Monday, 26 March 2012

How to balance chemistry equations

The Periodic Table

  • Understand the terms group and period
  • Recall the positions of metals and non-metals in the Periodic Table
  • Explain the classification of elements as metals or non-metals on the basis of their electrical conductivity and the acid-base character of their oxides
  • Understand why elements in the same group of the Periodic Table have similar chemical properties
  • Recall the noble gases (group 0) as a family of inert gases and explain the lack of reactivity in terms of their electronic configuration

  • The periodic table is a list of elements arranged in order of their increasing atomic (proton) number.
  • A period is a horizontal row of elements--the number of electron shells is the same as the period number of the element.
  • A group is a vertical column of elements--the number of valence electrons (outer shell electrons) is the same as the group number of the element.
    • Since elements with similar electronic configurations have similar chemical properties, we can deduce that elements in the same group have similar chemical properties.
  • The block of metals between Groups 2 and 3 are known as the transition elements/metals, these form coloured compounds.

Chemical Properties

  • Usually have 1-3 electrons in their outer shell
  • Lose their valence electrons easily
  • Form oxides that are basic
  • Are good reducing agents

  • Usually have 4-8 electrons in their outer shell
  • Gain or share valence electrons
  • Form oxides that are acidic
  • Are good oxidizing agents

Physical Properties

  • Good electrical and heat conductors
  • Malleable—can be hit and shaped
  • Ductile—can be stretched into wire
  • Possess metallic luster (shiny)
  • Opaque as thin sheet
  • Solid at room temperature (except Mercury [Hg]- liquid)

  • Poor conductors of heat and electricity
  • Brittle- if a solid
  • Non-ductile
  • Do not possess metallic luster
  • Transparent as a thin sheet
  • Solids, liquids or gases at room temperature


Noble Gases (Group 0):
  • Least reactive elements in the periodic table.
  • Apart from helium (which has 2), the rest have 8 valence electrons--all have full outer shells.
  • Their full electronic structures make them unreactive.
  • Group 0 elements also referred to as inert gases (because they are unreactive) or rare gases (because less than 1% of the air is made up of these gases).
  • Noble gases do not react to form compounds because their atoms have full outer shells of electrons, it is energetically easier to stay as they are. 
    The noble gases are:
  • Monatomic elements
  • All colorless gases at room temperature
  • Have low melting and boiling points that increase going down the group (As atoms get bigger, there is greater intermolecular attraction that require more energy to break.)
  • Insoluble in water
  • Unreactive
just thought this was cool :P 

Tuesday, 13 March 2012

Rates of Reaction

Different Speeds of Reaction

Different chemical reactions take place at different speeds.

Very fast:
  • explosion of a petrol-air mixture
  • precipitation reactions
  • fireworks going off
  • coal burning
Moderately fast:
  • reaction of metals or carbonates with dilute acids
  • rusting of iron in air
  • reaction of magnesium with cold water
  • oil forming
  • silver tarnishing
How do we calculate the speed of reaction? 

During a chemical reaction, the reactants get used up as products are formed. We can measure the speed of reaction by measuring the amount of a reactant used up per unit time, i.e.

Speed of reaction= amount of reactant used up/time taken

The speed of a reaction can also be measured in terms of the amount of a product formed/obtained per unit time, i.e. 

Speed of reaction= amount of product formed/time taken

For a chemical reaction that produces a gas, the speed of reaction can be found by measuring the volume of gas produced per unit time, i.e.

Speed of reaction= volume of gas produced/time taken

Measuring Speed of Reaction from Changes in Volume

The speed of a reaction can be found by measuring the following quantities at regular time intervals:
  • volume of gas produced by the reaction
  • the mass of the reactant that remains

Measuring Speed of Reaction from Changes in Mass

The speed of a reaction can also be found by measuring the changes in mass of a reaction mixture. This method works best for reactions which produce gases such as carbon dioxide. 

Factors Affecting Speed of Reaction

Many factors affect the speed of a chemical reaction. These include:
  • the concentration of the reactants
  • the pressure of the reactants (if the reactants are gaseous)
  • the particle size or surface area of the reactants
  • the temperature at which the reaction is occurring
Increasing the concentration means there's more of the reactants in the same volume. Thus with more particles there will be more collisions, and there will be higher chance of effective collisions, thus increasing the rate of reaction.

This is just like concentration, but it's more to do with gaseous reactants. With higher pressure--more particles--more collisions--higher chance of effective collisions--increase rate of reaction.

Surface area:
The bigger the surface area, the more chance of collisions, ditto above. Same thing. It's all about having more collisions, thus more effective collisions which increase rate of reaction. E.g. if you had a piece of metal, and you cut it into many smaller pieces, it would react faster with acid because the acid will have more area of the metal to collide with.

The higher the temperature, the more thermal energy is transferred to kinetic energy for the particles, thus they move faster and collide more often. If they have more energy, it is likelier that they will have minimum activation energy therefore there will be more effective collisions. And for a change, the rate of reaction increases... :P

For a reaction to occur between 2 particles: 
  1. the reacting particles must collide with each other
  2. they must collide with a certain minimum amount of energy known as the activation energy
In this way, collisions between reacting particles result in the formation of product particles. These collisions are known as effective collisions. 
Thus in a reaction between hydrogen and chlorine, only fast-moving molecules with energies equal to or greater than the activation energy will react on collision to form hydrogen chloride. 
  • In general, when any factor increases the rate of effective collisions between reacting particles, it will also increase the speed of reaction. 


4.13 understand the use of ΔH to represent molar enthalpy change for exothermic and
endothermic reactions

4.14 represent exothermic and endothermic reactions on a simple energy level diagram

4.15 recall that the breaking of bonds is endothermic and that the making of bonds is

Just remember this! To break smth, obviously you need energy, so you need to take it in, hence breaking of bonds is endothermic. When substances form bonds, it's usually to become more stable, and thus they release energy. E.g. reactive elements like sodium react with chlorine to form sodium chloride, your common table salt, and this is a very stable ionic compound, and the reaction is exothermic. (less energy, less violently reactive...makes sense right?)

Heat Changes in a Reaction

heat change/enthalpy change: the amount of energy involved in a reaction, measured in kilojoules (kJ) which is 1000 joules, and is represented by the symbol DH. (D is the Greek letter for 'delta', which means change. H means energy content. 

For an exothermic reaction DH is negative. This is because the chemicals have lost energy to the surroundings. 
For an endothermic reaction, DH is positive. This is because the chemicals have gained energy from the surroundings. 

1. When one mole of carbon is burnt in excess oxygen, 349 kJ of heat is produced. This is an exothermic reaction. So DH=-349kJ. 

 carbon + oxygen à carbon dioxide
C(s) + O2 à CO2 (g)   DH=-349kJ

2. When one mole of hydrogen reacts with one mole of iodine, 52 kJ of heat is absorbed from the surroundings. This is an example of an endothermic reaction. DH=+52 kJ

 hydrogen + iodine à hydrogen iodide
H2 (g) + I2 (g) à 2HI (g)   

Energy Level Diagrams for Exothermic and Endothermic Reactions

Energy level diagrams--convenient ways to express energy changes in a reaction

Exothermic Reaction 
Consider an exothermic reaction, heat energy is lost to surroundings. This means that total energy of the products is less than that of the reactants. 

The energy level diagram of an exothermic reaction

The difference between the energy levels of the products and the reactants is equal to the amount of energy given out by the reaction. 
i.e.  DH=Hproducts –  Hreactants  (as the energy of the reactants is larger, DH becomes negative. --> taking away a larger value from a smaller one gives a negative result. e.g. 2-4=-2) 

 DH= Hproducts –  Hreactants

Endothermic Reaction
Since an endothermic reaction absorbs heat from the surroundings, the products will have more energy than the reactants. The difference between the energy levels of the products and reactants is the energy absorbed during the reaction.

The energy level diagram of an endothermic reaction

Activation Energy "EaThe minimum energy required to initiate (start) a chemical reaction. Both endothermic and exothermic reactions require activation energy.

So when particles collide and react, this is called an effective collision. This only happens when the particles have the minimum activation energy, if not, they may collide but it wouldn't result in a reaction.

Exothermic Reaction
Endothermic Reaction
Gives out heat to the surroundings
Takes in heat from the surroundings
Causes an increase in temperature
Causes a decrease in temperature
Has a negative DH
Has a positive DH
Has products lower in energy than the reactants
Has products that have higher energy than the reactants

This is a great, concise video to summarise everything, watch! 

Thursday, 8 March 2012

Ionic Bonding

Spec 1.27 describe the formation of ions by the gain or loss of electrons

We think of atoms 'losing' an electron to another, what is really happening is that one of the atoms is attracted to the electron pair much more strongly than the other one. The electron pair is then pulled very close to that atom and away from the other one. Hence this atom gains an electron, whilst the other loses one.
[Note in a covalent bond, the electrons are shared between two atoms. Both nuclei are attracted to the same electron pair.]

The electrically charged particles are called ions. An ion is an atom which carries an electrical charge, either positive or negative.
  • a positive ion is called a cation
  • a negative ion is called an anion [tip: anion= A-Negative-ION]
Ionic bonding is bonding in which there has been a transfer of electrons from one atom to another to produce ions. The substance is held together by strong electrostatic attraction between the positive and negative ions. 

Covalent Bonding

What is a covalent bond?
This is a shared pair of electrons between two non-metal atoms. If each atom shares 2 electrons, it is called a double covalent bond. E.g. in oxygen, as each oxygen atom has 6 outer shell electrons it shares 2 electrons to have 8 as a full outer shell, hence oxygen is diatomic. O2
Each of the positively charged nuclei is attracted to the same negatively charged pair of electrons, which is why covalent bonds are so strong.

Why does hydrogen form molecules? 
Whenever a bond is formed (of whatever kind), energy is released, and that makes the things involved more stable than they were before. The more bonds an atom can form, the more energy is released and the more stable the system becomes.
In the case of hydrogen, each hydrogen atom has only one electron to share, so it can only form one covalent bond. The Hmolecule is still much more stable than two separate hydrogen atoms.

The significance of noble gas structures in covalent bonding
The formation of covalent bonds producing noble gas structures is quite common. When atoms bond covalently, they often produce outer electronic structures the same as noble gases-full outer shell. This is so they become stable and unreactive. The more electrons shared, the more covalent bonds there are, the more stable the molecule is.