rates & energy notes

8/12/2019 Rates & Energy Notes

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RATES OF REACTION

Increasing the rates of chemical reactions is important in industry because it helps toreduce costs.

The rate of reaction is the speed at which a chemical change takes place. It is followedby measuring the rate at which reactants are used up or the rate at which products areformed. This allows a comparison to be made of the changing rate of a chemicalreaction under different conditions.

Rate = amount of reactant used or Rate = amount of product formed time time

Chemical reactions can occur only when reacting particles collide with each other withsufficient energy. The minimum amount of energy which particles must have to react iscalled the activation energy .

Various factors alter the rate of a reaction: the state of division of the reactants i.e. the particle si!e of a solid": the smaller

the particle si!e# the faster the reaction is. the concentration of dissolved reactants or the pressure of gases: the higher the

concentration or the pressure# the faster the reaction is. the temperature of the reaction mi$ture: the higher the temperature# the faster the

reaction is. the addition of a catalyst speeds up the reaction

1. The Particle Size of Solid ReactantsThe reaction used to study the effect of particle si!e is the reaction of calciumcarbonate# in the form of marble# with dilute hydrochloric acid.

CaC% & s" ' ()Cl a*" CaCl ( a*" ' ) ( % l" ' C% ( g"

The reaction is followed by the change in mass of the reaction flask with time as carbondio$ide is given off. It would also be possible to measure the change in the volume ofcarbon dio$ide given off with time by collecting the gas in# for e$ample# a gas syringe.

Method: + constant mass of marble chips was weighed out and placed into a (,-cm & conical

flask. --cm & of / hydrochloric acid was added from a measuring cylinder. The flaskwas loosely stoppered with cotton wool to allow the gas to escape but to prevent theloss of li*uid splashes" then placed onto an electronic balance. The mass of the conicalflask was recorded every ,s for the first minute# then every &-s for a total of tenminutes. It is assumed that the temperature of the reaction mi$ture stayed constant.

The e$periment was repeated with three different si!es of marble chips# keeping allother variables the same. The results were tabulated and a graph of mass of carbondio$ide y0a$is" against time $0a$is" was plotted.

TOPIC 11.2. : RATES AN! ENER"# 1


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Res$lts:

1arge 2articles /edium 2articles 3mall 2articles


Time 4s /ass offlask4g/ass ofC% ( 4g

/ass offlask 4g

/ass ofC% ( 4g

/ass offlask 4g

/ass ofC% ( 4g

-

,

&-

5-

6-

(-

,-

7-

( -

(8-

(9-

&--

&&-

&5-

&6-

8(-

8,-

87-

, -

,8-

,9-

5--


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Inter%retation of Res$lts:The reaction occurs only on the surface of the calcium carbonate. or a given mass ofcalcium carbonate# the smaller the si!e of the particles# the greater is the surface areaand the faster the reaction. This is shown on the graph by the gradient becomingsteeper as the particle si!e decreases.

3ince the same *uantities of reactants are involved in all three reactions# the samemass of carbon dio$ide is given off in all of them# if the reaction is allowed to go tocompletion. This is shown by all three curves levelling off at the same total mass ofcarbon dio$ide.

The calcium carbonate is in e$cess# so the hydrochloric acid is used up completely as

the reaction takes place. 3ince its concentration decreases with time# the reactionbecomes slower and slower. This shown on the graph by a curve of steadily decreasinggradient.

The e$pected mass of carbon dio$ide can be calculated. 3ince the calcium carbonate isin e$cess# the mass of carbon dio$ide depends on the amount of hydrochloric acidused. --cm & of / )Cl contains -. mole of )Cl.

CaC% & s" ' ()Cl a*" CaCl ( a*" ' ) ( % l" ' C% ( g" ( moles mole -. mole -.-, mole -. mole -.-, $ 88 g -. mole (.(g

TOPIC 11.2. : RATES AN! ENER"# &

The smaller the particle size (the bigger the surface area) , thefaster the reaction


4/11

2. The Te'%erat$reThe reaction used to study the effect of temperature is the reaction of sodiumthiosulphate solution with dilute hydrochloric acid.

;a ( 3 ( %& a*" ' ()Cl a*" (;aCl a*" ' 3 s" ' ) ( % l" ' 3% ( g"

The reaction is followed by the appearance of colloidal sulphur as the reactionproceeds. The formation of sulphur begins as soon as the reactants are mi$ed# but ittakes time for observable amounts to be produced. The time taken to reach a particularpoint in the reaction can be determined by standing the reaction flask on a piece ofpaper marked with a feint cross and timing how long it takes for the cross to beobscured when looked at from above.

Method:-cm & of sodium thiosulphate stock solution concentration 8-g per litre" were

measured out into a measuring cylinder and poured into a (,-cm & conical flask. 8-cm &

of water were measured out in a similar way and added to the flask. The flask washeated gently over a


5/11

+n increase in temperature increases the rate of reaction in two ways:

The %articles collide 'ore ener(eticall)The particles of a substance have a range of different energies# the average energybeing proportional to the temperature in elvins. +s the temperature increases# theparticles move faster i.e. they have more kinetic energy"# and the proportion of particleswith higher energies increases. Therefore# the number of collisions with energy greaterthan or e*ual to the activation energy rises rapidly as the temperature increases# and sothe rate rises rapidly. This is the ma#or effect.

The %articles collide 'ore fre*$entl)The particles move around faster and therefore there is a greater chance that they willbe involved in a collision. This is a minor effect.

TOPIC 11.2. : RATES AN! ENER"# +

reactants

bondsforming

bondsbreaking

Reaction path

Chemical>nergy

products

>a

)

In the reaction between thiosulphateions and hydrogen ions# as the ionscollide covalent bonds are broken andnew bonds are formed. 3ince energy isneeded to break bonds# the collidingparticles must have a minimum energyon collision sufficient to break thebonds. This energy is known as theactivation energy ($ a).

%nly those collisions with energygreater than or e*ual to the activationenergy will result in a reaction.


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&. The Concentration of ReactantsThe reaction used to study the effect of concentration is the reaction of sodiumthiosulphate solution with dilute hydrochloric acid.

;a ( 3 ( %& a*" ' ()Cl a*" (;aCl a*" ' 3 s" ' ) ( % l" ' 3% ( g"

The reaction is followed by the appearance of colloidal sulphur as the reactionproceeds. The formation of sulphur begins as soon as the reactants are mi$ed# but ittakes time for observable amounts to be produced. The time taken to reach a particularpoint in the reaction can be determined by standing the reaction flask on a piece ofpaper marked with a feint cross and timing how long it takes for the cross to beobscured when looked at from above.

Method:,-cm & of sodium thiosulphate stock solution concentration 8-g per litre" weremeasured out into a measuring cylinder and poured into a (,-cm & conical flask. Theflask was placed on a piece of paper marked with a feint cross. ,cm & of (/ hydrochloricacid were measured out into a second measuring cylinder. +s the acid was poured intothe conical flask# a stopwatch was started and the mi$ture gently swirled. The stopwatchwas stopped when the cross# viewed from above# became obscured. The time wasrecorded.

The e$periment was repeated for five different concentrations# keeping all othervariables constant.

+ graph of --- 4time y0a$is" against volume of ;a ( 3 ( %& $0a$is" was plotted.

3ince the total volume of the reaction mi$ture is constant at ,,cm &" the concentration of

;a ( 3 ( %& is proportional to its volume.

Res$lts:

Constants: total volume of ;a ( 3 ( %& solution ,-cm &

(/ hydrochloric acid ,cm &

temperature 7 oCconical flask ? cross

%olume of &a ' ' *(+!g/l) /cm *

%olume of' /cm * Time /s !!!/ time /s

"

,- - &( & .&8- - 8- (,.-&- (- ,( 6.((- &- 96 (.9

- 8- 9( ,.7

TOPIC 11.2. : RATES AN! ENER"# ,


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. Addition of a Catal)stThe reaction used to study the effect of a catalyst is the decomposition of hydrogenpero$ide:

() ( %( () ( % ' % (

The reaction is catalysed by several metal o$ides@ the compound used here is

manganese IV" o$ide. The reaction is followed by collecting the o$ygen given off andmeasuring its volume at regular intervals of time. The gas may be collected either in agas syringe or over water in a burette.

Method:9cm & of (- volume hydrogen pero$ide were measured out using a measuring cylinderand poured into a --cm & conical flask. 8&cm & of water were measured out in a similarway and added to the conical flask. -.,g of powdered manganese IV" o$ide wasweighed out and added to the flask. The flask was connected by a delivery tube to a

--cm & gas syringe. + stopwatch was started. The volume of o$ygen# which hadcollected in the gas syringe# was recorded at &- second intervals for ten minutes.

eeping all other variables constant# the e$periment was repeated using -.9,g and .-gof the same manganese IV" o$ide.

Res$lts: Constants: (- volume ) ( %( 9cm &water 8&cm & temperature 7 oC

%olume of ' /cm *

Time /s !. g of Mn ' !.0 g of Mn ' .!g of Mn '- - - -

&- .7 (.6 &.(5- 8. 5.- ,.96- 5.( 9.6 6.8(- 7.& -.& (.-,- 6.6 (. 8.57- ., &.7 9.

( - (.7 ,.8 6.-(8- 8.8 5.7 ( .(9- 5.( 7.( (&.&&-- 9., 6.9 (,.&&- 7.7 (-.7 (5.7

&5- 6.6 ((. (7.,&6- ( .9 (&. &-.-8(- ((.7 (8.( & .58,- (&., (,.( &(.687- (,.- (5.& &8.(, - (5.- (9.8 &,.,,8- (9.- (7.& &5.5,9- (9.9 (6.- &9.95-- (7.7 &-. &6.-

TOPIC 11.2. : RATES AN! ENER"#


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+ll three reactions are being observed only in the early stages and are far fromcomplete. 3ince all three reactions use e$actly the same *uantity of hydrogen pero$ide#they should# when complete# give the same total volume of o$ygen.

+ graph of volume of o$ygen y0a$is" against time $0a$is" was plotted.

Inter%retation of Res$lts:The reaction is faster the more catalyst there is present@ this is shown by the increasingsteepness of the curves.

The reaction takes place on the surface of the catalyst. Increasing the mass of catalystincreases the surface area and therefore speeds up the rate.

+ catalyst can be used over and over again to speed up the conversion of reactants toproducts. Aifferent reactions need different catalysts.

TOPIC 11.2. : RATES AN! ENER"# /

reactants

Reaction path

Chemical>nergy

products

>a

)

>a cat

+ catalyst does not affect the outcome ofa reaction@ the same product is formed butin a shorter time. + catalyst works byweakening bonds# which lo1ers theactivation energy for the reaction. If theactivation energy is lowered# moreparticles have enough energy to react andtherefore the reaction goes faster.

+ catalyst is not used up in the reactionbut is recovered unchanged at the end.

2 catalyst is a substance 1hich increases the rate of a chemical reaction but isnot use up in the reaction.


10/11

ENER"# 0 C EMICA C AN"E

Auring a chemical reaction# e$isting chemical bonds are broken and new bonds areformed.

energy must be supplied to break e$isting bonds

energy is released when new bonds are formed

The overall change in energy# which takes place during the reaction# is called the heatof reaction and is given the symbol ) pronounced Bdelta ) ".

If the energy needed to break e$isting bonds is less than the energy released when newbonds are formed# the reaction will give out energy usually as heat" to its surroundings.The temperature of the reaction mi$ture will rise. This is called an e3othermic reaction.

or an e$othermic reaction# the sign of the heat change )" is negative. > a is called theactivation energy and is the minimum energy needed for a reaction to take place.Dhen a catalyst is added to a reaction# it provides an alternative reaction path with alower activation energy. If the activation energy is lowered# the reaction goes faster.

+ll reactions involving burning# e.g. the burning of magnesium# are e$othermic.

> a

Chemical >nergy

)

Reaction path

De can make use of e$othermic reactions is devices such as hand warmers and selfheating cans where heat may be needed in a remote environment.


reactants

products

bondsforming

bondsbreaking


11/11

If the energy needed to break e$isting bonds is greater than the energy released whennew bonds are formed# the reaction will absorb energy usually as heat" from its

surroundings. The temperature of the reaction mi$ture will fall. This is called anen othermic reaction. or an endothermic reaction# the sign of the heat change )" ispositive.

> a Chemical >nergy

Reaction path

The reaction of sodium hydrogencarbonate with hydrochloric acid is endothermic.

;a)C% & ' )Cl ;aCl ' ) ( % ' C% (

De can make use of endothermic reactions is devices such as chemical cold packs toput on an inEury to reduce swelling and numb pain.

If a reversible reaction is e$othermic in one direction it will be e*ually endothermic in theother. >g

Cu3% 8.,) ( - Cu3% 8. ' ,) ( -

Fou need to heat blue hydrated copper sulphate to produce white anhydrous coppersulphate so this process re*uires energy and is therefore endothermic.)owever# when water is added to white anhydrous copper sulphate a lot of heat is givenout as it turns blue and so is e$othermic.


reactants

products

)

rates & energy notes

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