperature on therate of reactionrate of reaction

1 Calcium carbonate reacts withhydrochloric acid to form carbon dioxideaccording to the following equation:

CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2(g)


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Figure 1.33 shows the graphs of total volume ofcarbon dioxide given off against the time taken forthe reaction between calcium carbonate and dilutehydrochloric acid at 25 C and 30 C.


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2. At low temperatures, particles of reactants move at aslower speed. However, when the temperature isincreased, the particles absorb the heat energy. As a result,the kinetic energy of the particles increases. Hence,

(a) the reacting particles move faster, and

(b) the number of reacting particles with the activationenergy required increases.


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3 Consequently, the frequency ofeffective collisions increases and hence,the rate of reaction also increases.


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4 Temperature has a great effect on therate of reaction. For most reactions, therate of reaction approximately doubles

when the temperature of reaction increasesby 10 C.


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Effect of catalysts on reactionEffect of catalysts on reactionratesrates

1 The decomposition of hydrogen peroxide towater and oxygen occurs very slowly at roomtemperature.

2H2O2(aq) 2H2O(l) +O2(g). In the presence of a catalyst, the decomposition of

hydrogen peroxide occurs rapidly.


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2 Figure 1.34 shows the rate of evolution of oxygen forthe decomposition of hydrogen peroxide without a catalystand in the presence of a catalyst such as manganese(IV)oxide or iron(III) oxide.


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3. A catalyst provides an alternative reaction route (orpathway) for the reaction to occur. In the presence of apositive catalyst, an alternative route has a loweractivation energy.


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In other words, a positive catalyst lowers the activationenergy required for the reaction (Figure 1.34). As aresult, more reacting particles possess sufficient energyto overcome the lower activation energy required foreffective collisions. Hence, the frequency of effective

collisions increases and the rate of reaction increases

Collision Theory by Abraham Hertanto

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