chem u5

1.5 Elements and the Periodic Table

The Periodic Law states that when elements are arranged in order of increasing atomic numbers, their physical and chemical properties show a periodic pattern. Periodicity is regularly repeating patterns or trends in the chemical and physical properties of the elements arranged in the periodic table.


Going down a group on the Periodic Table, each element has one more principal energy level filled with electrons than the element above it, so the outer electrons are farther away from the nucleus. This means the size of the atoms increases going down a group. Therefore the atomic radius increases going down a group.


Going from left to right across a period of the Periodic Table the valence electrons are all in the same principal energy level, but the number of protons in the nucleus increases from one element to the next. This means that the nucleus becomes more positively charged and attracts the electrons more strongly. Therefore, the atomic radius decreases going from left to right across a period.

2.5 Compounds and Bonding

Electron dot diagrams (Lewis dot diagrams) for an element show the number of valence electrons for that element.


Bonds form between atoms to form full valence shells to achieve stability. Atoms in covalent molecules achieve stability by forming covalent bonds that involve the sharing of electrons between atoms. 


The Lewis dot diagram for a covalent compound shows how the atoms in a molecule share electrons to gain a filled valence level. A filled valence level is called an octet. Lewis dot diagrams for most molecules generally follow the octet rule. Hydrogen is an exception, because it needs only two electrons to be stable. 


Each shared pair of electrons in a Lewis diagram represents a single covalent bond. 


A covalent molecule can also be represented by a structural formula in which each covalent bond is shown as a line joining two atoms. In other words, a line in a structural formula represents two electrons (a pair) shared by two elements.

3.5 Kinetic Theory

Collision theory states that particles must collide in order to break chemical bonds and react. An effective collision leads to the formation of products, while an ineffective collision does not. Thus, an effective collision occurs when particles collide at the proper orientation and with sufficient energy (activation energy) in order to react.


In chemical reactions effective collisions cause bonds to be broken which allows new bonds to form. The energy absorbed in breaking the bonds is never exactly equal to the energy released when the new bonds are formed. Therefore, all reactions are accompanied by a change in potential energy that can be measured and is represented by the symbol Δ H.


A potential energy diagram represents the energy change during a chemical reaction.


For Exothermic reactions (−ΔH), the energy required to break the bonds of the reactants is less than the energy released in making the bonds of the products. The products are more stable because they have less potential energy than the reactants.


For Endothermic reactions (+ΔH), the energy required to break the bonds of the reactants is greater than the energy released in making the bonds of the products. The reactants are more stable because they have less potential energy than the products.


Enthalpy (H) is the heat content of a system. The Enthalpy (Heat) of Reaction, ∆H, is the net amount of energy released or absorbed in a chemical reaction. The value for ∆H is given in kilojoules (kJ). 


ΔH = HProducts – HReactants


Exothermic reactions, ΔH (–)  and the HReact > HProd. The products have less potential energy than the reactants, so they are more stable.

 

Endothermic reactions, ΔH (+)  and the HProd > HReact. The reactants have less potential energy than the products, so they are more stable.

4.5 The Mole and Stoichiometry

Because matter cannot be created or destroyed, the total mass of the products is equal to the total mass of the reactants in a chemical reaction. 

The coefficients in a balanced equation indicate the mole ratios between each substance in the equation. These ratios can be used to predict the amount of product that can be formed from a given amount of reactant.

When given a molar amount of a substance, the molar mass of the desired substance and the mole ratio from a balanced chemical equation can be used as conversion factors to calculate the quantity of the desired unit in grams. 

5.5 Chemical Reactions

When two or more substances combine to form a single product, the reaction is called a synthesis reaction. For example, the formation of water from hydrogen and oxygen gases is a synthesis reaction:

                   2H2 (g)  + O2 (g)  → 2H2O (g)        synthesis


General formula :       A  +  B  →  AB


In a decomposition reaction, a compound breaks down into two or more simpler substances. For example, in electrolysis, water is broken down into hydrogen and oxygen gases:  


                 2H2O (l)  → 2H2 (g)  + O2 (g)        decomposition


General formula:         AB  →  A  +  B


Decomposition and synthesis are opposite chemical processes.

6.5 Solutions

The concentration of a solution is the amount of solute contained in a certain volume of solution. If a solution contains a small amount of solute it is called dilute, and if it contains a large amount of solute it is called concentrated.


In chemistry, concentration is given as molarity, the number of moles of the solute in one liter of solution and expressed as moles/liter or just M.   


[A] = molar concentration of A = mols of A per liter = mol A/L = M where A is a pure substance.


Medical professionals have to work with solution dilutions regularly. A stock solution with a known molarity is used to make varying dilute solutions, (standards). The dilute solution’s molarity is calculated using the following equation,

 

M1  x  V1  =  M2  x  V2


where M1 and V1 are the initial solution’s molarity and volume.  M2 and V2 are the final solution’s molarity and volume.  Use this equation to solve for the unknown variable, when three of the variables are known.

7.5 Experimentation

A volumetric flask is used to make solutions with precise molar concentrations. 


A pipette is used to add the final amount of solvent to a volumetric flask to ensure a precise concentration of a solution.

The theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. The actual yield is how much product is actually produced. From these two values the percent yield can be calculated using the formula to the right. 

8.5 Organic Chemistry

Many hydrocarbons contain groups of atoms besides carbon and hydrogen, called functional groups, which are responsible for the chemical properties of the hydrocarbon.

Replacing a part of a hydrocarbon with a functional group changes the structure, properties, and uses of the hydrocarbon.                                                                                                            

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Hydrocarbons containing halogens (group 17 elements) F, Cl, Br, and   I,  as functional groups  are called halogenated  compounds.  

                                          

                                    R represents the hydrocarbon part of the molecule.     X is F, Cl, Br, or  I