18.1 rates of reaction > 1 copyright © pearson education, inc., or its affiliates. all rights...

18.1 Rates of Reaction18.1 Rates of Reaction > >

1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Chapter 18Reaction Rates and Equilibrium

18.1 Rates of Reaction

18.2 The Progress of Chemical Reactions18.3 Reversible Reactions and Equilibrium18.4 Solubility Equilibrium18.5 Free Energy and Entropy



How can rusting be used to cook a meal?

There are products that use the rusting of an iron-magnesium alloy to heat packaged food. These products are known as Meals Ready to Eat, or MREs for short.

CHEMISTRY & YOUCHEMISTRY & YOU



Describing Reaction RatesDescribing Reaction Rates

Describing Reaction Rates

How is the rate of a chemical reaction expressed?




Describing Reaction Rates

How is the rate of a chemical reaction expressed?

– When you strike a match, it erupts into flame almost instantly and burns quickly.

– Millions of years were required for plants buried beneath Earth’s surface to be converted to coal.

• The speed of chemical reactions can vary from very fast to extremely slow.




A rate is a measure of how much something changes within a specified amount of time.

Rates of Change




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.




The figure below illustrates the progress of a typical reaction. Over time, the amount of reactant decreases and the amount of product increases.




A model called collision theory is used to relate the properties of particles to the rates of chemical reactions.

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.

Collision Theory




An effective collision of oxygen and hydrogen molecules produces water molecules.




An ineffective collision of oxygen and hydrogen molecules produces no reaction; the reactants bounce apart unchanged.




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.




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.





• 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.





• The lifetime of an activated complex is typically about 10-13 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.



The activation-energy barrier must be crossed before reactants are converted to products.

Interpret GraphsInterpret Graphs



Remember: An endothermic reaction absorbs heat, and an exothermic reaction releases heat.





Collision theory explains why some reactions are extremely slow at room temperature.

• Carbon and oxygen react when charcoal burns, but the reaction has a high activation energy.

• The O—O and C—C bonds must be broken to form the activated complex.

• At room temperature, the collisions of oxygen and carbon molecules are not energetic enough to break the bonds.

• Thus, the reaction rate of carbon with oxygen at room temperature is essentially zero.



What factor determines whether a molecular collision results in a reaction?



The molecules must collide with enough energy in order to react. The minimum amount of energy needed is called the activation energy.

What factor determines whether a molecular collision results in a reaction?



Factors Affecting Reaction RatesFactors Affecting Reaction Rates

Factors Affecting Reaction Rates

What four factors influence the rate of a chemical reaction?

• By varying the conditions, you can modify the rate of almost any reaction.




Factors that can affect the rate of a chemical reaction are temperature, concentration, particle size, and the use of a catalyst.




Temperature

• Usually, raising the temperature speeds up a reaction.

• Lowering the temperature usually slows down a reaction.




Temperature

• The frequency of collisions increases along with the percentage of particles that have enough kinetic energy to slip over the activation-energy barrier.

• Thus, an increase in temperature causes products to form faster.

At higher temperatures, particles move faster.




Concentration

• Cramming more particles into a fixed volume increases the concentration of reactants, and, thus, the frequency of collision.

• Increased collision frequency leads to a higher reaction rate.

The number of particles in a given volume affects the rate at which reactions occur.




The lighted splint glows in air and soon dies out because air is only 20% oxygen.

When the glowing splint is plunged into pure oxygen, it immediately bursts into flame. The increased concentration of oxygen greatly speeds up the combustion reaction.




The total surface area of a solid or liquid reactant affects the rate of a reaction.

Particle Size




• The smaller the particle size, the greater the surface area is for a given mass of particles.


Particle Size




• The smaller the particle size, the greater the surface area is for a given mass of particles.

• The result of an increase in surface area is an increase in the frequency of collisions and the reaction rate.


Particle Size




Only atoms at the surface of the metal are available for reaction.

Dividing the metal into smaller pieces increases the surface area and the number of collisions.

When a piece of magnesium is placed in dilute acid, hydrogen ions can collide with magnesium atoms.

Mg(s) + 2H+(aq) Mg2+(aq) + H2(g)




Another way to increase the surface area of solids is to dissolve them.

• In a solution, particles are separated and more accessible to other reactants.

Particle Size




• Small dustlike particles, however, can be dangerous when suspended in air.

You can also increase the surface area of a solid by grinding it into a fine powder.

Particle Size




An explosion destroyed this sugar refinery. Particle Size

The tiny size of the reactant particles (sugar dust) caused the reaction of the sugar with oxygen in the air to be explosive.




Catalysts

Increasing the temperature is not always the best way to increase the rate of a reaction. A catalyst is often better.




Catalysts


• Recall that a catalyst is a substance that increases the rate of a reaction without being used up during the reaction.




Catalysts


• Recall that a catalyst is a substance that increases the rate of a reaction without being used up during the reaction.

• Catalysts permit reactions to proceed along a lower energy path.



• When the barrier is lower, a greater fraction of reactants have the energy to form products within a given time.


The activation-energy barrier for the catalyzed reaction is lower than that of the uncatalyzed reaction.



When salt water is added to the metal alloy in an MRE, the rate of the rusting reaction increases, and heat is produced rapidly. Which factor that can affect reaction rates is being applied in this situation?




When salt water is added to the metal alloy in an MRE, the rate of the rusting reaction increases, and heat is produced rapidly. Which factor that can affect reaction rates is being applied in this situation?

Salt acts as a catalyst for the reaction between the metal and water, speeding up the reaction without being consumed.





Catalysts

• The rate of reaction of hydrogen and oxygen at room temperature is negligible.

• But with a small amount of platinum (Pt) as a catalyst, the reaction is rapid.

2H2(g) + O2(g) 2H2O(l)Pt




Catalysts

• A catalyst is not consumed during a reaction. • Therefore, it does not appear as a reactant in

the chemical equation.• Instead, the catalyst is often written above the

yield arrow, as in the equation above.

2H2(g) + O2(g) 2H2O(l)Pt




At normal body temperature (37C), reactions in the body would be too slow without catalysts.

• The catalysts that increase the rates of biological reactions are called enzymes.

• When you eat a meal containing protein, enzymes in your digestive tract help break down the protein molecules in a few hours.

Catalysts




An inhibitor is a substance that interferes with the action of a catalyst.

Catalysts




• Some inhibitors work by reacting with, or “poisoning,” the catalyst itself.


Catalysts




• Some inhibitors work by reaction with, or “poisoning,” the catalyst itself.

• Thus, the inhibitor reduces the amount of catalyst available for a reaction.


Catalysts




• 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.


Catalysts



Which of the following factors could be increased in order to decrease a reaction rate?

A.Catalyst concentration

B.Concentration

C.Temperature

D.Particle size



A.Catalyst concentration

B.Concentration

C.Temperature

D.Particle size

Which of the following factors could be increased in order to decrease a reaction rate?



Key ConceptsKey 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.



Glossary TermsGlossary 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



Glossary TermsGlossary 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



Chemical Reactions, Matter, and Energy

BIG IDEABIG IDEA

• 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.



END OF 18.1END OF 18.1

18.1 rates of reaction > 1 copyright © pearson education, inc., or its affiliates. all rights...

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