Thermodynamics - Chemistry HW

A thermochemical equation is a balanced stoichiometric chemical equation which shows both mass relationships and enthalpy change (delta H) between products and reactants. In variable form, a thermochemical equation would look like this:

A + B → C

ΔH = (±) #

Where {A, B, C} are the usual agents of a chemical equation with coefficients and “(±) #” is a positive or negative numerical value, usually with units of kJ.

 

     An endothermic reaction refers to a chemical reaction in which a system receives heat from its environment. They must absorb energy in order to proceed. They cannot occur spontaneously. They are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH). The endothermic chemical reaction creates a product that has a higher energy level than the original materials, causing the reactant's stored energy to decrease. The resulting product of the reaction is less stable because, the higher the energy bond, the less strength its molecules possess.

     It is the opposite of an endothermic reaction.

 

Exothermic reactions refers to chemical reactions which release energy in the form of heat, light, or sound. They may occur spontaneously and result in higher randomness or entropy of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy. The reactants contain more stored energy than the product because energy from external sources is not required, but given off. This gives the product more stability due to the lower amount of energy needed.

 

At a more advanced level, heat change is called the enthalpy change. It is denoted by delta H, ΔH.

Enthalpy is considered as the heat content of the system.

ΔH represents the difference between the enthalpy of the system before and after the process and is represented as :

• ΔH is negative (-ve) for exothermic reactions i.e. heat energy is given out and lost from the system to the surroundings which warm up.
• ΔH is positive (+ve) for endothermic reactions i.e. heat energy is gained by the system and taken in from the surroundings which cool down OR, as is more likely, the system is heated to provide the energy needed to effect the change.

                                                                               

     The first law of thermodynamics, often called as the law of conservation of energy, states that energy can be transformed, but cannot be created nor destroyed. This law suggests that energy can be transferred from one system to another in many forms. The amount of energy in the universe is constant – merely changing from one form to another. However, this energy cannot be created from nothing or reduced to nothing. Every natural process transforms energy and moves energy, but cannot create or eliminate it.

     It is usually formulated by stating that the total energy lost by the system is equal to the total energy absorbed by its surroundings. Likewise, the total energy absorbed by the system is equal to the energy lost bythe surroundings.

Chemical Reactions - Chemistry HW 11/4/10

Combination / Synthesis

In a synthesis reaction, two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. These reactions come in the general form of:

A + B ---> AB

example : 

combination of iron and sulfur to form iron (II) sulfide: 8 Fe + S₈ ---> 8 FeS 

            simple hydrogen gas combined with simple oxygen gas can produce a more complex substance-----water!

1.      Metal  +  oxygen  →   metal oxide

EX. 2Mg_(s)  +  O_2(g)  →    2MgO_(s)

2.      Nonmetal  +  oxygen  →    nonmetallic oxide

EX. C_(s)  +  O_2(g)  →    CO_2(g)

3.      Metal oxide  +  water  →    metallic hydroxide

EX. MgO_(s)  +   H₂O_(l)  →    Mg(OH)_2(s)

4.      Nonmetallic oxide  +  water  →    acid

EX. CO_2(g)  +  H₂O_(l)  →    ; H₂CO_3(aq)

5.      Metal + nonmetal  →    salt

EX. 2 Na_(s)  +  Cl_2(g)  →    2NaCl_(s)

6.      A few nonmetals combine with each other.

EX. 2P_(s)  +  3Cl_2(g)  →    2PCl_3(g)

 

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Decomposition / Analysis Reaction

     A decomposition reaction is the opposite of a synthesis reaction.  In a decomposition reaction, a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Compounds that are unstable will decompose quickly without outside assistance.

         These reactions come in the general form:         AB ---> A + B

a.Thermal Decomposition – It is the most common type of decomposition caused by a rise in temperature

EX. CaCO_3(s)  →    CaO_(s)  +  CO_2(g)

1.      Most metallic hydroxides, when heated, decompose into metallic oxides and water.

EX. Ca(OH)_2(s)  →    CaO_(s)  +  H₂O_(g)

2.      Metallic chlorates, when heated, decompose into metallic chlorides and oxygen.

EX. 2KClO_3(s)  →    2KCl_(s)  +  3O_2(g)

3.      Some acids, when heated, decompose into nonmetallic oxides and water.

EX. H₂SO₄  →    H₂O_(l)  +  SO_3(g)

4.      Some oxides, when heated, decompose.

EX. 2HgO_(s)  →    2Hg_(l)  +  O_2(g)

b. Photo-chemical Decompositions – Chemical decompositions that occur with light as a factor

c. Electrolytic Decomposition – It is a reaction where electricity is used to decompose compounds. They are frequently used in the industrial manufacture or purification of many elements and compounds. 

                        EX. 2H₂O_(l)  →    2H_2(g)  +  O_2(g)

EX. 2NaCl_(l)  →    2Na_(s)  +  Cl_2(g)

Substitution Reaction / Single displacement / Single Replacement /

In a single replacement reaction, a single uncombined element replaces another in a compound. A more reactive element takes the place of another element in a compound and sets the less active one free. Two reactants yield two products. These reactions come in the general form of:

A + BC ---> AC + B or  AX  +  Y  →    AY + X

examples :      Mg + 2 H₂O ---> Mg(OH)₂ + H₂

2 Na_(s) + 2 HCl_(aq) → 2 NaCl_(aq) + H_2(g)

Fe_(s)  +  CuSO_4(aq)  →    FeSO_4(aq)  +  Cu_(s)

2Na_(s)  +  2H₂O_(l)  →    2NaOH_(aq)  +  H_2(g)

Cl_2(g)  +  2NaBr_(aq)  →    2NaCl_(aq)  +  Br_2(l)

 

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Double Replacement /Double displacement  / Metathesis / Ionic

In a double replacement reaction, the anions and cations of two different molecules switch places, forming two entirely different compounds. These reactions are in the general form:

AB + CD ---> AD + CB

·         Two reactants yield two products. The reaction will produce at least one of the following:

1.      a precipitate

EX. BaCl_2(aq)  +  Na₂ SO_4(aq)  →    2NaCl_(aq)  +  BaSO_4(s)

1.      a gas

EX. HCl_(aq)  +  FeS_(s)  →    FeCl_2(aq)  +  H₂S_(g)

2.      water or some other non-ionized substance.

Example:     Pb(NO₃)₂ + 2 KI ---> PbI₂ + 2 KNO₃

CaCO_3(s)  +  HCl_(aq)  →    CaCl_2(aq)  +  CO_2(g)  +  H₂O_(l)

 

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Acid-base Neutralization 

     This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H⁺ ion in the acid reacts with the OH^- ion in the base to form water and an ionic salt.

Example: reaction of hydrobromic acid (HBr) with sodium hydroxide: 

HBr + NaOH ---> NaBr + H₂O

HA + BOH ---> H₂O + BA

·         Hydrochloric acid and sodium hydroxide ---> sodium chloride and water
HCl + NaOH ---> NaCl + H₂O 

·         Sulphuric acid and potassium hydroxide ---> potassium sulphate and water
H₂SO₄ + 2KOH ---> K₂SO₄ + 2H₂O 

·         Nitric acid and calcium hydroxide ---> calcium nitrate and water
2HNO₃ + Ca(OH)₂ ---> Ca(NO₃)₂ + 2H₂O 

·         Ethanoic acid and sodium hydroxide ---> sodium ethanoate and water
(Acetic acid and sodium hydroxide ---> sodium acetate and water , using the older names)
CH₃COOH + NaOH ---> NaOOCCH₃ + H₂O

 

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Endothermic Reactions
      Chemical reactions in which energy is absorbed are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy. Adding electrical energy to metal oxides can separate them into the pure metal and oxygen. Adding electrical energy to sodium chloride can cause the table salt to break into its original sodium and chlorine parts.

 

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Exothermic Reactions
     Chemical reactions in which energy is released are exothermic. The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product(s) to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction.

Oxidation-Reduction or Redox Reaction
      Redox is an abbreviation of reduction/oxidation reactions. In a redox reaction, the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical substances. One substance gains oxygen (or loses hydrogen) and another loses oxygen (or gains hydrogen).

example: 2 S₂O₃²⁻(aq) + I₂(aq) --> S₄O₆²⁻(aq) + 2 I⁻(aq)

·         Iron and oxygen ---> iron oxide, where the iron gets oxidised
4Fe + 3O₂ ---> 2Fe₂O₃

  

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Combustion / burning

 A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat.  Usually in A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. The term combustion is usually used for only large-scale oxidation of whole molecules.

Combustion is similar to a decomposition reaction, except that oxygen and heat are required for it to occur. If there is not enough oxygen, the reaction may not occur. Sometimes, with limited oxygen, the reaction will occur, but it produces carbon monoxide (CO) or even soot. In that case, it is called incomplete combustion.

Example: burning of napthalene:

C₁₀H₈ + 12 O₂ ---> 10 CO₂ + 4 H₂O

• CH₂S + 6 F₂ → CF₄ + 2 HF + SF₆ 

   

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Disproportionation - a redox reaction in which one reactant forming two distinct products varying in oxidation state.

2 Sn²⁺_(aq) → Sn_(s) + Sn⁴⁺_(aq)

   

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Organic reactions encompass a wide assortment of reactions involving compounds which have carbon as the main element in their molecular structure. The reactions in which an organic compound may take part are largely defined by its functional groups.