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Section 6.3
Nature of Reactions
• Demonstrate factors that influence the direction of a reaction.
• Classify factors that influence the rate of a reaction.
Section 6.3
Nature of Reactions
synthesis: reaction in which two or more substances combine to form a single product.
Section 6.3
Nature of Reactions
equilibrium
dynamic equilibrium
Le Châtelier’s principle
soluble
insoluble
activation energy
concentration
limiting reactant
catalyst
enzyme
inhibitor
External factors modify the direction and rate of chemical reactions.
Section 6.3
Reversible Reactions
• When no net change occurs in the amount of reactants and products, a system is said to be in equilibrium.
• A dynamic equilibrium is a system in which opposite actions are taking place at the same rate.
Section 6.3
Reversible Reactions (cont.)
• The reversible reaction will favor the direction that produces the most stable products, which are those that are least likely to change.
• Le Châtelier’s principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
Section 6.3
Reversible Reactions (cont.)
• A product that does not dissolve in water can be removed if all other products and the reactants dissolve in water.
– A compound is soluble in a liquid if it dissolves in it.
– A compound is insoluble if it does not dissolve in a liquid.
Section 6.3
Reversible Reactions (cont.)
• Adding or removing energy, usually in the form of heat, can also influence the direction of a reaction.
Section 6.3
Activation Energy
• For a reaction to occur between two substances, particles of those substances must collide with each other with enough force to cause a change to take place.
• The amount of energy the particles must have when they collide is called the activation energy of the reaction.
Section 6.3
Speed Rate
• To determine how fast a reaction is taking place, you can measure how quickly one of the reactants disappears or how quickly one of the products appears.
• Most reactions go faster at higher temperatures.
Section 6.3
Speed Rate (cont.)
• Raising the concentration, or amount of the substance present in a certain volume, will speed up a reaction because there are more particles per volume.
Section 6.3
Speed Rate (cont.)
• When the limiting reactant, or the reactant in limited supply, is used up, the reaction stops and no new product is formed.
Section 6.3
Speed Rate (cont.)
• A catalyst is a substance that speeds up the rate of a reaction without being permanently changed or used up itself.
– Biological catalysts are called enzymes.
– A substance that slows a reaction is called an inhibitor.
Section 6.3
Section Assessment
What is a substance that slows a reaction called?
A. inhibitor
B. enzyme
C. limiting reactant
D. insoluble
Section 6.3
Section Assessment
Which of the following does not affect the speed rate of a reaction?
A. raising the temperature
B. raising the concentration
C. removing a catalyst
D. none of the above
Section 6.3
Section Assessment
Lowering concentration ___ the rate of reaction.
A. decreases
B. increases
C. equalizes
D. does not affect
Study Guide 1
Key Concepts
• Chemical equations—used to represent reactions—are written using symbols and formulas for elements and compounds.
• Chemical equations can tell you how elements and compounds change during a reaction and whether a reaction is endothermic or exothermic.
• Equations are balanced by changing coefficients.
• A balanced chemical equation reflects the law of conservation of mass.
Study Guide 2
Key Concepts
• Although thousands of individual chemical reactions are known, most can be classified into five major classes that are based on patterns of behavior of reactants and products.
• The five general classes of reactions are synthesis, decomposition, single-displacement, double-displacement, and combustion.
• Sometimes classes of reactions overlap. For example, some combustion reactions are also synthesis reactions.
Study Guide 3
Key Concepts
• Reversible reactions are those in which the products can react to reform the reactants.
• Equilibrium occurs when forward and reverse reactions take place at the same rate.
• At equilibrium, there is no net change in the amounts of products and reactants.
• According to Le Châtelier’s principle, if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress.
Chapter Assessment 1
In which kind of reaction is energy absorbed?
A. endothermic
B. exothermic
C. esothermic
D. ergothermic
Chapter Assessment 2
What do scientists use to represent the number of parts involved in a chemical reaction?
A. product
B. coefficient
C. reactant
D. symbols
Chapter Assessment 3
What do scientists use to express chemical reactions?
A. word equations
B. chemical equations
C. symbols to indicate the physical state of the substances
D. all of the above
Chapter Assessment 4
A water-based solution is indicated by which symbol?
A. (s)
B. (l)
C. (g)
D. (aq)
Chapter Assessment 5
Which of the following is not a chemical reaction?
A. a piece of wood burning
B. a car rusting
C. an ice cube melting
D. red litmus paper turning blue
STP 1
What is the coefficient of bromine in the equation 2Al(s) + 3Br2(l) → 2AlBr3(s)?
A. 1
B. 2
C. 3
D. 6
STP 2
The law of conservation of atoms requires what in a chemical reaction equation?
A. both side of the equation to contain the same substances
B. the reactants to have the same amount of molecules
C. both sides to have the same amount of atoms of each element
D. the products to have fewer molecules than the reactants
STP 3
How can the rate of a reaction be determined?
A. by how quickly a reactant disappears
B. by how quickly the product disappears
C. by the amount of activation energy it releases
D. by how quickly it reverses direction
STP 4
Reactions that release heat energy are called____.
A. esothermic
B. exothermic
C. endothermic
D. ergothermic
STP 5
Atoms in a chemical reaction do not change but are rearranged.
A. true
B. false
