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Chapter 6: Kinetics
6.1 Rates of reaction
- Rate of reaction = decrease in concentration of reactants or
increase in concentration of products per unit time.
- Units of rate of reaction = mol dm–3 s–1.
- Rate of reaction at time t = gradient of tangent to curve [R] or [P] vs time at time.
6.2 Collision theory
- Absolute temperature is a measure of average kinetic energy.
- Activation energy, Ea, is the minimum KE particles must have in order to react.
- Collision theory: collisions between reactant particles will lead to reaction when:
- i) the particles have KE > Ea and
- ii) the particles have the correct collision geometry.
- Catalysts speed up reactions by providing an alternate reaction route with a lower activation energy. Enzymes are biological catalysts.
16.1 Rate expression
- For a reaction with reactants A and B:
Rate = k [A]m [B]n
where k = rate constant
m and n are the orders of reaction with respect to A and B respectively
m + n = overall order of reaction.
- The units of k depend on the order of the reaction.
- Constant half-life ¬ ® first order reaction.
16.2 Reaction mechanism
- Many reactions proceed in a series of small steps known as elementary steps.
- Molecularity = the number of particles taking part in an elementary step.
- The rate-determining step = the slowest step in the reaction.
- The order of the reaction is determined by the molecularity of the rate-determining step.
16.3 Activation energy
- The Arrhenius equation (given in the Data booklet) shows the temperature dependence of the rate constant.
- The Arrhenius plot can be used to calculate Ea.
Get it straight
- All substances at the same temperature have the same average kinetic energy.
- The rate equation cannot be predicted from the overall stoichiometry of the reaction.
- The units of the rate constant k vary – they depend on the order of the reaction.
Unit 5: Kinetics important definitions
Absolute temperature The temperature in kelvin (K). It is proportional to the average kinetic energy of the particles in the substance.
Activation energy Ea The minimum value of kinetic energy that particles must possess in order to react.
Arrhenius constant The constant, A, in the Arrhenius equation. It is sometimes called the frequency factor and takes into account the frequency with which successful collisions occur, based on collision geometry and energy requirements.
Arrhenius equation The equation which shows the relationship between the rate constant, the activation energy and the temperature of a reaction: k = A e –Ea/RT. It shows how the temperature dependence of the rate constant depends on the activation energy for the reaction. This equation is given in the IB Data booklet.
Arrhenius plot A graph of ln k against 1 / T, which can be used to derive the activation energy for a reaction. The graph is a straight line with gradient – Ea / R.
Bimolecular reaction An elementary step that involves two reactant particles.
Catalyst A catalyst is a substance that increases the rate of a reaction without itself undergoing chemical change. Most catalysts work by providing an alternate reaction route of lower activation energy.
Collision geometry Refers to the orientation of the particles with respect to each other when they collide. It is one of the criteria for a collision between particles to be successful and lead to reaction.
Collision theory The theory that particles must collide in order to react. Collisions will only lead to reaction when the particles have values for kinetic energy greater than the activation energy, and the correct collision geometry.
Elementary step A single step in a reaction usually involving a small number of particles that collide. The sequence of elementary steps gives the reaction mechanism.
Enzyme A biological catalyst. All enzymes are protein molecules which form a temporary association with the reactant, known as the substrate.
First order reaction A reaction in which the sum of the powers of the concentrations of the reactants in the rate expression is one.
Half-life The time it takes for a concentration of reactant to decrease to half its original value. For first order reactions, the half-life is constant.
Initial rates method A method to determine the order of a reaction by finding out the effect of changing concentration of reactant on reaction rate. A number of experiments on the same reaction are carried out, with different initial concentrations of reactants. The initial rate of each reaction is measured and used as the basis of comparison.
Kinetic energy The energy associate with movement of particles. Its value depends on the mass and the square of the velocity of the particles.
Maxwell–Boltzmann distribution curve The distribution curve of kinetic energy values for the particles in a sample of a gas. The area under the curve represents the total number of particles in the sample.
Molecularity The number of reactant particles taking part in an elementary step.
Order of reaction The overall order of a reaction is the sum of the powers of the concentrations of the reactants in the rate expression. The order with respect to a single reactant is the power to which its concentration is raised in the rate expression.
Quenching A technique used to stop a reaction at the moment a sample is withdrawn. It is used to analyze the concentrations in the reaction mixture at a known interval of time.
Rate The reciprocal of time. It has the units sec–1.
Rate constant This is the constant in a rate equation and it has the symbol k. Its value depends on the reaction and on the temperature. Its units vary, depending on the order of the reaction.
Rate determining step The elementary step in a reaction that is the slowest step. It determines the overall rate of the reaction.
Rate expression Also known as the rate law. It is an equation which shows the relationship between the rate of a reaction and the concentrations of reactants. It is derived experimentally as it depends on the reaction mechanism, and cannot be deduced from the stoichiometry of the equation.
Rate law Also known as the rate expression. It is an equation which shows the relationship between the rate of a reaction and the concentrations of reactants. It is derived experimentally as it depends on the reaction mechanism, and cannot be deduced from the stoichiometry of the equation.
Rate of reaction The increase in concentration of products or the decrease in concentration of reactants per unit time. It has the units mol dm–3 s–1.
Reaction intermediates Species which are both formed and used during a reaction mechanism. They do not appear in the overall reaction equation.
Reaction mechanism The sequence of elementary steps which show how reactants are converted into products.
Second order reaction A reaction in which the sum of the powers of the concentrations of the reactants in the rate expression is two.
Transition state This is sometimes referred to as the activated complex. A short-lived reaction intermediate which breaks down to form products.
Unimolecular reaction An elementary step that involves a single reactant particle.
Zero-order reaction A reaction in which the sum of the powers of the concentrations of the reactants in the rate expression is zero. It is relatively uncommon. A reaction is zero order with respect to a particular reactant when that reactant does not take part in the rate-determining step.