Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 1

Hwa Chong Institution Sec 4 Chemistry Name: _______________________ ( ) Class: 4 ( ) Date: Topic: Rate of Reaction 1. The Importance of Reaction Rate

The rate of reaction for a reactant or product in a particular chemical reaction is defined as how fast a reaction takes place. For example, the oxidation (rusting) of iron under the atmosphere is a slow reaction which can take many years, but the combustion of butane in a fire is a reaction that takes place in fractions of a second.

Speeds of reactions are also important in industry. Many industrial reactions must be fast so that large amounts of products are formed in a short time. A slow reaction means slow production which means less profit.

2. Measuring the Rate of Reaction

By rate of reaction, we mean how quickly a reactant is used up or how quickly a product is formed.

The rate of reaction can be measured by measuring: 1) the time taken for reaction to be complete 2) the amount of reactant used up in a period of time

Rate of reaction = 3) the amount of product produced in a period of time

Rate of reaction = 4) For a chemical reaction that produces a gas,

Amount of reactant used up Time taken

Amount of product obtained Time taken

Volume of gas produced Time taken

Rate of reaction =

Pupils should be able to: (a) describe the effect of concentration, pressure, particle size and temperature on the

speeds of reactions and explain these effects in terms of collisions between reacting particles

(b) describe how the above factors are used to explain the danger of explosive combustion with fine powders (e.g. in flour mills) and combustible gases (e.g. in mines)

(c) define the term catalyst and describe the effect of catalysts (including enzymes) on the rates of reactions

(d) explain how pathways with lower activation energies account for the increase in rates of reactions

(e) state that some compounds act as catalyst in a range of industrial processes and that enzymes are biological catalyst

(f) suggest a suitable method for investigating the effect of a given variable on the rate of a reaction

(g) interpret data obtained from experiments concerned with rate of reaction

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 2

The method chosen to measure the rate of a reaction depends on the particular chemical reaction being investigated.

•• TThhee rreeaaccttiioonn ________________________ wwhheenn aacciidd iiss ppoouurreedd iinnttoo tthhee tteesstt--ttuubbee wwhhiicchh ccoonnttaaiinnss ccaallcciiuumm ccaarrbboonnaattee.

•• RReeaaccttiioonn iiss ccoommpplleettee wwhheenn tthheerree iiss __________________________________________________________..

•• TTiimmee ttaakkeenn ffoorr tthhee rreeaaccttiioonn iiss mmeeaassuurreedd ffrroomm wwhheenn aacciidd iiss aaddddeedd ttoo wwhheenn

nnoo mmoorree eeffffeerrvveesscceennccee oobbsseerrvveedd..

•• RRaattee ooff rreeaaccttiioonn iiss iinnvveerrsseellyy pprrooppoorrttiioonnaall ttoo tthhee ttiimmee ttaakkeenn ffoorr rreeaaccttiioonn ttoo bbee ccoommpplleetteedd

test-tube

Hydrochloric acid

calcium carbonate

RRaattee αα 11//ttiimmee

Method 2: To measure the volume of CO2 evolved at regular time intervals

EE..gg.. RReeaaccttiioonn bbeettwweeeenn ccaallcciiuumm ccaarrbboonnaattee aanndd hhyyddrroocchhlloorriicc aacciidd

EEqquuaattiioonn:: CCaaCCOO33 ((ss)) ++ 22HHCCll ((aaqq)) ÆÆ CCaaCCll22 ((aaqq)) ++ CCOO22 ((gg)) ++ HH22OO ((ll))

Method 1: To measure time taken for reaction to be completed

calcium carbonate

hydrochloric acid

gas syringe

stop-watch

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 3

TThhee ffoolllloowwiinngg oobbsseerrvvaattiioonn ccaann bbee mmaaddee ffrroomm tthhee ggrraapphh::

((11)) TThhee rraattee cchhaannggeess tthhrroouugghhoouutt tthhee rreeaaccttiioonn.. IItt iiss hhiigghheesstt aatt tthhee ssttaarrtt,, bbuutt ddeeccrreeaasseess aass tthhee rreeaaccttiioonn pprroocceeeeddss..

((22)) TThhee ccuurrvvee bbeeccoommeess ffllaatt wwhheenn tthhee rreeaaccttiioonn iiss oovveerr.. Gradient is zero at the end of the reaction.

Inference:

RRaattee ooff rreeaaccttiioonn iiss eeqquuaall ttoo tthhee ________________________ ooff tthhee ttaannggeenntt ddrraawwnn ttoo tthhee ccuurrvvee aatt tthhaatt ttiimmee.. TThhee ______________________ tthhee ggrraaddiieenntt,, tthhee ffaasstteerr iiss tthhee rreeaaccttiioonn..

Graph of volume of CO2 produced against time VVoolluummee ooff CCOO22 eevvoollvveedd // ccmm33

TTiimmee // mmiinn

Method 3: To measure the change in mass against time

(Calcium carbonate)

Explain the shape of the graph.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 4

QQuueessttiioonnss:: 11.. WWhhyy wwiillll tthheerree bbee aa cchhaannggee iinn mmaassss oovveerr aa ppeerriioodd ooff ttiimmee?? 22.. WWhhaatt iiss tthhee uussee ooff tthhee pplluugg ooff ccoottttoonn wwooooll?? 33.. PPlloott aa ggrraapphh ttoo sshhooww hhooww tthhee mmaassss ooff tthhee ffllaasskk aanndd iittss ccoonntteennttss cchhaannggeess wwiitthh ttiimmee.. 44.. PPlloott aa ggrraapphh ooff lloossss iinn mmaassss ooff tthhee ffllaasskk aanndd iittss ccoonntteennttss aaggaaiinnsstt ttiimmee.. Other methods of measuring rate of reaction:

EE..gg.. RReeaaccttiioonn bbeettwweeeenn ssooddiiuumm tthhiioossuullpphhaattee aanndd ddiilluuttee hhyyddrroocchhlloorriicc aacciidd

Equation: Na2S2O3(aq) + 2HCl(aq) Æ 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

Measure the time taken for the ‘X’ to disappear from view.

•• MMeeaassuurriinngg tthhee ttiimmee ttaakkeenn ffoorr aa pprreecciippiittaattee ttoo ffoorrmm

Time/min

Mass of flask and contents /g

Time/min

Loss in mass of flask and contents /g

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 5

EE..gg.. RReeaaccttiioonn bbeettwweeeenn magnesium and dilute hhyyddrroocchhlloorriicc aacciidd 3. The Collision Theory Consider the reaction between hydrogen and chlorine to form hydrogen chloride. The collision theory can be used to explain why some hydrogen and chlorine molecules react to form hydrogen chloride but others do not.

1. The collision theory states that for a reaction to occur between 2 particles,

a) the reacting particles must collide with each other, and b) they must collide with a certain minimum energy known as the activation

energy, and with the correct orientation. Activation energy is the minimum amount of energy that reacting molecules must have in order for a chemical reaction to occur.

2. A collision that results in the formation of product molecules is called an effective collision. 3. In order for a chemical reaction to occur, effective collisions must take place between

the reacting particles. 4. Thus, fast moving hydrogen and chlorine molecules with energies equal to or

greater than the activation energy will react on collision to form hydrogen chloride.

•• MMeeaassuurriinngg tthhee cchhaannggeess iinn ppHH

Equation: Mg(s) + 2H2SO4(aq) → MgSO4(aq) + H2 (g)

ppHH

TTiimmee // mmiinn 11

77

Measure the pH of the reacting mixture at regular time intervals

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 6

4. Factors Affecting Rate of Reaction

5. Use the following set-up to study the effect of different factors on the

rate of reaction between calcium carbonate and hydrochloric acid.

EEqquuaattiioonn:: CCaaCCOO33 ((ss)) ++ 22HHCCll ((aaqq)) ÆÆ CCaaCCll22 ((aaqq)) ++ CCOO22 ((gg)) ++ HH22OO ((ll))

Factor

Increases the number of effective collisions per unit

Increases the frequency of effective collisions between reacting particles

Increases rate of reaction

Factors to consider: 1. Concentration of solution 2. Temperature 3. Particle size (relating to surface area) 4. Pressure of gas 5. Catalyst

calcium carbonate

hydrochloric acid

gas syringe

stop-watch

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 7

5.1 Concentration of solution EExxaammppllee 11:: TToo ssttuuddyy tthhee eeffffeecctt ooff ccoonncceennttrraattiioonn ooff aacciidd oonn tthhee rreeaaccttiioonn bbeettwweeeenn ccaallcciiuumm

ccaarrbboonnaattee aanndd hhyyddrroocchhlloorriicc aacciidd VVaarriiaabblleess:: Independent variable:

• Concentration of dilute hydrochloric acid Constant variables:

• ________________ of dilute hydrochloric acid • ________________ of dilute hydrochloric acid • Mass of calcium carbonate • Particle size

Dependent variable: • Volume of carbon dioxide produced

Result (1):

Comments:

a) Graph for reaction A is more __________ than that for B. b) Reactants in reaction ____ are used up faster than reactants in reaction ___.

c) The limiting reactant in both reactions is ________________________

d) Volume of carbon dioxide produced is ________________ in both reactions.

RReeaaccttiioonn AA:: CCaaCCOO33 ++ 22 mmooll//ddmm33 ooff HHCCll RReeaaccttiioonn BB:: CCaaCCOO33 ++ 11 mmooll//ddmm33 ooff HHCCll

VVoolluummee ooff CCOO22 eevvoollvveedd // ccmm33

TTiimmee // mmiinn

11 mmooll//ddmm33 HHCCll

22 mmooll//ddmm33 HHCCll

RReeaaccttiioonn BB hhaass ccoommpplleetteellyy rreeaacctteedd

RReeaaccttiioonn AA hhaass ccoommpplleetteellyy rreeaacctteedd

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 8

Result (2): Hydrochloric acid is the limiting reactant.

Comments:

a) Calcium carbonate is in _____________. b) No. of moles of hydrogen ions used in reaction A is ______________ the no. of

hydrogen ions used in reaction B. c) Volume of carbon dioxide produced in reaction A is ________________ the volume

of carbon dioxide produced in reaction B.

Time/min

Volume of CO2 / cm3

Explanation • An increase in concentration of the acid will cause an increase in

the ____________ of hydrogen ions present _____________________ of the acid.

• This will result in an increase in the ________________ of

_____________ collisions between the reacting particles (in this case, hydrogen ions and calcium carbonate).

• Thus, the rate of reaction increases.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 9

EExxaammppllee 22:: TToo ssttuuddyy tthhee eeffffeecctt ooff ccoonncceennttrraattiioonn ooff aacciidd oonn tthhee rreeaaccttiioonn bbeettwweeeenn ssooddiiuumm tthhiioossuullpphhaattee aanndd ddiilluuttee hhyyddrroocchhlloorriicc aacciidd

Equation: Na2S2O3(aq) + 2HCl(aq) Æ 2NaCl(aq) + S(s) + SO2(g) + H2O(l)

RReeaaccttiioonn AA:: ssooddiiuumm tthhiioossuullpphhaattee ssoolluuttiioonn ++ 22 mmooll//ddmm33 ooff HHCCll

RReeaaccttiioonn BB::ssooddiiuumm tthhiioossuullpphhaattee ssoolluuttiioonn ++ 11mmooll//ddmm33 ooff HHCCll TTiimmee ttaakkeenn ffoorr tthhee ccrroossss ttoo ““ddiissaappppeeaarr”” iiss sshhoorrtteerr iinn RReeaaccttiioonn AA tthhaann iinn RReeaaccttiioonn BB RRaattee ∝∝ 11//ttiimmee RRaattee ooff RReeaaccttiioonn AA iiss ffaasstteerr tthhaann RReeaaccttiioonn BB

Explanation • An increase in concentration of the acid will cause an increase in

the ____________ of hydrogen ions present _____________________ of the acid.

• This will result in an increase in the ________________ of

_____________ collisions between the reacting particles (in this case, hydrogen ions and thiosulphate ions).

• Thus, the rate of reaction increases.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 10

5.2 Temperature EExxaammppllee:: TToo ssttuuddyy tthhee eeffffeecctt ooff tteemmppeerraattuurree ooff aacciidd oonn tthhee rreeaaccttiioonn bbeettwweeeenn ccaallcciiuumm

ccaarrbboonnaattee aanndd hhyyddrroocchhlloorriicc aacciidd VVaarriiaabblleess:: Independent variable:

• ____________________of dilute hydrochloric acid Constant variables:

• __________________ of dilute hydrochloric acid

• __________________ of dilute hydrochloric acid

• Mass of calcium carbonate

• Particle size

Dependent variable:

• Volume of carbon dioxide produced Result: Conclusion: Reaction _____ is faster than reaction ________.

RReeaaccttiioonn AA:: 3300ooCC RReeaaccttiioonn BB:: 4400ooCC

Time/min

Volume of CO2 / cm3

Explanation • At temperature is higher, the reacting particles move faster, and collide with

each other more frequently. • As the particles have more kinetic energy, there is an increase in the number of

particles colliding with energy equal to or greater than the activation energy. • This results in an increase in the frequency of effective collisions between the

reacting particles and an increase in rate of reaction In general, the ssppeeeedd ooff rreeaaccttiioonn ddoouubblleess ffoorr eevveerryy 1100ooCC iinnccrreeaassee iinn tteemmppeerraattuurree..

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 11

5.3 Particle Size EExxaammppllee:: TToo ssttuuddyy tthhee eeffffeecctt ooff ppaarrttiiccllee ssiizzee oonn tthhee rreeaaccttiioonn bbeettwweeeenn ccaallcciiuumm

ccaarrbboonnaattee aanndd hhyyddrroocchhlloorriicc aacciidd VVaarriiaabblleess:: Independent variable:

• Particle size of calcium carbonate Constant variables:

• ________________ of dilute hydrochloric acid

• ________________ of dilute hydrochloric acid

• ________________ of dilute hydrochloric acid

• Mass of calcium carbonate

Dependent variable:

• Volume of carbon dioxide produced Result: Conclusion: Reaction _____ is faster than reaction ________.

LLaarrggee ppiieecceess ooff CCaaCCOO33

PPoowwddeerreedd CCaaCCOO33 VVoolluummee ooff CCOO22 eevvoollvveedd // ccmm33

TTiimmee // mmiinn

RReeaaccttiioonn AA:: llaarrggee ppiieecceess ooff CCaaCCOO33 ++ 11 mmooll//ddmm33 ooff HHCCll RReeaaccttiioonn BB:: ppoowwddeerreedd CCaaCCOO33 ++ 11 mmooll//ddmm33 ooff HHCCll

Explanation • When powdered CaCO3 is used, the total surface area of the carbonate

particles that is exposed for reaction with the acid ___________. • This increases the __________________ of _____________

________________ between hydrogen ions and calcium carbonate. • Thus, the rate of reaction increases.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 12

Questions:

1. What is the advantage of cooking food in small pieces? 2. Why is coal dust dangerous in a mining area?

3. Why are antacid tablets for gastric patients often chewed before being swallowed?

5.4 Pressure of Gas Example: N2 (g) + 2H2(g) Æ 2NH3 (g)

Apply pressure

Reaction A – big solid particles

Reaction B – smaller solid particles

Explanation • Increasing the pressure of the gas _______________ the no. of gaseous molecules

per unit volume.

• At higher pressures, the gaseous molecules are packed closer together.

• This increases the __________________ of _____________ ________________

between gaseous molecules.

• Thus, the rate of reaction increases.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 13

5.5 Catalysts

A catalyst is a substance which increases the speed of a chemical reaction, but remains chemically and quantitatively unchanged at the end of the reaction.

CChhaarraacctteerriissttiiccss::

• only a ________ amount of a catalyst is needed to change the rate of a reaction

• catalysts remain chemically unchanged after a reaction has taken place, but they

can be changed physically

• catalysts are very ____________ to a particular chemical reaction

•• catalysts increase the rate but not the yield of a chemical reaction, i.e. the same

amount of products is formed whether a catalyst is used or not.

Example: 2H2O2 (aq) Æ 2H2O (l) + O2 (g) RReessuulltt::

CCoonncclluussiioonn::

•• RReeaaccttiioonn BB wwiillll bbee ffaasstteerr tthhaann rreeaaccttiioonn AA.. •• PPrreesseennccee ooff ccaattaallyysstt ddooeess nnoott aaffffeecctt tthhee ffiinnaall aammoouunntt ooff pprroodduuccttss ffoorrmmeedd

RReeaaccttiioonn AA:: AAqquueeoouuss H2O2 wwiitthhoouutt mmaannggaanneessee ((IIVV)) ooxxiiddee MMnnOO22 RReeaaccttiioonn BB:: AAqquueeoouuss H2O2 (aq) wwiitthh 11gg ooff mmaannggaanneessee ((IIVV)) ooxxiiddee MMnnOO22

Time/min

Volume of O2 / cm3

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 14

On the same axes, draw a second curve to show the energy profile for the catalysed reaction. Label the new activation energy.

EEnneerrggyy EEaa ((wwiitthhoouutt ccaattaallyysstt))

Progress of reaction

rreeaaccttaannttss pprroodduuccttss

Explanation

• Catalysts provide an alternate reaction path which has lower activation energy than the original reaction.

• Thus, a larger number of particles could have sufficient energy to

overcome the new energy barrier.

• Hence, the rate of reaction increases.

• At the end of the reaction, the catalyst is released and can be reused.

Explanation (for HETEROGENEOUS catalysts)

A solid catalyst provides active sites on their surface for reaction to occur.

The active sites serve as meeting points for the reacting molecules to come

together and form product molecules.

Activation energy is lowered as bonds are weakened at the active sites and

the reaction occurs faster.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 15

NOTE: You are not expected to use the terms – heterogeneous catalyst.

CCoommmmoonn ccaattaallyyssttss yyoouu nneeeedd ttoo kknnooww::

PPrroocceessss CCaattaallyysstt Haber Process 33HH22 ++ NN22 22NNHH33

IIrroonn

Contact Process 22SSOO22 ++ OO22 22SSOO33

VVaannaaddiiuumm ooxxiiddee,, VV22OO55

MMaannuuffaaccttuurree ooff mmaarrggaarriinnee VVeeggeettaabbllee ooiill ++ hhyyddrrooggeenn ÆÆ mmaarrggaarriinnee

NNiicckkeell

AAcciidd aanndd mmeettaall Zn + H2SO4 Æ ZnSO4 + H2

CCooppppeerr ((**))

**NNoottee:: AAqquueeoouuss ccooppppeerr ((IIII)) ssuullpphhaattee iiss aaddddeedd ttoo tthhee rreeaaccttiinngg mmiixxttuurree.. CCooppppeerr pprroodduucceedd ccaattaallyysseess tthhee rreeaaccttiioonn bbeettwweeeenn zziinncc aanndd ddiilluuttee ssuullpphhuurriicc aacciidd.. ZZnn ++ CCuuSSOO44 Æ ZnSO4 + Cu

Example of HETEROGENEOUS catalyst (for info only)

Consider the production of ammonia from nitrogen and hydrogen in the Haber process. In this reaction, finely divided iron is used as a catalyst. N2 (g) + 3H2 (g) Æ 2NH3 (g) • The reacting molecules are adsorbed onto the "active sites" on the

surface of the iron particles.

• Once adsorbed, the bonds in the reacting molecules are weakened. The atoms formed can combine faster as they are close to each other on the iron surface.

• Once the ammonia molecules are formed, they are desorbed from the active sites.

• In this way, the weakening of the triple bond in nitrogen, and the faster

combination of nitrogen and hydrogen atoms, cause an increase in the rate of formation of ammonia.

Seck-tanbc/ rate of reaction adapted from Mr Vincent Tan’s lecture notes 16

Enzymes: Biological Catalysts Enzymes have the following properties: 1. They are made from proteins. 2. They are very specific in their actions. The human body contains many different

kinds of enzymes and each one does a single specific job. Amylases, for example, the enzyme present in saliva, catalyses the reaction which changes starch into sugar.

3. Enzymes are sensitive to temperatures. They operate most effectively at body

temperature (35 – 40 °C). Too low a temperature will render enzymes inactive while too high a temperature will cause enzymes to be denatured.

4. They are sensitive to pH changes. Most enzymes work best under pH 7. Uses of enzymes 1. In the manufacture of wine and other alcoholic drinks, enzymes produced by

yeast (known as zymase) are used to catalyse the conversion of sugar or starch to ethanol (fermentation)

C6H12O6(aq) 2C2H5OH(aq) + 2CO2(g) Glucose solution ethanol carbon dioxide

2. Modern detergents (biological washing powders) contain enzymes which are very

effective in removing stains caused by protein-based substances such as food and blood.

3. Enzymes can be used to make tough meat tender. One such enzyme is found in

the papaya fruit. You can make meat tender by squeezing juice from unripe papaya onto the fresh meat.

4. Antibiotics are compounds that kill harmful bacteria. They are produced by

enzymes in fungi. The first antibiotic, penicillin, was discovered in 1928. Today, penicillin and other antibiotics are extracted on a large scale from special strains of fungi.

Bibliography

• http://en.wikipedia.org/wiki/Catalysis • http://www.chemguide.co.uk/physical/catalysis/introduction.html#top

yeast