1. Chapter 18 Reaction Rates and Equilibrium 18.1 Rates of Reaction
18.2 The Progress of Chemical
Reactions
18.3 Reversible Reactions
and Equilibrium
18.4 Solubility Equilibrium
18.5 Free Energy and Entropy
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2. Describing Reaction Rates
In chemistry, the rate of a chemical reaction, or the reaction rate, is usually expressed as the change in the amount of reactant or product per unit time.
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3. Describing Reaction Rates
The figure below illustrates the progress of a typical reaction. Over time, the amount of reactant decreases and the amount of product increases.
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4. Describing Reaction Rates
Collision Theory
A model called collision theory is used to relate the properties of particles to the rates of chemical reactions.
According to collision theory, atoms, ions, and molecules can react to form products when they collide if the particles have enough kinetic energy.
Particles that do not have enough energy to react bounce apart unchanged when they collide.
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5. Describing Reaction Rates
The minimum energy that colliding particles must have in order to react is called the activation energy.
You can think of the activation energy for a reaction as a barrier that reactants must cross before products can form.
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6. Describing Reaction Rates
When two reactant particles collide, they may form an activated complex.
An activated complex is an unstable arrangement of atoms that forms for a moment at the peak of the activation-energy barrier.
The activated complex forms only if the colliding particles have enough energy and if the atoms are oriented properly.
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7. Describing Reaction Rates
When two reactant particles collide, they may form an activated complex.
The lifetime of an activated complex is typically about seconds.
Its brief existence ends with the reformation of the reactants or with the formation of products.
Thus, the activated complex is sometimes called the transition state.
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8. Interpret Graphs
The activation-energy barrier must be crossed before reactants are converted to products.
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9. Interpret Graphs
Remember: An endothermic reaction absorbs heat, and an exothermic reaction releases heat.
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10. Factors Affecting Reaction Rates
Factors that can affect the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.
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11. Factors Affecting Reaction Rates
Particle Size
An explosion destroyed this sugar refinery.
The tiny size of the reactant particles (sugar dust) caused the reaction of the sugar with oxygen in the air to be explosive.
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12. Interpret Graphs
The activation-energy barrier for the catalyzed reaction is lower than that of the uncatalyzed reaction.
When the barrier is lower, a greater fraction of reactants have the energy to form products within a given time.
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13. Factors Affecting Reaction Rates
Catalysts
An inhibitor is a substance that interferes with the action of a catalyst.
Some inhibitors work by reaction with, or “poisoning,” the catalyst itself.
Thus, the inhibitor reduces the amount of catalyst available for a reaction.
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14. Factors Affecting Reaction Rates
Catalysts
An inhibitor is a substance that interferes with the action of a catalyst.
Some inhibitors work by reaction with, or “poisoning,” the catalyst itself.
Thus, the inhibitor reduces the amount of catalyst available for a reaction.
Reactions slow or even stop when a catalyst is poisoned.
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15. Key Concepts
In chemistry, the rate of a chemical reaction, or the reaction rate, is usually expressed as the change in the amount of reactant or product per unit time.
Factors that can affect the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.
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16. rate: describes the speed of change over an interval of time
Glossary Terms
rate: describes the speed of change over an interval of time
collision theory: atoms, ions, and molecules can react to form products when they collide, provided that the particles have enough kinetic energy
activation energy: the minimum energy colliding particles must have in order to react
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17. inhibitor: a substance that interferes with the action of a catalyst
Glossary Terms
activated complex: an unstable arrangement of atoms that exists momentarily at the peak of the activation-energy barrier; an intermediate or transitional structure formed during the course of a reaction
inhibitor: a substance that interferes with the action of a catalyst
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18. Chemical Reactions, Matter, and Energy
BIG IDEA
Chemical Reactions, Matter, and Energy
The rate of a chemical reaction can be controlled by adjusting temperature, concentration, or particle size.
Adding a catalyst speeds up a reaction by lowering the activation energy.
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