Geochemistry on the Earth’s surface for analytical geochemists 3b. Speed of processes Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie č. CZ.02.2.69/0.0/0.0/16_018/0002593 Questions 1. How can we determine which processes will take place spontaneously in nature? 2. What will tell us in which direction? 3. From what can we determine the equilibrium state of the system/process? 4. Which processes will not reach equilibrium, but will run (more or less) completely? Reaction kinetics • It studies the rate of reactions that are feasible from thermodynamic point of view. • The rate of a reaction is basically the rate at which the reactants are consumed and the products are formed. • How reactions occur is explained by the collision theory Collision theory • For the reaction to take place, several conditions must be met: 1. The reactant particles must collide. 2. The colliding particles must have sufficient energy to react. 3. In some cases, particles need to collide with a specific orientation (e.g., a functional group). Activation energy • The minimum total kinetic energy that the particles must have for a reaction to occur. • Each reaction has a different value of EA. • In the case of slow reactions, the activation energy is much higher than the average energy of the particles – collisions rarely lead to the reaction. • Sometimes the activation energy can be supplied externally as an initiator, and then the reaction continues to be driven by the energy released during it. Factors affecting reaction rate • What factors will play a role in the final rate of chemical reactions? • How will this be reflected in geological processes? Factors affecting reaction rate • There are several, but 4 generally apply to all reactions: 1. Physical nature of reactants. 2. Concentration. 3. Temperature. 4. The presence of catalysts. Physical nature of reactants • State, but also the size of the particles. • Reaction rate: gas > liquid > solid • Solids react on the surface: – The larger the surface, the faster the reaction. – The smaller the grains, the faster the reaction. • Ex.: Consider the burning of the same amount of wood in different forms Increase in surface area = increase in contact between reactants Increase in burning rate Trunk - boards - chips - sawdust Concentration • An increase in reactants leads to an increase in the reaction rate. • Substances burn faster in pure oxygen than in air. • A higher concentration increases the chance of particle collision. • The effect of concentration varies depending on the substance, there is no general relationship and each substance needs to be tested experimentally. • E.g.: Influence of the content of CO2, O2 or organic acids in the soil on weathering and soilforming processes. Temperature • Increases the probability of collisions because the particles have higher kinetic energy. • Particles with higher energy are closer to exceeding the activation energy (EA). • The reaction rate roughly doubles for every 10°C. • Ex.: Food is refrigerated to slow down decomposition processes. • Weathering in cold/warm climates. The presence of catalysts • A catalyst is a substance that increases the rate of a reaction but is itself unchanged after the reaction is complete. • They provide alternative reaction pathways with lower activation energy. • It is part of the reaction in the same way that a table is part of the lunch – it provides space and the right conditions, but the reaction does not consume it. Catalysis Homogeneous catalyst • Same phase as the reactants, usually forming an intermediate. cat A + B + cat -> A–B -> product + catalyst Heterogeneous catalyst • A separated phase, usually a solid. • It is assumed to provide the environment for the reaction. – The reactants bind to the surface of the solid phase at a certain angle and thus facilitate the reaction. Use of catalysts • Intensive use in the chemical industry in the production of chemicals -> low energy consumption during reactions is economically very advantageous. • High temperatures are demanding and expensive! • Exhaust gas cleaners in combustion engines of cars. • Enzymes are important in biochemistry. • Ozone depletion. SPEED OF PROCESSES A natural process • We observe that the concentration of a substance in a solution changes over time • There is a process going on • How can I find out and express its speed? Process speed • We usually use the change in content over time – concentration, weight, amount of substance, volume d[A]/dt • The tangent of the concentration-time curve 0th order reaction A => B 0th order reaction Adapted from Ryan (2014) 1st order reaction A => B 1st order reaction Adapted from Ryan (2014) 2nd order reaction 2A => B A + B => C 2nd order reaction Adapted from Ryan (2014) Dependence of k on temperature Adapted from Ryan (2014) Retention time • Inverse of the rate constant k = 1/τ CALCITE DISSOLUTION Example process Dissolution/precipitation Ω = 𝑄 𝐾 = 𝐶𝑎2+ 𝐶𝑂3 2− 𝐾 An empirical model • They can vary significantly • Effect of particle size or mineral surface condition? Mechanical model • The dissolution mechanism is pH dependent (EditedofParkhurstetal.al.1979) pH dependentindependent of pH Processes on the surface of calcite −++ +⎯→⎯+ 3 2 3 1 HCOCaHCaCO k −+ +⎯→⎯+ 3 2 323 22 HCOCaCOHCaCO k +−+ +⎯→⎯+ HCaCOHCOCa k 33 2 4 −−++−+ ++→++⎯→⎯+ OHHCOCaHCOCaOHCaCO k 3 22 3 2 23 3 1. 2. 3. 4. Driven by diffusion of H+ to the surface pH < 5 Controlled by diffusion of products from the surface pH > 5 A RESERVOIR AS A REACTOR Extension to the natural environment Rezervoár ByWilliamWarby-Flickr,CCBY2.0, https://commons.wikimedia.org/w/index.php?curid=41817787 PhotobyAdamVradenburg. Reservoir balance 𝐴 = σ𝑖 𝐼 + σ𝑖 𝑃 − σ𝑖 𝑂 − σ𝑖 𝐿 A … increase/decrease of substance in the system I … input P … production O ... output L … loss/consumption Balance • If A is zero, the system is in dynamic equilibrium (steady state) • Will the content of the system grow? • Will system content go down? • How long will it take to reach balance? Retention time in the reactor • The average time spent by the particle in the system • At a steady state, transport by advection only 𝜏 = celková hmota hmotová výměna = 𝑀𝑡𝑜𝑡 Τ𝑑𝑀𝑖𝑛(𝑜𝑢𝑡) 𝑑𝑡 = 𝑉 𝐹 τ … retention time M tot … total content of the reactor M in(out) … quantity entering/exiting the reactor V ... reactor volume F … flux to/from the reactor (they are in equilibrium) Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie č. CZ.02.2.69/0.0/0.0/16_018/0002593 Resources • Images without credit are mine or with a free license. • RYAN, Peter Crowley. Environmental and lowtemperature geochemistry. Chichester, West Sussex, UK: Wiley Blackwell, 2014. ISBN 978-1- 118-86735-8.