Category: chemistry

Physical Changes Vs Chemical Changes

Chemical Reactions Nuggets

Chemical Reactions Nuggets 
last updated 19th Jan 2014
v1.12

 Physical and Chemical Changes
  1. Physical reactions/changes doesn’t form new substances and such changes are reversible. (e.g. melting ice, boiling water)
  2. In physical reactions, little or no heat/light is taken in or given out.
  3. Chemical reactions involves chemical changes which forms new substances. (e.g. burning of paper, photosynthesis)
  4. In general, these new substances are different in color, texture when compared to the reactants. Their states(solid/liquid/gas) may also be different. **(do note that in some cases, chemical reactions don’t involve any colour change. for e.g. methane burning in oxygen gas to form carbon dioxide and water vapour. In this reaction, all the reactants and products are colourless. CH4(g) + 2O2(g) —> CO2(g) + 2H2O(g))
  5. In chemical changes, the products usually don’t resemble the reactions. (refer to point 4 for exceptional cases)
  6. We use a word equation or a chemical equation to represent chemical reactions. 
  7. Carbon + Oxygen –> Carbon Dioxide (Word Equation)
  8. C + O2 –> CO2 (Chemical equation)
  9. In a chemical reaction, the total mass of reactants before the reaction is equal to the total mass of the products after the reaction. 
  10. In a chemical reaction, no atoms are created or destroyed. They simply re-arrange among themselves to form new molecules. 
  11. Chemical reactions are usually irreversible but some are reversible. (refer to the list below for such examples)
  12. Chemical reactions can be started by heat, mixing, light, electric current.
  13. Some signs (or evidence) of chemical change are:
    • a gas is produced,
    • the temperature changes,
    • a substance disappears,
    • a solid is formed
    • a colour change occurs,
    • a new odour is produced.
Examples of Physical Changes
  • water turning into ice (freezing)
  • salt dissolves in water (dissolution)
  • evaporation (liquid -> gas)
  • Zinc oxide turns from white to yellow upon heating and turns white again upon cooling
  • magnetising and demagnetising a magnet
  • expansion and contraction of materials under the effect of heat
Examples of Chemical Changes that are reversible

( —><— sign refers to reversible reactions)
  • Sulphur dioxide  + Water   —><—  Sulphurous acid 
  • Ammonia + Water —><— Ammonium hydroxide
  • Iron + Steam —><—  Iron oxide + Water 
  • Calcium Carbonate —><—  Calcium Oxide + Carbon dioxide
  • Ammonium Chloride —><—  Ammonia Gas + Hydrogen Chloride Gas (this reaction is commonly mistake for a sublimation process)

Different Types/Categories of Chemical Reactions 

  1. Combination (Synthesis)
  2. Decomposition (thermal, light)
  3. Single Displacement
  4. Double Displacement (metathesis reaction)
  5. Complete Combustion and Incomplete Combustion
  6. Hydrolysis
  7. Oxidation
  8. Reduction
  9. Neutralisation (Acid-Alkali)
  10. Precipitation
  11. Electrolysis
Types of Chemical Reactions by Heat
  • combination
  • combustion
  • thermal decomposition
  • oxidation (may also happen without heat)
Explanation for the different types of Chemical Reactions

Combination 

This involves 2 or more substances going through a chemical reaction to form a single new product.

Examples:
  • Fe + S —> FeS (involving elements)
  • C + O2 —> CO(involving elements)
  • Fe + O2 —> Fe2O(involving elements)
  • 2Na + Cl2 —> 2NaCl (involving elements)
  • PCl3 + Cl2 —> PCl(involving compounds and elements)
Recall:
In this reaction Fe + S —> FeS , we know this is a chemical change because the product looks different form the reactants. Fe is a silvery metal and S is a yellow solid. But the product is a black solid.

Complete Combustion

Combustion is also known as burning. Combustion is also known as a type combination reaction. In a combustion reaction, oxygen is combined with other reactants in the presence of heat. The end product of combustion reactions are CO2 and H2O.

Examples: 
  • combustion of methane
  • CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
  • burning of naphthalene
    C10H8 + 12O2 → 10CO2 + 4H2O

  • combustion of ethane
    2C2H6 + 7O2 → 4CO2 + 6H2O
Incomplete Combustion
Combustion reactions can also sometimes be incomplete. In such cases, carbon dioxide (CO2) will not be formed. Instead, carbon monoxide (CO) is formed. CO is a poisonous gas. It is also odourless which makes it hard to be detected. 
Single Displacement

Single displacement reactions has this general pattern. A + BC —> AB + C 
In single displacement reactions, one of the elements in the compound is replaced by another element.

Li(s) + NaCl(aq) > LiCl(aq) + Na(s)

F2(aq) + 2KI(aq) —> I2(aq) + 2KF(aq)


Double Displacement

The general pattern of a double displacement reaction is:

AB + CD —> AD + CB

Examples
  • 2KI(aq) + Pb(NO3)2(aq) —> PbI2(s) + 2KNO3(aq)
  • MgCl2(aq) + Ca(OH)2(aq) > Mg(OH)2(s) + CaCl2(aq)










Nuggets: Alkanes

last update 3rd Oct
V1.1
  1. Organic chemistry is the study of the properties of compounds containing carbon. (carbon compounds)
  2. hydrocarbon is a compound containing carbon and hydrogen only.
  3. An organic compound contains the element carbon. (except CO, CO2, and carbonates)
  4. Organic compounds may also contain other elements like oxygen and nitrogen.
  5. saturated compound contains only single covalent bonds
  6. An unsaturated compound contains double covalent bonds.
  7. homologous series is a group of compounds with a general chemical formula, which follows a regular structural pattern in which each successive member differs from the other by a -CH2- group.
  8. Most chemical properties of organic compound (except for alkanes) are due to the presence of the function group. (e.g. alkenes have the C = C functional group)
  9. General formula for alkanes: CnH2n+2
  10. Name Formula State
    Methane CH4 gas
    Ethane C2H6 gas
    Propane C3H8 gas
    Butane C4H10 gas
    Pentane C5H12 liquid
    Hexane C6H14 liquid
    Heptane C7H16 liquid
    Octane C8H18 liquid
    Nonane C9H20 liquid
    Decane C10H22 liquid
  11. C1-4 (gas), C5-17(liquid), C18 onwards (solid: e.g bitumen)
  12. Characteristics of an alkane homologous series:
  13. (a) all members share a general chemical formula,
  14. (b) each successive member differs in its molecular formula by the addition of a -CH2- gropup,
  15. (c) its relative molecular mass differs from each group by 14,
  16. (d) all members have similar chemical properties ( (i) undergoes combustion to form CO2 and H20, (ii) undergoes substitution reaction),
  17. (e) physical properties of each member changes gradually as the number of carbon atoms increases. (as the carbon atom increases, the boiling point also increases, the compound becomes less flammable and become less volatile, the compound gets more viscous as the longer molecules gets entangled more easily)
  18. Physical Properties of Alkanes
  19. (i) insoluble in water
  20. (ii) does not conduct electricity
  21. (iii) low melting and boiling point for C1-4.
  22. Chemical Properties of Alkanes
  23. (i) All alkanes are generally unreactive because the C-C and C-H covalent bonds are strong.
  24. However they can undergo
  25. (ii) combustion: easily flammable when in gaseous and liquid states. less flammable in solid state. When solid alkane(alkanes with larger molecule size, e.g. candle wax) is burnt, it produces a smoky flame due to incomplete combustion of carbon atoms in the molecules. The smoky flame is caused by soot (carbon). This combustion is an exothermic reaction (energy released). Incomplete combustion also produces CO instead of CO2. In an incomplete combustion, the products formed are CO and H2O. In some other cases, incomplete combustion can also lead to the formation of carbon. (e.g. CH4 + O2 -> C + 2H20)
  26. (iii) (Halogenation) Substitution reactions with halogens (chlorine/bromine)
  27. Alkanes react with halogens in the presence of ultraviolet light(UV)
  28. The UV light acts as a catalyst. The UV light breaks up the Cl2 bonds.
  29. CH4 + Cl2 –> CH3Cl + HCl (hydrogen atom is replaced by chlorine atom)
  30. Substitution reaction can be repeated to form CH2Cl2, CHCl3, CCl4
  31. Isomerism (only for Pure Chem)
  32. Isomerism is where two or more organic molecules have the same molecular formula but different structural formulae. These different molecules are called isomers.
  33. Isomers have different melting and boiling points.
  34. The branched-chain or side-chain of an alkane is called alkyl groups. The formula of the alkyl groups is CnH2n+1

Nutshell: Bases and Alkalis

This is meant to be a simple explanation to differentiate base from alkali. This is a common question among students.

A base is usually a metal oxide. For example, sodium oxide, potassium oxide, calcium oxide.

If the base can be dissolved in water, we give them another term called alkali. For example, sodium oxide dissolves in water to form sodium hydroxide. Now we can call sodium hydroxide to be an alkali.

In general, any chemical name ending with the word hydroxide can be treated to be an alkali.

Nutshell: Sublimation

Sublimation is the process of changing a substance from the solid state directly into the gaseous state. It doesn’t form the liquid state before converting to the gaseous state.

For example, if you heat solid dry ice, it changes into the gaseous state.

Examples of substances that can undergo sublimation process are

1. Dry ice (solid carbon dioxide)
2. Ammonium chloride (it doesn’t really sublimes. Its a chemical reaction that is reversible upon heating)
3. Iodine
4. Napthalene
5. Camphor
6. Mercuric Chloride

Sublimation is also used to describe a method used to separate mixtures containing substances that sublimes.

For example, to separate a mixture of iodine and sand, we can use sublimation to separate the iodine from the salt. When we heat the mixture, the iodine sublimes and became gaseous state leaving the sand behind. To collect the iodine back in the form of solid, we can place a filter funnel above the mixture. The gaseous iodine when touches the filter funnel will change back to the solid state.

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Nutshell: Solutions and Suspensions

Solutions are mixtures. Solutions are formed when the solute dissolves in a solvent. The solutes are usually in the solid state and the solvent in the liquid state.(however that is not always the case). The solvent can also be in the form of a solid or a gas.

Water is commonly known as a universal solvent as it dissolves many substances. More substances are known to dissolve in water than in any other solvents.

However, water is a poor solvent for organic compounds thus we say organic compounds are insoluble in water.

Examples (solid and liquid mixture)

Format:
solute + solvent = solution

  • sugar + water = sugar solution
  • salt + water = salt solution
  • iodine + alcohol = iodine solution
  • Suspensions are mixtures too. However, the solute doesn’t dissolve in the solvent. In suspension, you will find that the solid is either floating around in the liquid or it sinks to the bottom of the liquid.

    List: Volatile Liquids

    Volatile liquids have low boiling point. As a result, they evaporate very easily at room temperature. Because of their low boiling points, volatile liquids convert to the gas phase at a lower temperature and the gas molecules can diffuse faster than the molecules of a non-volatile liquid. This is the reason why perfumes use volatile liquids so that their scent can disperse more quickly into the air.

    When volatile liquids come in contact with human skin, it gives us a cooling sensation.

    Some other examples of volatile liquids include acetone, ammonia, dichloromethane, and diethylether.

    Examples of volatile liquids:

  • alcohol
  • petrol
  • methylated spirits
  • acetone
  • chloroform