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Reaction Rates & Equilibrium. Chemistry Chapter 18. Section 1 - Rates of Reaction. Introduction.
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Reaction Rates & Equilibrium Chemistry Chapter 18
Section 1 - Rates of Reaction Introduction • The heat given off by the corrosion reaction of an iron-magnesium alloy with salt water can produce a hot meal. The rate of reaction is increased by adding salt water, so heat is produced rapidly. You will learn some ways in which the rate of a reaction can be increased.
18.1 1. Collision Theory • A rate is a measure of the speed of any change that occurs within an interval of time. • In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time. • Rates of chemical reactions are often measured as a change in the number of moles during an interval of time.
18.1 Collision Theory (cont.) • According to collision theory, atoms, ions, and molecules can react to form products when they collide with one another, provided that the colliding particles have enough kinetic energy. • An effective collision of reactant molecules has sufficient kinetic energy and therefore produces a product. • An ineffective collision of reactant molecules does not have sufficient kinetic energy and therefore produces no reaction, and the reactants bounce apart unchanged. Effective Collision vs. Ineffective Collision
18.1 Collision Theory (cont.) • The minimum energy that colliding particles must have in order to react is called the activation energy. • The activation energy is the barrier that reactants must cross to be converted to products. • When two reactant particles with the necessary activation energy collide, a new entity called the activated complex may form.
18.1 Collision Theory (cont.) • An activated complexis an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. • Only forms if the colliding particles have sufficient energy and if the atoms are oriented properly. • The lifetime of an activated complex is typically about 10-13 s and ends with the re-formation of the reactants or with the formation of the products. • The two outcomes are equally likely, thus activated complex is sometimes called the transition state.
18.1 2. Factors Affecting Reaction Rates • Every chemical reaction proceeds at its own rate. • The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst. • Temperature • Usually, raising the temperature speeds up the reactions. • Increasing temperature increases both the frequency of collision and the number of particles that have enough kinetic energy to slip over the activation-energy barrier to become products. • Storing foods in a refrigerator keeps them fresh longer. • Low temperatures slow microbial action.
18.1 Factors Affecting Reaction Rates In air, a lighted splint glows and soon goes out. • Concentration • The number of particles in a given volume affects the rate at which reactions occur. • Cramming more particles into a fixed volume increases the concentration of reactants and consequently the frequency of collision. • Increased collision frequency leads to higher reaction rate. When placed in pure oxygen (higher oxygen concentration), the splint bursts into flame.
18.1 Factors Affecting Reaction Rates (cont.) • Particle Size • The total surface area of a solid or liquid reactant affects the reaction rate. • One way to increase surface area of solid reactants is to dissolve them. • In a solution, particles are separated and more accessible to other reactants • Another way is to grind the reactant into a fine powder. • Small dust-like particles can be dangerous when suspended in air. The minute size of the reactant particles (grain dust), and the mixture of the grain dust with oxygen in the air caused the reaction to be explosive, destroying the grain elevator.
18.1 Factors Affecting Reaction Rates (cont.) • Catalysts • A catalyst is something that speeds up the reaction without being used up during the reaction. • They permit reactions to proceed along a lower energy path. • With a lower activation-energy barrier, more reactants have the energy to form products within a given time. • Because a catalyst is not consumed, it does not appear as a reactant or product in a chemical equation and is written above the yield arrow. 2 H2(g) + O2(g) Pt2 H2O(l)
18.1 Factors Affecting Reaction Rates • An inhibitor is a substance that interferes with the action of a catalyst. • Some inhibitor molecules work by reacting with, or “poisoning”, the catalyst itself. • Thus the inhibitor reduces the amount of functional catalyst available. • Reactions slow or even stop when a catalyst is poisoned Antioxidants and antimicrobials used in drying fruits and preserving fruit juices slow the action of microbes and limit contact with air.
