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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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1.31.3 The Collision TheoryThe Collision Theory
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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1.31.3 The Collision TheoryThe Collision Theory
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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1.31.3 The Collision TheoryThe Collision Theory
4 Collisions of particles that are unsuccessful inproducing a chemical reaction are calledineffective collisions.
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1.31.3 The Collision TheoryThe Collision Theory
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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1.31.3 The Collision TheoryThe Collision Theory
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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Activation EnergyActivation Energy
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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Activation EnergyActivation Energy
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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Activation EnergyActivation Energy
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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Activation EnergyActivation Energy
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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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
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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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
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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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
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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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
Total surface area
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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
2. The smaller the particle size, the greater thetotal surface area exposed for reaction to occur.
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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
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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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
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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Effect of surface area on theEffect of surface area on thereaction ratereaction rate
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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Effect of concentration on theEffect of concentration on therate of reactionrate of reaction
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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Effect of concentration on theEffect of concentration on therate of reactionrate of reaction
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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Effect of pressure on the rate ofEffect of pressure on the rate ofreactionreaction
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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Effect of pressure on the rate ofEffect of pressure on the rate ofreactionreaction
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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Effect of pressure on the rate ofEffect of pressure on the rate ofreactionreaction
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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Effect of temperature on theEffect of temperature on therate of reactionrate of reaction
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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Effect of temperature on theEffect of temperature on therate of reactionrate of reaction
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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Effect of temperature on theEffect of temperature on therate of reactionrate of reaction
3 Consequently, the frequency ofeffective collisions increases and hence,the rate of reaction also increases.
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Effect of temperature on theEffect of temperature on therate of reactionrate of reaction
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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Effect of catalysts on reactionEffect of catalysts on reactionratesrates
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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Effect of catalysts on reactionEffect of catalysts on reactionratesrates
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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Effect of catalysts on reactionEffect of catalysts on reactionratesrates
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