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1. The periodic table is a way of classifying atoms according to both their chemical properties and their (atomic) mass. Metals are found on the left and non-metals on the right. The bigger the atomic mass the larger the atom.
2. Covalent bond are formed between two non-metals. Ionic bonds are generally between metals and non-metals
3. In covalent bonding atoms will share electrons in order to get to an arrangement where each atom has a full outer shell of electrons
4. Remember the innermost shell of electrons can contain 2, the next shell contains 8 and the third shell also contains 8. All atoms want to get themselves into a situation where they have full outer shells of electrons.
5. In ionic bonding the metal atom will lose one or more electrons and form a positive ion in order to get to an arrangement where it has a full outer shell of electrons . The non-metal will gain one or more electrons and form a negative ion in order to get to an arrangement where it has a full outer shell of electron
6. The ions that are formed will then be held together by a electrostatic force of attraction between opposite charges. This holds each positive ion close to it's neighboring negative ions on all sides
7. The table below compares the properties of ionic and covalent compounds:
8. When naming covalent compounds:
the least electronegative element is named first
the name of the second element has the suffix 'ide'
if the compound contains hydrogen, hydrogen is named first
the number of atoms of each element is indicated by a prefix - see below
if the first element only has one atom the prefix is not used
9. When naming ionic compounds:
the metal element is named first
the non-metal comes second and ends in 'ide' if there is only one type of non-metal atom
Transition metals can take on a number of different ion charges within a compound. This is shown by roman numerals (in brackets) following the metal's name - see below
Some ions contain more than one type of element, we call these Polyatomic ions. They are usually negatively charged ions so follow the metal when you name them or write down formula. Ammonia is one example of a positive ion though and will replace the metal in an ionic compound. You do not need to memorize all the formula for polyatomic ions, but you do need to recognize from their names that they are polyatomic and then look up their formula (e.g. NaOH is Sodium Hydroxide. The Sodium ion will be +1 and the Hydroxide ion will be -1. Together they give a neutral compound).
10. Balancing Chemical Equations:
When a chemical reaction occurs, the reactants are converted to products. However, the number of atoms, remains the same before and after the reaction as per the law of conservation of mass. When you write a chemical equation, a necessary step is to balance the number of atoms on both sides of the equation.
Take the unbalanced equation and make a note of the elements present in each side of the equation.
Now, count the number of molecules of each element present on both sides of the equation.
Here comes the task of balancing the chemical equations. You should see that same numbers of all elements and ployatomic ions are present on the reactants side as well as the products side.
While balancing the equations you should only change the co-efficient of the chemical formula but not the subscripts. Changing the subscripts will change the components.
Start by balancing one element at a time. Finally check if all the elements are balanced.
11. A Mole is a unit of measurement and represents a certain quantity of a substance. A mole is the quantity of a substance that contains the same number of particles found in 12.000 grams of carbon-12. That number of particles is called Avogadro's number, which is 6.02x1023
We often use moles instead of grams to quantify the amount of a substance as it makes the mathematics a bit simpler (e.g. when Na reacts with Cl to form NaCl it requires 1 mole of Na for every 1 mole of Chlorine. It would require 23g of Na for every 35g of Chlorine though which is more complicated)
12. The periodic table gives us the relative atomic mass (or weight) of each element (Remember that the mass number is the biggest of the two numbers shown for each element on the periodic table, e.g. Hydrogen is 1, Carbon is 12, Oxygen is 16). This is their mass relative (or compared to) a proton or neutron, which each have a mass of 1. Electrons within an atom have such a small mass that we ignore them.
We can use the relative atomic mass of elements to create a relative formula mass (also called relative molecular mass for covelant compounds). For example:
Water has the formula H2O so has a formula mass of 18 (1+1+16)
Methane has the formula CH4 so has a formula mass of 16 (12+1+1+1+1)
Magnesium oxide has the formula MgO so has a formula mass of 40 (24+16)
These formula masses tell you how many grams of a substance are in a mole. e.g. 1 mole of water weighs 18 grams. We write this as 18g/mol (grams per mole)
13. Converting between mass and moles:
Use this formula to convert between moles of a substance and mass.
For example: How many moles are in 72g of water?
First find the relative formula mass for water = 16 + 1 +1 = 18g/mol
Secondly cover up amount in the triangle above and you are left with amount = mass / relative formula mass
Thirdly carry out the calculation: amount = 72 / 18 = 4 mol
14. Avogadro's law states that equal volumes of gas (at the same temperature and pressure) contain the same number of particles.
1 mole of any gas occupies 24 litres (or dm3) of volume. This is assuming that the gas is at Room Temperature and Standard Pressure. Room temperature is 25 ° C or 298 K. Standard pressure is 1 atmosphere or 101,000 Pascals.
15. A 1 molar solution (1M) is a solution that contains 1 mole of a substance per dm3 (litre). If you dissolve 1 mole of Copper Sulphate in 10 dm3 of water you get a 0.1M solution
16. In practice, it is not always possible to get the calculated amount of product in a reaction. This could be because....
reversible reactions may not go to completion
some product may be lost when it is removed from the reaction mixture
some of the reactants may react in an unexpected way
The yield of a reaction is the actual mass of product obtained. The percentage yield can be calculated:
For example, the maximum theoretical mass of product in a certain reaction is 20g, but only 15g is actually obtained.
percentage yield = 15⁄20 × 100 = 75%
17. A reaction stops when all the particles of one of the reactants are used up. In a reaction involving two reactants:
The limiting reactant is the one that is all used up at the end of the reaction
The reactant in excess is still there at the end of the reaction (although in a smaller amount than at the start)
For example, magnesium reacts with hydrochloric acid. When the reaction is over:
Magnesium is the limiting reactant if it is all gone at the end
Hydrochloric acid is the limiting reactant if some magnesium is left at the end
If you know the mass of each reactant you can work out which is the limiting agent. You must use each mass of reactant to calculate the mass of product formed. The one that gives the smallest mass of product is the limiting reactant.
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