Chemistry 18.1

Rates of Reaction 18.1 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.

Collision Theory How is the rate of a chemical change expressed? 18.1

18.1 Collision Theory In chemistry, the rate of chemical change, or the reaction rate, is usually expressed as the amount of reactant changing per unit time.

18.1 Collision Theory A rate is a measure of the speed of any change that occurs within an interval of time. Speed is measured as a change in distance in a given interval of time. A world-class sprinter might cover 100 meters in 11.5 seconds; the speed, or rate, is 100 m/11.5 s = 8.70 m/s.

18.1 Collision Theory Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. As time passes, the amount of reactant (red squares) decreases and the amount of product (blue spheres) increases. Rates of chemical reactions are often measured as a change in the number of moles during an interval of time. Interpreting Diagrams Assuming equal time intervals between the boxes, how can you tell that the rate of conversion of reactant to product is not constant throughout this reaction?

18.1 Collision Theory 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.

Effective Collision 18.1 Collision Theory If colliding particles have enough kinetic energy and collide at the right orientation, they can react to form a new product. a) An effective collision of reactant molecules produces product molecules. b) An ineffective collision of reactant molecules produces no reaction, and the reactants bounce apart unchanged.

Ineffective Collision 18.1 Collision Theory Ineffective Collision If colliding particles have enough kinetic energy and collide at the right orientation, they can react to form a new product. a) An effective collision of reactant molecules produces product molecules. b) An ineffective collision of reactant molecules produces no reaction, and the reactants bounce apart unchanged.

18.1 Collision Theory The minimum energy that colliding particles must have in order to react is called the activation energy. The activation-energy barrier must be crossed before reactants are converted to products. INTERPRETING GRAPHS a. Navigate Which are at a higher energy, the reactants or products? b. Read Is energy absorbed or released in progressing from the reactants to the activated complex? c. Interpret Once the activated complex is formed, will it always proceed to form products? Explain.

The activated complex is sometimes called the transition state. 18.1 Collision Theory An activated complex is an unstable arrangement of atoms that forms momentarily at the peak of the activation-energy barrier. The activated complex is sometimes called the transition state.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Factors Affecting Reaction Rates What four factors influence the rate of a chemical reaction?

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates The rate of a chemical reaction depends upon temperature, concentration, particle size, and the use of a catalyst.

Factors Affecting Reaction Rates Animation 22 Explore several factors that control the speed of a reaction.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Temperature Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Refrigeration Storing foods in a refrigerator keeps them fresh longer. Low temperatures slow microbial action. Meat kept at freezing temperatures has a lifetime of months.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Concentration a. In air, a lighted splint glows and soon goes out. b. When placed in pure oxygen (higher oxygen concentration), the splint bursts into flame. The rate of a reaction depends upon the concentrations of the reactants. a) In air, a lighted splint glows and soon goes out. b) When placed in pure oxygen, the splint bursts into flame. Inferring What accounts for the difference in reactivity?

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Particle Size 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. An explosion destroyed this grain elevator. 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.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates Catalysts A catalyst increases the rate of a reaction by lowering the activation-energy barrier. INTERPRETING GRAPHS a. Navigate How does the catalyst affect the magnitude of the activation energy? b. Read Does the catalyst change the amount of energy released in the reaction? c. Interpret Along which of the two reaction paths are reactants converted more rapidly to products?

Factors Affecting Reaction Rates Simulation 23 Explore the effects of concentration, temperature, and a catalyst on reaction rate.

Factors Affecting Reaction Rates 18.1 Factors Affecting Reaction Rates An inhibitor is a substance that interferes with the action of a catalyst. Antioxidants and antimicrobials used in drying fruits and preserving fruit juices slow the action of microbes and limit contact with air.

18.1 Section Quiz. 18.1.

18.1 Section Quiz. 1. The units below that would be appropriate to measure the rate of a chemical reaction is mmol/s. mol/L. kJ/mol. h/mol.

18.1 Section Quiz. 2. In a chemical reaction, the energy of reactants is always greater than the energy of the products. more than the activation energy. less than the activation energy. less than the energy of the products.

18.1 Section Quiz. 3. An increase in which one of the following will NOT increase the reaction rate? temperature concentration of reactants total mass of reactants surface area of reactants

18.1 Section Quiz. 4. A catalyst works because it lowers the activation energy. increases the temperature. is permanently changed in a reaction. supplies energy to a reaction.

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