ERT 316: REACTION ENGINEERINGCHAPTER 3RATE LAWS & STOICHIOMETRY Lecturer: Miss Anis Atikah Ahmad Email: firstname.lastname@example.org Tel: +604 976 3245
Outline • PART 1: Rate Laws • Relative Rates of Reaction • Reaction Order & Rate Law • Reaction Rate Constant, k • PART 2: Stoichiometry • Batch System Stoichiometric Table • Flow System Stoichiometric Table • Calculation for Concentration in terms of Conversion
1. Relative Rates of Reaction Reaction Stoichiometry EXAMPLE If NO2 formed at 4 mol/m3/s (r NO2= 4 mol/m3/s), what is the rate of formation of NO?
1. Relative Rates of Reaction If NO2 formed at 4 mol/m3/s (r NO2= 4 mol/m3/s), what is the rate of formation of NO?
1. Relative Rates of Reaction • The Reaction: is carried out in a reactor. If at a particular point, the rate of disappearance of A is 10 mol/dm3/s, what are the rates of B and C? EXERCISE
1. Relative Rates of Reaction • The relative rates are • Given, the rate of disappearance of A, -rA, is 10mol/dm3/s • Thus, solving the rates of B & C; r A= -10 mol/dm3/s
Rate law is a kinetic expression that gives the relationship between reaction rate, -rA, and concentration. 2. Reaction Order & Rate Law • The reaction rate (rate of disappearance) depends on temperature and composition. • It can be written as the product of reaction rate constant, kAand a function of concentrations (activities) of the reactants involved in the reaction:
Rate law is a kinetic expression that gives the relationship between reaction rate, -rA, and concentration. 2. Reaction Order & Rate Law • For reaction in which the stoichiometric coefficient is 1 for ALL species: we shall delete the subscript on the specific reaction rate, (e.g.; A in kA) to let
2.1 Power Law Models & Elementary Rate Laws • Power Law Model: The rxn is 𝛂 order wrt reactant A AND The rxn is 𝛃 order wrt reactant B The overall order of the reaction, n;
2.1 Power Law Models & Elementary Rate Laws • The unit of the specific reaction, k, will vary with the order of reaction. Products Zero order (n=0) First order (n=1) Second order (n=2) Third order (n=3)
2.1 Power Law Models & Elementary Rate Laws • Elementary reaction: a chemical reaction in which one or more of the chemical species react directly to form products in a single reaction step and with a single transition state. • Elementary rate law: The rxn is said to follow the elementary rate law if the stoichiometic coefficients are IDENTICAL to the reaction order of each species. Products Unimolecular reaction Products Bimolecular reaction Non-elementary rxn But follows the elementary rate law!
2.2 Non-Elementary Rate Laws • Non-elementary rate laws: reactions that do not follow simple rate laws (power rate laws). • Example 1: Homogeneous Rxn The kinetic rate law is: Rxn order: first order wrt to CO, three-halves order wrt Cl2, five-halves order overall. Gas phase synthesis of phosgene
2.2 Non-Elementary Rate Laws Gas-solid catalyzed rxn: Hydrodemethylation of toluene (T) • Example 2: Heterogeneous Rxn The rate of disappearance of toluene per mass of catalyst is: where KB & KT is the adsorption constants. In terms of partial pressure rather than concentrations
2.3 Reversible Reactions ⇌ • For reversible rxn, all rate laws must reduce to the thermodynamic relationship relating the reacting species concentrations at equilibrium. Thermodynamic Equilibrium Relationship
2.3 Reversible Reactions EXAMPLE: combination rxn of 2 mol of benzene to form 1 mol H2 and 1 mol diphenyl. kB ⇌ k-B kB ⇌ symbolically; k-B The rate of disappearance of benzene; OR The reverse rxnbtweendiphenyl & hydrogen; k-B ⇌ The rate of formation of benzene (in reverse direction);
2.3 Reversible Reactions The net rate of formation of benzene is; Multiplying both sides by -1, we obtain the rate law of disappearance of benzene, -rB
2.3 Reversible Reactions Replacing the ratio of the reverse & forward rate law constant by equilibrium constants; where Concentration equilibrium constant
3. The Reaction Rate Constant Arrhenius equation A= preexponential factor or frequency factor E= activation energy, J/mol or cal/mol R=gas constant = 8.314 J/mol-K = 1.987 cal/mol-K T= absolute temperature, K -no of collision -probability that the collision will result in a reaction
3. The Reaction Rate Constant • Activation energy is a measure of the minimum energy that the reacting molecules must have in order for the reaction to occur (energy required to reach transition state). Transition state - no of collision that result in a rxn -total no of collision Energy barier probability that - the collision will result in a rxn Reactants Products
3. The Reaction Rate Constant Taking a natural logarithm; The larger the activation energy, the more temperature sensitive k and thus the reaction rate. • E ⬆, k ⬆, -r = ⬆
4. Batch Systems Stoichiometric Table • Purpose of developing stoichiometric table: • To determine the no of moles of each species remaining at a conversion of X.
4. Batch Systems Stoichiometric Table refers to moles of species reacted or formed • Components of stoichiometric table:
aA + bB cC + dD 4. Batch Systems Stoichiometric Table • Recall from Chapter 2: • Factorizing; moles of A reacted moles of A remaining in the reactor at a conversion of X
4. Batch Systems Stoichiometric Table Moles B reacted, NB Moles B reacted Moles A reacted Moles A reacted Moles C formed, NC Moles D formed, ND
4. Batch Systems Stoichiometric Table moles B remaining in the system, NB moles of B reacted moles of B initially in the system NC moles of C formed ND moles of D formed
4. Batch Systems Stoichiometric Table • Total no of moles per mole of A reacted can be calculated as: where Change in the total number of moles per mole of A reacted
4. Batch Systems Stoichiometric Table Can we express concentration of each species??
4. Batch Systems Stoichiometric Table • Concentrationof each species in terms of conversion can be expressed as: Recall from stoichiometric table
4. Batch Systems Stoichiometric Table EXAMPLE Given the saponification for the formation of soap from aqueous caustic soda & glycerylstearate is: Letting X the conversion of sodium hydroxide, set up a stoichiometric table expressing the concentration of each species in terms of its initial concentration and the conversion.
4. Batch Systems Stoichiometric Table EXAMPLE We know that this is a liquid-phase reaction. Therefore, V=V0
5. Flow Systems Stoichiometric Table • Purpose of developing stoichiometric table: • To determine the effluent flow rate of each species at a conversion of X.
5. Flow Systems Stoichiometric Table • Components of stoichiometric table:
QUIZ 5 • Given a liquid phase reaction: A+ 2B C + D The initial concentration of A and B are 1.8 kmol/m3 and 6.6 kmol/m3 respectively. Construct a stoichiometric table for a flow system considering A as the basis of calculation.
Answer For Quiz 5 A+ 2B C + D Given: From stoichiometry, we know that, Since C & D are products.
Answer for quiz 5 Substituting the numerical values;
6. Concentration in terms of conversion 1. For liquid phase: • Batch System:
6. Concentration in terms of conversion 1. For liquid phase: • Flow System -
6. Concentration in terms of conversion 2. For gas phase: • Batch System Need to substitute V from gas law equation From equation of state; At any time t, At initial condition (t=0) T= temperature, K P= total pressure, atm (1 atm= 101.3 kPa) Z= compressibility factor R= gas constant = 0.08206 dm3-atm/mol-K (1) (2)
6. Concentration in terms of conversion 2. For gas phase: • Batch System Dividing (1) by (2); (1) (2) Recall from stoichiometric table (4) (3) Dividing (4) by NT0 ;