Lecture 11 Atoms and electron configurations Chemical Reactions Energy and Chemical Reactions

Chemical Reactions Chemical reactions = interactions of the electrons between atoms. Nuclear reactions = interactions in the atomic nucleus. Chemical Bond: the sharing or transfer of electrons to attain a stable electron configuration. Molecules: made from two or more chemically bonded atoms.

Electrons move within regions called principle shells. Each shell can hold a specific number of electrons. Outermost electrons are called valence electrons -involved in chemical bonding.

Compound: a substance consisting of chemically bonded atoms of different elements. Elements to Compounds

Chemical formula: shows the proportion by which elements combine to form a compound. CompoundFormula Sodium chloride NaCl Ammonia NH 3 Water H2OH2O Elements to Compounds Silica SiO 2

Ionic Bond: a chemical bond in which valence electrons are transferred. Ionic compounds: compounds containing ions.

Chemical Bonds Covalent Bond: a chemical bond in which valence electrons are shared – an overlap of outer electron shells. –Forms molecules in which atoms are bonded together without a complete transfer of electrons.

Physical change: a change in the physical properties of a substance.

Chemical change: the transformation of one or more substances into others.

Physical Change of Potassium Chromate

Chemical Change in which Ammonium dichromate is transformed into new substances.

Chemical Changes During a chemical change, new substances are formed as atoms rearrange themselves into new configurations

O The transformation of oxygen (O 2 ) into ozone (O 3 ) is an example of O=O O O O O O oxygen ozone 1.a physical change. 2.a chemical change. 3.a nuclear reaction. 4.all of the above.

Chemical Equations Chemical Equation: shows the reactants and products of a chemical reaction and their proportions. Reactants Products

Chemical Equations Coefficient in front of a chemical formula indicates proportions of compounds and elements in reaction. Example: 2 H 2 O = 2 water molecules Reactants Products

In a balanced chemical equation, there must be the same number of each type of atom on both sides of the arrow. Law of Mass Conservation: no atoms are gained or lost during any reaction.

Is the following chemical equation balanced? 1.No, because the molecules have changed. 2.No because there are more hydrogen atoms in the reactants. 3.No, because there are more oxygen atoms in the products. 4.Yes, because the same atoms appear before and after. 1 CH O 2  2 H 2 O + 1 CO 2

Using Chemical Equations Combustion of Methane: 1 CH 4 + 2O 2  2 H 2 O + 1 CO 2 Ratios: 1 CH 4 : 2 H 2 O = 1:2 2 O 2 : 2 H 2 O = 1:1 1 CH 4 : 1 CO 2 = 1:1 2 O 2 : 1 CO 2 = 2:1 Balanced chemical equations make it possible to determine: the proper proportions of reactants needed. the amount of product produced.

Reaction Rates Reaction rate: speed with which products form from the reactants. Affected by Concentration Temperature Catalyst Reactant molecules have to make physical contact with each other for a reaction to occur.

Concentration Greater concentration of nitrogen and oxygen = greater likelihood that N 2 and O 2 molecules will collide and form nitrogen monoxide.

Temperature Slow-moving molecules may collide without enough force to break the bonds. In this case, they cannot react to form product molecules.

Catalysts Catalyst: any substance that increases the rate of a chemical reaction by lowering its activation energy. Activation Energy: minimum amount of energy required for reactant molecules to transform to product molecules.

Activation Energy = E a

Catalysts Transformation of Ozone to Oxygen by Chlorine in atmosphere. E a is reduced by Chlorine catalyst.

Cl + O 3  ClO + O 2 ClO + O 3  Cl + 2O 2 Cl atoms in the stratosphere act as a catalyst in the destruction of the ozone layer. Cl is a byproduct of human-made CFCs. One chlorine atom is estimated to catalyze the transformation of 100,000 ozone molecules to O 2 molecules in 1 or 2 years before is removed by natural processes.

Carefully examine the following reaction sequence of the catalytic formation of ozone, O 3, from molecular oxygen, O 2. Which chemical is behaving as the catalyst? O NO  2 NO 2 2 NO 2  2 NO + 2 O 2 O + 2 O 2  2 O 3

O NO  2 NO 2 2 NO 2  2 NO + 2 O 2 O + 2 O 2  2 O 3 Carefully examine the following reaction sequence for the catalytic formation of ozone, O 3, from molecular oxygen, O 2. Which chemical compound is behaving as the catalyst? Explanation: The NO reacts with O 2 to form O, which reacts with O 2 to form O 3. In other words, the NO gets this reaction going. As another clue, note how NO is regenerated after the second reaction.

Exothermic reaction: a chemical reaction that results in the net release of energy. reactants products + energy Endothermic reaction: a chemical reaction in which there is a net consumption of energy. energy + reactants products Energy and Chemical Reactions

Chemical reaction representing the combustion of gasoline: 2C 8 H O 2  16CO H 2 O + Heat Energy

Is the Following Reaction an endothermic or exothermic reaction? 1.Exothermic 2.Endothermic

Energy and Chemical Reactions In exothermic reactions: the energy released goes into: –increasing the speeds (kinetic energy) of reactant molecules and atoms –often into electromagnetic radiation (ex. light).

Energy and Chemical Reactions Bond Energy: –Energy absorbed in order to break a chemical bond = energy released when the same chemical bond is formed.

1 mole of a chemical compound = 6.02 x molecules. Table below shows bond energy per mole (kJ/mole) of molecules.

Counting Atoms and Molecules By Mass Units of Moles relate the mass to the number of molecules or atoms. The mass (grams) of 1 mole of atoms of an element = atomic mass number of element. The mass (grams) of 1 mole of molecules = the sum of the atomic masses of all atoms in the molecules (molecular mass) x = 1 mole

Products have lower chemical potential energy than reactants. Energy released in reaction = difference in potential energies = difference in bond energies. Exothermic Reaction Energy absorbed in breaking chemical bonds in reactants Energy released in forming chemical bonds in products <

Products have greater chemical potential energy than reactants. Energy absorbed in reaction = difference in potential energies = difference in bond energies. Endothermic Reaction > Energy absorbed in breaking chemical bonds in reactants Energy released in forming chemical bonds in products

It is the natural tendency of energy to disperse from where it is concentrated to where it is dilute. Entropy: term used to describe the degree to which energy has become dispersed. Reactions that result in an increase in entropy (energy dispersal) tend to occur on their own or are spontaneous. Chemical Reactions Are Driven By Entropy

Which type of chemical reaction leads to a greater dispersal of energy? 1.Exothermic 2.Endothermic

Exothermic reactions tend to be self-sustaining because they lead to large increases in entropy. Endothermic reactions tend to require the continual input of energy. Example: A campfire Example: Photosynthesis Chemical Reactions Are Driven By Entropy

Transformation of sunlight into stored chemical energy:

Transformation of stored chemical energy to heat energy through combustion: Plant + O 2  CO 2 + H 2 O + C + Other Gases + Energy

Crystal Growth and Entropy Formation of crystals results in the alignment of atoms or molecules into an orderly framework. Result in a release of energy into the surroundings - adding to an increase in entropy. –Salt crystals from solution. –Ice crystals from liquid water in atmosphere. –Silicate mineral crystals from magma.