Monday 4 April 2016

Notes on Reaction and their types

Chemical reactions: Chemical equation, Balanced chemical equation, implications of a balanced chemical equation, types of chemical reactions: combination, decomposition, displacement, double displacement, precipitation, neutralization, oxidation and reduction.

What is a chemical change?


  • Definition: A chemical change occurs whenever compounds are formed or decomposed.
    • reactants disappear as chemical change occurs.
    • products appear as chemical change occurs.
    • catalysts speed up the reaction, but aren't produced or consumed.
  • Microscopic view: A chemical change occurs whenever there is a rearrangement of atoms that makes or breaks chemical bonds.

Writing chemical equations

  1. Write a sentence describing the reaction that includes the names and states of all reactants and products, and any special conditions the reaction requires.
"Ozone gas decomposes into oxygen gas when exposed to ultraviolet light."
  1. Separate reactants & products with a process symbol:
    rightarrowreaction goes to completion; sometimes used for one-step reactions
    doublearrowreaction is reversible; at equilibrium, reactants and products are mixed
    =net reaction; reaction occurs in several steps.
    Catalysts are usually written on top of the process symbol.
Ozone gasrightarrowoxygen gas
  1. Replace names with formulas; put plus signs between formulas.
O3 gasright arrowO2 gas
  1. Include symbols for physical states
    (g)gas
    (l)liquid
    (s)solid
    (aq)aqueous (dissolved in water)
O3 (g)right arrowO2(g)

  1. "Balance" the equation by adding coefficients so atoms and charge are conserved. Each side of a balanced equation must have:
    • the same number of atoms of each type
    • the same total charge
2 O3 (g)right arrow3 O2(g)
Types of Reactions
There are four main types of reactions:
Combination Reactions
Decomposition Reactions
Single-Replacement Reactions
Double-Replacement Reactions

Combination Reaction
A combination reaction combines two or more reactants in to a single product.

The general pattern for this reaction is:

A + B      AB
Examples of substances that often combine in synthesis reactions:

metal + nonmetal yields a salt compound             Ex: Ca + Cl2         CaCl2

metal + oxygen yields a  metal oxide                   Ex: 2Mg + O2         2MgO



Decomposition Reaction
Decomposition reactions break down, or “decompose” a single reactant into multiple products.

The general pattern for decomposition reactions:
AB       A + B

“AB” is always a compound
A and B (products) may be single elements or smaller compounds
Some common types of decomposition reactions:

metal oxides yield metal + oxygen                     Ex: 2HgO           2Hg + O2

ternary acids yield nonmetal oxide + water      Ex: H2CO3          CO2 + H2O

hydrates yield a compound + water                    Ex: BaCl2•H2O        BaCl2 + H2O


Single-replacement Reaction
In a single-replacement reaction, a reactive element replaces a less reactive element in a compound.

The general pattern for single-replacement reactions:

A + BX à B + AX

A = reactive element
The B in BX is less reactive than A.

Common single-replacement reactions:

*More active metal replaces the less active:
Zn (s) + CuCl2 (aq) à Cu (s) + ZnCl2 (aq)

*Metal replaces the H in an acid:
Mg (s) + 2HCl (aq) à MgCl2 + H2 (g)

*More active halogen replaces less reactive:
Cl2 (g) + 2NaBr (aq) à 2NaCl + Br2

Single-replacement reactions occur because the element that does the replacing is a more active element, based on its tendency to lose or gain electrons.

If a compound is placed in contact with a more active element, a reaction is likely.

To determine whether or not a single-replacement reaction will occur, refer to the “activity series.”

If the free element is above the other (in the compound) on the activity series, a reaction will occur.

If the free element is below the other on the activity series, a reaction will not occur.
Will a Reaction Occur?
Au (s) + NaCl (aq) à ?

Zn (s) + H2SO4 à ?

Cl2 (g) + MgBr2 à ?

Mg (s) + Ca(OH)2 à ?

Double-Replacement Reactions
In a double-replacement reaction, two compounds switch partners.

The general pattern of this reaction:
AY + BX à AX + BY

Most double-replacement reactions take place when both reactants are aqueous (dissolved in water).

When two aqueous compounds are mixed, and partners “swap,” a precipitate is often formed.

Example:
Pb(NO3)2 (aq) + K2CrO4 (aq) à PbCrO4 (s) + 2KNO3 (aq)

Another example double-replacement reaction:

HCl (aq) + KOH (aq) à HOH (l) + KCl (aq)

If we break apart the compounds into ions, we get:

H+ + Cl- + K+ + OH- à HOH + K+ + Cl-

**This is called the “ionic equation.”

H+ (aq) + Cl- (aq) + K+ (aq) + OH- (aq) à HOH (l) + K+ (aq) + Cl- (aq)

K+ and Cl- are “spectator ions”
If you cross out what does not change from reactants to products (spectator ions), you get the “net ionic equation.”

H+ + OH- à HOH

Balanced and unbalanced chemical equations:

A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
Count the number of atoms of all the elements in the reactants and products separately.

In reactants
In products
No. of Zn atoms
1
1
No. of H atoms
2
2
No. of S atoms
1
1
No. of O atoms
4
4

Characteristics of Chemical reactions

  • The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be easily observed easily.
  • The important characteristics of chemical reactions are:
    1. Evolution of a gas
    2. Formation of a precipitate
    3. Change in colour
    4. Change in temperature
    5. Change in state

Evolution of a Gas:

Some chemical reactions are characterized by the evolution of a gas.
The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.
  • Take some zinc granules in a conical flask.
  • Add dilute sulphuric acid over zinc granules.
  • We will see the bubbles of hydrogen gas being formed around zinc granules.
  • If we touch the conical flask with our hand, we will find that it is somewhat hot. So, a change in temperature also occurs in this chemical reaction.

Formation of a precipitate:

Some chemical reactions are characterized by the formation of precipitate. A precipitate is a ‘solid product’ which separates out from the solution during a chemical reaction.
The chemical between potassium iodide and lead nitrate is characterized by the formation of a yellow precipitate of lead iodide.
  • Take some lead nitrate solution in test tube.
  • Add potassium iodide solution to it.
  • A yellow precipitate of lead iodide is formed at once.
  • A change in colour also takes place in this chemical reaction.

Change in colour:

Some chemical reactions are characterized by a change in colour.
The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized by a change in colour from purple to colourless.
  • Take some dilute potassium permanganate solution in a test tube. It has purple colour.
  • Add some lemon juice (it contains citric acid) to it with the help of a dropper and shake the test tube.
  • The purple colour of potassium permanganate solution goes on fading and ultimately it becomes colourless.

Change in temperature:

Some chemical reactions are characterized by a change in temperature.
The chemical reaction between quicklime and water to form slaked lime is characterized by a change in temperature.
  • Take a little of quicklime in a hard-glass beaker.
  • Add water to it slowly.
  • Touch the beaker.
  • The beaker feels to be quite hot.

Change in state:

  • Some chemical reactions are characterized by a change in state.
  • When wax is burned (in the form of wax candle), then water and carbon dioxide are formed.
  • Now, wax is a liquid whereas carbon dioxide is a gas. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and gas.

Chemical equations:

  • The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.
  • Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
  • This is known as word equation. Putting the symbols and formulae of all the substances in the above word equation, we get the following chemical equation:
  • The substances which combine or react are known as reactants.
  • The new substances produced in a reaction are known as products.
  • A chemical equation is a short-hand method of representing a chemical reaction.

  • Oxidation and Reduction reactions:

    Oxidation:
    The addition of oxygen to a substance is called oxidation.
    The removal of hydrogen from a substance is called oxidation.
    Reduction:
    The addition of hydrogen to a substance is called reduction.
    The removal of oxygen from a substance is called reduction.
    • The oxidation and reduction reactions are also called redox reactions.
      Example:
      When copper oxide is heated with hydrogen, then copper metal and water are formed.
      In the above reaction, copper oxide (CuO) is changing into copper (Cu), so copper oxide is being reduced to copper.
      Hydrogen is changing into water (H2O), so hydrogen is being oxidized to water.
      Copper oxide is giving oxygen required for the oxidation of hydrogen, therefore, copper oxide is oxidizing agent and hydrogen is reducing agent.
    • Effect of oxidation reactions in everyday life:
      Oxidation has damaging effect on metals as well as on food. There are two common effects of oxidation reactions which we observe in daily. These are:
      1. Corrosion of metals:
        Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
        • Corrosion is caused mainly by the oxidation of metals by oxygen of air. Rusting of iron metal is the most common form of corrosion.
        • During the corrosion of iron (rusting of iron), iron metal is oxidized by the oxygen of air in the presence of water (moisture) to form hydrated iron (III) oxide called rust.
        • Corrosion weakens the iron and steel objects and structures such as railings, car bodies, bridges and ships, etc., and cuts short their life.
      2. Rancidity
        • When the fats and oils present in food materials get oxidized by the oxygen (of air), their oxidation products have unpleasant smell and taste.
        • The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
        • Rancidity spoils the food materials prepared in fats and oils which have been kept for a considerable time and make them unfit for eating.
        • The development of rancidity of food can be prevented or retarded (slowed down) in the following ways:
          1. Rancidity can be prevented by adding anti-oxidants to foods containing fats and oils:
            Anti-oxidant is a substance (or chemical) which prevents oxidation. Anti-oxidants are actually reducing agent.
            The two common anti-oxidants used in foods to prevent the development of rancidity are BHA (Butylated Hydroxy – Anisole) and BHT (Butylated Hydroxy-Toluene).
          2. Rancidity can be prevented by packaging fats and oils containing foods in nitrogen gas:
            When the packed is surrounded by unreactive gas nitrogen, there is no oxygen to cause its oxidation and make it rancid.
            The manufacturers of potato chips fill the plastic bags containing chips with nitrogen gas.
          3. Rancidity can be retarded by keeping food in a refrigerator.
          4. Rancidity can be retarded by storing food in air-tight containers.
          5. Rancidity can be retarded by storing food away from light.
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