Chemical Compositions and Reactions

Question 1

Atom

Answer 1

The smallest unit of an element, made up of three different subatomic particles:
Protons
Neutrons
Electrons

Question 2

Elements

Answer 2

An element is a group of all the same type of atom. Atoms of different elements are different due to a unique number of protons in the nucleus.

To tell elements apart, we give them different names and symbols.

Question 3

Periodic Table of Elements

Answer 3

The top number of each square is the atomic number, this is equal to the number of protons in the nucleus.
The name of the element is below that, and above the chemical symbol which contains one uppercase letter and sometimes a lowercase letter.
The bottom number is the relative atomic mass, which is equivalent to the number of protons and neutrons in the nucleus.

Question 4

Relative Atomic Mass (RAM)

Answer 4

Protons and neutrons each have a mass of 1 atomic mass unit.
Electrons have a very small mass and contribute very little to the mass of the element.
Some elements have different isotopes which have different numbers of neutrons.

Question 5

Molecules and Compounds

Answer 5

A molecule exists when two or more atoms are bound together.
A compound is a molecule that contains at least two different atoms.
All compounds are molecules but not all molecules are compounds.
In a chemical structure, the lines between the atoms represent the bonds which hold the molecule together.

Question 6

Chemical Formulae (1)

Answer 6

The chemical formulae indicates which elements are contained in the molecule, and how many molecules of each there are.
Formulae can be written in different ways.

With more complicated chemical formulae, brackets may also be present.

Question 7

Moles, Mass and RMM

Answer 7

The relative molecular mass (RMM) is the sum of the relative atomic masses of the elements making up a molecule in the proportions they occur in the molecule.

Question 8

Moles, Mass and RMM (Equations)

Answer 8

Mass = moles x RMM
RMM = mass/moles
Moles = mass/RMM

Question 9

Chemical Reactions

Answer 9

In a chemical reaction, atoms are neither made nor destroyed, so the same number of each should occur on both sides. The equation should be balanced.

For a chemical reaction to have taken place, a new substance must be produced. Evidence of an occurrence may include:
Bubbles
A colour change
A large change in energy

Question 10

Sample Preparation

Answer 10

It is often necessary to prepare samples to be analysed. One of the most common requirements is to produce a liquid sample that can be introduced into an instrument for analysis.

Sometimes it is necessary to extract the components of interest from a mixture.

Question 11

Polarity

Answer 11

For a solid to dissolve in a liquid, the polarities of the two need to be similar.
Polar solids will dissolve in polar liquids.
Non-polar solids will dissolve in non-polar liquids.

Polarity refers to the presence of a slight positive or negative charge in part of a molecule.

This property arises due to the differing abilities of atoms in a molecule to pull electrons towards themselves.

Question 12

Polarity - Ionic and Covalent Bonding

Answer 12

Ionic and covalent are different types of bonding, which hold together the atoms within a molecule of a compound.

Polar compounds contain ionic bonds. Ionic bonding involves atoms losing and gaining electrons with the resulting +/- charges holding the atoms together.

Non-polar compounds only contain covalent bonds. Covalent bonding involves the sharing of electrons between atoms.

Question 13

Polarity - General Rule

Answer 13

Although this doesn’t work all of the time, it is a good general rule to use:

Polar compounds - combinations of metal and non-metal elements.

Non-polar compounds - combinations of non-metal elements only.

Question 14

Dissolving Polar Compounds

Answer 14

Water and other polar solvents dissolve polar compounds by stabilising the charges that exist within the solid.

The solvent molecules surround the ions and their slight charge cancels out the charge on the ion.

Question 15

Dissolving Non-Polar Compounds

Answer 15

As with polar compounds, there are weak intermolecular forces which hold together molecules of non-polar compounds.

The much weaker forces come about from the random movement of electrons in adjacent molecules - causing temporary and very weak +/- regions in the molecule.

Non-polar compounds do not have strong attractive forces from the permanent +/- areas present within polar molecules.

Question 16

Dissolving Non-Polar Compounds

Answer 16

Non-polar compounds are not very soluble in polar solvents. The strong forces holding together polar molecules means they would rather remain together, than allow a non-polar molecule to come between them.

However, it is possible to assist their dissolution. Addition of a surfactant compound that contains both polar and non-polar sections. These produce a protective shell, allowing dissolution in a normally unsuitable solution.