Chemical Reactions

Chapter 7, Section 1 - The Nature of Chemical Reactions                                                                                                      

Chemical reactions happen when substances change to form new substances.
  • Reactant - A substance that participates in a chemical reaction.
  • Product - A substance that forms in a chemical reaction.
In a chemical reaction, the atoms are rearranged.
  • Atoms are not created or destroyed.
  • There are the same number and types of atoms before and after the reaction.
Chemical reactions always involve changes in energy.
  • Energy must be added to break bonds.
    • This can be heat, light, sound or electricity.
    • The reaction occurs when molecules collide with enough energy to break bonds.
  • Forming new bonds releases energy.
    • This can be heat, light, sound or electricity - depending on the reaction.
  • Energy is conserved.  (Energy is not created or destroyed.)
    • Chemical Energy - Energy that is stored in chemical bonds.
      • It is released when new bonds are formed.
  • Exothermic Reaction - A chemical reaction in which energy is released as heat.
    • Respiration is exothermic - it creates our body heat.
  • Endothermic Reaction - A chemical reaction that requires energy to occur.
    • Photosynthesis is endothermic - it requires energy from sunlight.

    Pg. 224 #1-6.

Chapter 7, Section 2 - Chemical Equations                                                                                                                                 

Chemical Equations use symbols to represent chemical reactions and to show the relationship between reactants and products.

2H2  +  O2  =>  2H2O

Chemical equations must be balanced.  The same number and type of atoms on each side of the arrow.  (Law of Conservation of Mass.)

Mole Ratio - In a chemical reaction, the proportion of reactants and products.  (In other words, how many moles of each reactant is needed to make a certain amount of product.)
  • A balanced chemical equation tells you the mole ratio of the reactants and products.
  • The mole ratio is the same, no matter how much of the reactants are actually combined.
  • Law of Definite Proportions
    • A compound always contains the same elements in the same proportions (regardless of how it is made or how much of it is formed).
  • Mole ratios tell you the relative amounts of reactants and products.

Converting mole ratio to mass.
  • Multiply the mole ratio by the molecular masses (see figure at right).

    Pg. 229 #1-5.

Chapter 7, Section 3 - Reaction Types                                                                                                                                       

You can use patterns to identify kinds of chemical reactions and to predict the products.

Synthesis Reaction - Two or more substances combine to form a new substance.  (A + B => AB)

2H2  +  O2  =>  2H2O

2Na  +  Cl2  =>  2NaCl

Decomposition Reaction - A compound breaks down into two or more simpler substances.  (AB => A + B)

2H2O  =>  2H2  +  O2

Combustion Reaction - The oxidation reaction of an organic compound, in which heat is released.
Oxygen is always a reactant in combustion reactions.

CH4  +  2O2  =>  CO2  + 2H2O

Organic Compound - A covalently-bonded compound that contains carbon (except for carbonates and oxides).

If there is not enough oxygen present, CO is formed.  CO is colorless, odorless and poisonous!

Single-Displacement Reaction - One element takes the place of another in a compound.  (AX + B => A + BX)

3CuCl2  +  2Al  =>  2AlCl3  +  3Cu

The more-reactive element tends to take the place of the less-reactive one.
All alkali metals undergo single-displacement reactions with water.

2K  +  2H2O  =>  2KOH  + H2                        (Think of it like:   2K  +  2HOH  =>  2KOH  + H2  )

Double Displacement Reaction - Two compounds exchange ions/atoms.  (AX + BY => AY + BX)

Pb(NO3)2  + K2CrO4  =>  PbCrO4  +  2KNO3

Double-displacement can form...
  • a gas
  • a solid precipitate
  • a covalent compound

Free-radical reactions and redox reactions can be understood as changes in the numbers of electrons that atoms have.

Free Radical - An atom or group of atoms with one unpaired electron.
Electrons tend to form pairs with other electrons, so free radicals are very reactive (they quickly form covalent bonds).

Oxidation-Reduction (Redox) Reaction - Any chemical reaction in which one substance loses electrons (oxidation) and another gains electrons (reduction).
Oxidation and reduction always happen together.

Fe  --in water-->  Fe+3 ions    (oxidation)
O --in water-->  O-2 ions       (reduction)
Fe+3  +  O-2  =>  Fe2O3 (rust)

Some redox reactions do not involve ions...
Oxidation = gain O / lose H
Reduction = gain H / lose O
In combustion, carbon atoms are oxidized (form CO2), oxygen atoms are reduced (form H2O).

    Pg. 237 #1-6, 8.

Chapter 7, Section 4 - Reaction Rates and Equilibrium                                                                                                                

Chemical reactions happen when particles collide with enough force to break/form bonds.  These reactions happen at different rates.

The rate of a chemical reaction depends on the amount of contact between particles.
Increasing the amount of contact between particles will increase the rate of (speed up) the reaction.
Decreasing the contact will decrease the rate of (slow down) the reaction.

Factors that affect reaction rates include...
  • Temperature
    • Higher temperature speeds up reaction rate.
      • Particles move faster at higher temperatures - this means more collisions and more energetic collisions.
  • Surface Area
    • Greater surface area speeds up reaction rate.
      • Only the particles on the surface are available to interact.  Greater surface area means more particles can participate in the reaction.
  • Concentration
    • Higher concentration of reactants speeds up reaction rate.
      • Higher concentration means more particles are available to interact.  This means more collisions.
  • Pressure
    • Higher pressure speeds up reaction rate.
      • Gas that is under higher pressure is more concentrated.  Particles have less room to move and have more collisions.
  • Particles Size
    • More massive, bulky particles react more slowly.
      • More massive particles tend to move slower than less massive particles, so they have fewer collisions.  (Kinetic Theory of Matter.)
      • Large, bulky molecules often need to be hit in just the right place in order to react, so not every collision will cause a reaction.

Catalyst - A substance that changes the rate of a reaction, without being changed by the reaction.
Inhibitor - A catalyst that slows down a reaction.

Enzyme - A protein that acts as a catalyst in biochemical reactions (reactions that involve living things).
Substrate - The material that an enzyme acts on.
Enzymes are...
  • Substrate-Specific - They only work on certain reactants.
  • Temperature-Specific - They break down if they get too hot or too cold.
  • pH-Specific - They break down if they get put in solutions that are too acidic or too alkaline.
Enzymes have a specific shape that fits with their substrates, like a lock and key.  The place where the substrate fits is called the active site.

Some chemical reactions only go in one direction.  Others go in both directions, which results in an equilibrium system.

Chemical Equilibrium - A state of balance where the rate of the forward reaction is equal to the rate of the reverse reaction.  Since the rates are the same, the concentrations of reactants and products do not change.

Le Chatelier's Principle - If a change is made to a system at chemical equilibrium, the balance will shift to oppose the change until a new balance is reached (equilibrium is restored).
  • If heat is added, the balance will shift to favor the reaction that absorbs heat.
  • If heat is removed, the balance will shift to favor the reaction that gives off heat.
  • If one substance is added, the balance will shift to favor the reaction that uses that substance.