CHEMICAL REACTIONS

• A chemical reaction is a rearrangement of atoms in which some of the original bonds are broken and new bonds are formed to give different chemical structures.
• In a chemical reaction, atoms are neither created, nor destroyed.
• A chemical reaction, as described above, is supported by Dalton's postulates.
  

  

• A chemical reaction can be detected by one of the following evidences:

1. Change of color
2. Formation of a solid
3. Formation of gas
4. Exchange of heat with surroundings

THE CHEMICAL EQUATION

• A chemical equation is a shorthand expression for a chemical reaction.

• A chemical equation consists of the following information:

1. Reactants separated from products by an arrow ( → ):

2. Coefficients are placed in front of substances to balance the equation:

3. Reaction conditions are placed over the arrow:

4. The physical state of the substances are indicated by symbols (s), (l), (g) and (aq):

WRITING & BALANCING EQUATIONS

• A balanced equation contains the same number of atoms on each side of the equation, and therefore obeys the law of conservation of mass.
• Many equations are balanced by trial and error; but it must be remembered that coefficients can be changed in order to balance an equation, but not subscripts of a correct formula.

1. Write the unbalanced equation

• Make sure the formula for each substance is correct

2. Balance by inspection

• Count and compare each element on both sides of the equation.

• Balance elements that appear only in one substance first.

• When finally done, check for the smallest coefficients possible.

Examples:

TYPES OF CHEMICAL REACTIONS

• Chemical reactions are classified into five types:

1. Synthesis or Combination

• Two elements or compound combine to form another compound.

2. Decomposition

• A compound breaks up to form elements or simpler compound.

3. Single Replacement

• A more reactive element replaces a less reactive element in a compound.
• These reactions will be discussed in more detail later in this chapter.

TYPES OF CHEMICAL REACTIONS

4. Double Replacement

• Two compounds interact to form two new compounds.
• These reactions will be discussed in more detail later in this chapter.

5. Combustion Reactions:

• A reaction that involves oxygen as a reactant and produces large amounts of heat is classified as a combustion reaction.

Examples:

DOUBLE REPLACEMENT REACTIONS

• Double replacement reactions can be subdivided into one of the following subgroups:

1. Precipitation:

3. Unstable product:

DOUBLE REPLACEMENT REACTIONS

Examples:

OXIDATION-REDUCTION REACTIONS

• Reactions known as oxidation and reduction (redox) have many important applications in our everyday lives. Rusting of a nail or the reaction within your car batteries are two examples of redox reactions.
• In an oxidation-reduction reaction, electrons are transferred from one substance to another. If one substance loses electrons, another substance must gain electrons.
  

  

• Oxidation is defined as loss of electrons, and reduction is defined as gain of electrons. One way to remember these definitions is to use the following mnemonic:

• Combination, decomposition, single replacement and combustion reactions are all examples of redox reactions.

OXIDATION-REDUCTION REACTIONS

• In general, atoms of metals lose electrons to form cations, and are therefore oxidized, while atoms of non-metals gain electrons to form anions, and are therefore reduced.
• For example, in the formation of calcium sulfide from calcium and sulfur

• Therefore, the formation of calcium sulfide involves two half-reactions that occur simultaneously, one an oxidation and the other a reduction.
• Similarly, in the reaction of magnesium metal with hydrochloric acid

• In every oxidation-reduction reaction, the number of electrons lost must be equal to the number of electrons gained.

Examples:

1. Identify each of the reactions below as oxidation or reduction:
   a)
   b)
   c)
   d)

OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS

• Many important biological reactions involve oxidation and reduction. In these reactions, oxidation involves addition of oxygen or loss of hydrogen, and reduction involves loss of oxygen or gain of hydrogen.
• For example, poisonous methyl alcohol is metabolized by the body by the following reaction:

• The formaldehyde is further oxidized to formic acid and finally carbon dioxide and water by the following reactions:

• In many biochemical oxidation-reduction reactions, the transfer of hydrogen atoms produces energy in the cells.
• For example, cellular respiration is an oxidation-reduction process that transfers energy from the bonds in glucose to form ATP.
  

  

• The oxidation of a typical biochemical molecule can involve the transfer of hydrogen atoms to a proton acceptor such as coenzyme FAD to produce its reduced form FADH2.
  

  

OXIDATION-REDUCTION IN BIOLOGICAL SYSTEMS

• In summary, the particular definition of oxidation-reduction depends on the process that occurs in the reaction. A summary of these definitions appears below:

Examples:

1. Linoleic acid, an unsaturated fatty acid, can be converted to a saturated fatty acid by the reaction shown below. Is linoleic acid oxidized or reduced in this reaction?

2. The reaction of succinic acid provides energy for the ATP synthesis and is shown below:

ENZYMES IN BIOLOGICAL SYSTEMS

• In biochemical reactions, enzymes are necessary to oxidize glucose and other foods.
• For example, oxidation of glucose involves the transfer of hydrogen atoms and electrons to an enzyme, such as NAD to produce its reduced form NADH.
  

  

• Similarly, oxidation of methanol involves transfer of 2 hydrogen atoms and 2 electrons to NAD to form the reduced form NADH.
  

  

• Molecules such as NAD are called "electron carriers" since they carry electrons in their reduced form.

ENZYMES IN BIOLOGICAL SYSTEMS

• The electron carriers collectively are called electron transport chain.
• As electrons are transported down the chain, ATP is generated.
  

  

THE MOLE CONCEPT

• Chemists find it more convenient to use mass relationships in the laboratory, while chemical reactions depend on the number of atoms present.
• In order to relate the mass and number of atoms, chemists use the SI unit mole (abbreviated mol).
• The number of particles in a mole is called Avogadro's number and is .
  

  

1 mol of H atoms..........contains: 6.02 x 10 23 H atoms
1 mol of H2 molecules..........contains: 6.02 x 10 23 H2 molecules
2 x(6.02x10 23) H atoms
1 mol of }\mp@subsup{\mathbf{H}}{2}{}\mathbf{O}\mathrm{ molecules....contains: 6.02x10 23 }\mp@subsup{\textrm{H}}{2}{}\textrm{O}\mathrm{ molecules
    2 x (6.02 x10 23) H atoms
    1 x (6.02 x10 23) O atoms
1 mol of Na 

• The atomic mass of one atom expressed in amu is numerically the same as the mass of one mole of atoms of the element expressed in grams.

• Mole is a counting unit for atoms similar to other counting units used for other objects

MOLAR MASS

• The mass of one mole of a substance is called molar mass and is measured in grams.

Examples:

1. Lithium carbonate
2. Salicylic acid

CALCULATIONS USING THE MOLE CONCEPT

• The molar mass to convert between mass and moles.
• Avogadro's number to convert between moles and number of entities.
  

  

Examples:

1. How many moles of iron are present in 25.0 g of iron?

2. What is the mass of 5.00 mole of water?
3. How many magnesium atoms are present in 5.00 g of Mg ?

4. How many molecules of HCl are present in 25.0 g of HCl ?

MOLES OF ELEMENTS IN A FORMULA

• The subscripts in a chemical formula of a compound indicate the number of atoms of each type of element. For example, in a molecule of aspirin, , there are 9 carbon atoms, 8 hydrogen atoms and 4 oxygen atoms.
• The subscript also indicates the number of moles of each element in one mole of the compound. For example, one mole of aspirin contains 9 moles of carbon atoms, 8 moles of hydrogen atoms and 4 moles of oxygen atoms.
  

  

• Using the subscripts from the aspirin formula, one can write the following conversion factors for each of the elements in 1 mole of aspirin:

Examples:

1. Determine the moles of C atoms in 1 mole of each of the following substances:
   a) Acetaminophen used in Tylenol,
   b) Zinc dietary supplement,
2. How many carbon atoms are present in 1.50 moles of aspirin, ?

SUMMARY OF MASS-MOLE CALCULATIONS

Mass

Moles

Particles

Atoms or ions

Molecules or
formula units
of compound Molecules or formula units of compound

STOICHIOMETRY

• Stoichiometry is the quantitative relationship between the reactants and products in a balanced chemical equation.
• A balanced chemical equation provides several important information about the reactants and products in a chemical reaction. For example:

mole moles moles

This is the MOLE RATIO between REACTANTS and PRODUCTS

Summary of Stoichiometric Calculations in Chemistry

Examples:

1. Determine each mole ration based on the reaction shown below:

STOICHIOMETRIC CALCULATIONS

Mole-Mole Calculations:

• Relates moles of reactants and products in a balanced chemical equation

Examples:

1. How many moles of nitrogen will react with 2.4 moles of hydrogen to produce ammonia as shown in the reaction below?

2. How many moles of ammonia can be produced from 32 moles of hydrogen? (Assume excess nitrogen present)

3. In one experiment, 6.80 mol of ammonia are prepared. How many moles of hydrogen were used up in this experiment?

STOICHIOMETRIC CALCULATIONS

Mass-Mole Calculations:

• Relates moles and mass of reactants or products in a balanced chemical equation

Examples:

1. How many grams of ammonia can be produced from the reaction of 1.8 moles of nitrogen with excess hydrogen as shown below?

2. How many moles of hydrogen gas are required to produce 75.0 g of ammonia?

3. How many moles of ammonia can be produced from the reaction of 125 g of nitrogen as shown above?

STOICHIOMETRIC CALCULATIONS

Mass-Mass Calculations:

• Relates mass of reactants and products in a balanced chemical equation
  

  

Examples:

1. What mass of oxygen will be required to react completely with 96.1 g of propane, , according to the equation below?

2. What mass of carbon dioxide will be produced from the reaction of 175 g of propane, as shown above?

LIMITING REACTANT

• When 2 or more reactants are combined in non-stoichiometric ratios, the amount of product produced is limited by the reactant that is not in excess.
• This reactant is referred to as limiting reactant.
• When doing stoichiometric problems of this type, the limiting reactant must be determined first before proceeding with the calculations.

Analogy:

• When solving limiting reactant problems, assume each reactant is limiting reactant and calculate the desired quantity based on that assumption.
• Compare your answers for each assumption; the lower value is the correct assumption.

LIMITING REACTANT

Guide to Calculating Product from a Limiting Reactant

Examples:

1. How many moles of can be produced by reacting 4.0 mol of hydrogen and 3.0 mol of oxygen gases as shown below:

Notes:

1. Even though mass of is greater than , there are fewer moles of it due to its larger molar mass.
2. Limiting reactant calculations must always be done when amount of both reactants are given.

LIMITING REACTANT

Examples:

3. How many moles of can be produced by reacting 16.8 g of Fe with 10.0 g of as shown below:

PERCENT YIELD

• The amount of product calculated through stoichiometric ratios is the maximum amount of product that can be produced during the reaction, and is thus called theoretical yield.
• The actual yield of a product in a chemical reaction is the actual amount obtained from the reaction.
• The percent yield of a reaction is obtained as follows:

Example:

1. In an experiment forming ethanol, the theoretical yield is 50.5 g and the actual yield is 46.8 g . What is the percent yield for this reaction?

2. Silicon carbide can be formed from the reaction of sand ( ) with carbon as shown below:

PERCENT YIELD

Examples:

3. In an experiment to prepare aspirin, the theoretical yield is 153.7 g and the actual yield is 124.0 g . What is the percent yield of this reaction?

4. Carbon tetrachloride ( ) was prepared by reacting 100.0 g of with excess carbon disulfide ( ), as shown below. If 65.0 g was prepared in this reaction, calculate the percent yield.

ENERGY CHANGES IN CHEMICAL REACTIONS

• Heat is energy change that is lost or gained when a chemical reaction takes place.
• The system is the reactants and products that we are observing. The surroundings are all the things that contain and interact with the system, such as the reaction flask, the laboratory room and the air in the room.
• The direction of heat flow depends whether the products in a reaction have more or less energy than the reactants.
• For a chemical reaction to occur, the molecules of the reactants must collide with each other with the proper energy and orientation. The minimum amount of energy required for a chemical reaction to occur is called the activation energy.
• The heat of reaction is the amount of heat absorbed or released during a reaction and is designated by the symbol .
  

  

Endothermic reaction

• When heat is released during a chemical reaction, it is said to be exothermic. For exothermic reactions, is negative and is included on the right side of the equation.

• When heat is gained during a chemical reaction, it is said to be endothermic. For endothermic reactions, is positive and is included on the left side of the equation.

CALCULATING HEAT IN A REACTION

• The value of in a chemical reaction refers to the heat lost or gained for the number of moles of reactants and products in a balanced chemical equation. For example, based on the chemical equation shown below: