C2aL11 Compounds

Key Words

Atom - smallest particle into which an element can be chopped.

Chemically combined - some change involving electron shells has taken place to join atoms.

Element - substance consisting of one type of atom only.

Formula - short-hand showing the elements and the number of atoms there are in the compound.

Molecules - elements or compounds consisting of at least two atoms covalently bonded.

Symbol equation - equation that describes a reaction, written in symbols rather than words.

Word equation - equation that describes a reaction, written in words rather than symbols.

Test Yourself

Homework
Subject Page
Home

Grade E

Elements are substances that consist of one kind of atom only.  Compounds are substances that consist of two or more atoms chemically combined.

Sodium chloride is a compound that is ionically bonded.  In its solid form, it is a crystal lattice containing many millions of Na+ ions and Cl- ions.

The formula for sodium chloride is NaCl, which means that for every sodium ion, there is one chloride ion.

Glucose is a molecule held together by covalent bonds:

The glucose molecule has the formula C6H12O6.  This is a short-hand way of writing that in each molecule, there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms.

Please remember that the numbers are written subscript.

A mixture is where two or more substances are put together, but do not react.  A compound is where a chemical reaction has taken place

Grade C

An important skill for any chemist is to write reactions in the form of equations.  A word equation sums up the reaction in words:

glucose + oxygen ® carbon dioxide + water

A symbol equation tells us not only the formulae for the substances, but also how many atoms are involved.  Remember that you can't make or destroy atoms; you have to have as many at the end as you did at the start.

C6H12O6 + 6O2 ® 6CO2 + 6H2O

  • There are 6 carbon atoms at the start, so there has to be 6 carbon atoms at the end.

  • There are 12 hydrogen atoms which go to make 6 molecules of water.

  • There are 18 oxygen atoms (6 from the glucose, and 12 from the oxygen in the air) to be shared out - 12 in the carbon dioxide and 6 in the water.

Consider this reaction:

Iron oxide + carbon monoxide ®  iron + carbon dioxide

Fe2O3 + CO ® Fe + CO2

On the left hand side we have 4 oxygen atoms and on the right hand side, we have just 2 oxygen atoms.  We can't destroy the other two, so we need to balance up the equation:

  • If we wrote Fe2O3 + CO ® 2Fe + 2CO2 then that would be wrong; we have got 1 carbon on the left hand side, and two on the right.

  • Let's double the iron oxide - 2Fe2O3 + 2CO ® 4Fe + 2CO2 .  That is still wrong because we have oxygen atoms left over.

You can waste a lot of time fiddling about getting things right by trial and error.  A better way is to look at the ratio of the oxygen atoms:

  • There are three atoms in the iron oxide and one in the carbon monoxide.  So we need 3 carbon monoxide molecules to take away the three oxygen atoms from the iron oxide:

  • So we will have 6 oxygen atoms on the left hand side, and 6 oxygen atoms on the right hand side to give us:

Fe2O3 + 3CO ® 2Fe + 3CO2

This equation is balanced. 

Grade A

Balancing equations is not easy to start off with, and needs practice.  It is an important skill for the chemist, especially when thinking about industrial processes.