FACTORS AFFECTING REACTION RATES

Transcription

1 QUESTIONSHEET 1 a) Rate at which the molar concentration of a reactant decreases with time FACTORS AFFECTING REACTION RATES Or r = - dc dt where dc is a concentration change occurring in a time interval dt b) (i) III Half the molar amount of acid, half the amount of CO 2 Less concentrated acid, lower rate of reaction / gradient less steep (ii) II Greater surface area, greater rate of reaction / steeper gradient (iii) IV Half the amount of acid, half the amount of CO 2 Higher temperature, greater rate of reaction / steeper gradient (iv) V One quarter the amount of acid, one quarter the amount of CO 2 Less concentrated acid, lower rate of reaction / gradient less steep c) Pressure Catalysts Light

2 QUESTIONSHEET 2 COLLISION THEORY a) Colliding molecules must have a certain minimum kinetic energy and orientation must be correct Head-on or sideways collision is productive but glancing collision is not Maximum 3 marks b) (i) Reason 1 Average / mean kinetic energy increases with temperature collisions occur with greater energy / are more likely to lead to the breaking of chemical bonds Or more molecules have acquired the necessary activation energy Reason 2 Average velocity increases with temperature collision frequency / number of collisions per second increases (ii) Collisions occurring with greater energy / number of molecules with activation energy c) (i) More molecules per unit volume collision frequency increases / more collisions per unit time (ii) In concentrated HCl(aq), there are more H + ions per unit volume in unit time, a given surface area of iron is struck by more H + ions (iii) In the gas phase, molecules are relatively far apart Successful gas phase reaction depends on a multi-body high energy collision / simultaneous collision of one N 2 and at least two H 2 molecules Such collisions are rare / most collisions are simpler than this In the adsorbed state, nitrogen and hydrogen molecules / atoms are closer together Maximum 3 marks

3 QUESTIONSHEET 3 MAXWELL-BOLTZMANN DISTRIBUTION I - Effect of temperature a) (i) Vertical axis fraction / number of molecules Horizontal axis (kinetic) energy (ii) Because there are no molecules with no energy (iii) M marked on horizontal axis below maximum (iv) Because they have recently been struck by several other molecules travelling in approximately the same direction (v) Because there is no theoretical upper limit to the energy that molecules may possess (vi) Vertical should be drawn at E a and area to RHS shaded This area is very small compared with the total area under curve only a small proportion of molecules is theoretically able to react together on collision b) (i) New curve should be lower and maximum displaced to the right (ii) Mean kinetic energy increases with temperature maximum is displaced to the right Curve must be lower because area under the curve must be the same as before since this is related to the number of molecules in the sample which is constant Maximum 4 marks (iii) Shaded area to right of vertical at E a is approximately twice the original approximately twice as many molecules have the necessary activation energy

4 QUESTIONSHEET 4 MAXWELL-BOLTZMANN DISTRIBUTION II - Effect of concentration & catalysts a) (i) New curve should be higher but maximum must correspond to the same kinetic energy as the original (ii) Mean kinetic energy No different because the temperature is constant Area under the curves Greater for c 2 because there are more molecules in the sample (iii) Vertical should be drawn to the right of the maximum and labelled E a Areas under the curves to the right of the vertical should be compared That for c 2 > that for c 1 more molecules have the necessary activation energy Maximum 3 marks b) (i) A substance which can alter / increase the rate of a chemical reaction but which can be recovered chemically unchanged / unchanged in mass at the end of the reaction (Do not accept: plays no part in the reaction ) (ii) Catalyst provides a different route / mechanism with a lower activation energy which should be marked on the graph to the left of the original but to the right of the maximum Areas under the curves to the right of the original E a and new E a should be compared (iii) New shaded area for b) (ii) is large compared with the new shaded area for a) (iii) in the presence of a catalyst many more molecules have the necessary activation energy

5 QUESTIONSHEET 5 ACTIVATION ENERGY AND REACTION PROFILES a) The minimum energy which must be supplied to reactant molecules over and above that which they already possess so that they are theoretically able to react together on collision b) (i) & (ii) Potential energy/ energy/enthalpy Reactants E a H Products Reaction coordinate/reaction path/distance along reaction path (iii) A transition state / activated complex / reaction intermediate (iv) Profile lower Profile must start and end in the same places as the uncatalysed one Two humps c) Reactants E a Products H Products shown at higher energy level than reactants

6 QUESTIONSHEET 6 CATALYSIS I a) (i) Oxidation state change so accepts electrons (ii) 2Fe 3+ (aq) + 2I - (aq) 2Fe 2+ (aq) + I 2 (aq) S 2 O 8 2- (aq) + 2Fe 2+ (aq) 2SO 4 2- (aq) + 2Fe 3 (aq) in either order (iii) Without the catalyst involves reaction of similar charged ions that repel Catalysed stages involves reaction of oppositely charged ions (iv) energetically feasible transition between oxidation states so can accept and donate electrons in successive stages (v) homogeneous in the same state as reactants/involved in reaction stages heterogeneous in different state to reactants/surface catalyst b) Uses less energy/works at a lower temperature/lower energy costs Increases rate/yield c) Esterification acid Fermentation enzyme

7 QUESTIONSHEET 7 CATALYSIS II a) MnO 4- (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) C 2 O 42 (aq) - 2CO 2 (g) + 2e b) Large activation energy c) (i) increases (ii) meaning the reaction is catalysed by one of the products formed catalyst manganese (II) ion/mn 2+ reason the rate increases as the reaction proceeds d) (i) Provides alternative reaction route with lower activation energy allowing more reactant particles to react/achieve the necessary energy for reaction no marks for lowers activation energy (ii) form powder reason greater surface area

8 QUESTIONSHEET 8 CATALYTIC CONVERTERS a) (i) Carbon monoxide Incomplete combustion of hydrocarbons Nitrogen monoxide Combination of N 2 and O 2 from the air (ii) Formation of low level ozone and photochemical smog b) (i) Platinum / palladium / rhodium (ii) Heterogeneous catalysis (iii) To offer a large surface area to the reacting gases c) (i) I 50NO(g) + 2C 8 H 18 (g) 16CO 2 (g) + 18H 2 O(g) + 25N 2 (g) II 2CO(g) + 2NO(g) 2CO 2 (g) + N 2 (g) (ii) ON of C in CO = +2 (½) ON of C in CO 2 = +4 (½) ON of N in NO = +2 (½) ON of N in N 2 = 0 (½) (iii) NO is reduced CO is oxidised d) The catalyst becomes poisoned / less efficient e) Organic sulfur compounds in the fuel on combustion produce sulfur dioxide / SO 2 In the presence of a catalytic converter, this is reduced to hydrogen sulfide / H 2 S

9 QUESTIONSHEET 9 TEST QUESTION I a) (i) A substance which increases / alters rate of reaction without being consumed / chemically altered Homogeneous acts in the same phase as the reactants (ii) It effectively lowers the activation energy by providing an alternative reaction pathway which proceeds via an intermediate b) (i) [CH 3 COOH] mol dm Time/s Correctly labelled axes Suitable scales Correctly plotted points Curve of best fit (ii) Tangent drawn at time = 0 Correct calculation, e.g /300 = 9.3 x 10-5 mol dm -3 s -1 Allow 8.75 x 10-5 to 9.75 x 10-5 (iii) Increasing temperature skews the distribution to the right / increases range of kinetic energies of particles / shifts maximum kinetic energy of particles to right This increases the proportion of particles possessing the necessary activation energy which in turn increases the rate of reaction

10 QUESTIONSHEET 10 TEST QUESTION II a) (i) Platinum-rhodium (ii) Increases surface area of catalyst hence increases its efficiency (iii) Heterogeneous (iv) Stage 1 Adsorption Stage 2 Reaction Stage 3 Desorption (v) Poisons the catalyst by blocking up active sites b) (i) Fraction of molecules with energy E E a Molecular energy E (ii) Activation energy E a marked Shaded area labelled (iii) Catalyst provides an alternative route with a lower activation energy the fraction of molecules which may react is increased