Science. Reactions and compounds. Chemical reactions

Transcription

1 Science Chemical reactions When elements react, their atoms join with other atoms to form compounds [compound: A compound is a substance formed by the chemical union (involving bond formation) of two or more elements. ]. Chemical bonds form when this happens, which involves atoms transferring or sharing electrons. Reactions and compounds New substances are formed by chemical reactions. When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur react together to form a compound called iron sulfide. Mixture of iron (grey) and sulfur (yellow) powders. The mixture is heated in a test tube. 1/5

2 A chemical reaction occurs and iron sulfide is formed. Compounds usually have different properties from the elements they contain. Ionic bonds Chemical bonds involve electrons from the reacting atoms. Compounds formed from metals and non metals consist of ions. Ions are charged particles that form when atoms (or clusters of atoms) lose or gain electrons: metal atoms lose electrons to form positively charged ions non metal atoms gain electrons to form negatively charged ions The ionic bond is the force of attraction between the oppositely charged ions. This animation shows how ions form when sodium atoms react with chlorine atoms to form sodium chloride. Covalent bonds Compounds formed from non metals consist of molecules. The atoms in a molecule are joined together by covalent bonds. These bonds form when atoms share pairs of electrons. Chemical formulas The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound. For example, in iron sulfide every iron atom is 2/5

3 joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na 2 O. Notice that the 2 is written as a subscript, so Na2O would be wrong. This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is written as CO 2. Carbon dioxide units contain one carbon atom and two oxygen atoms Sometimes you see more complex formulae such as Na 2 SO 4 and Fe(OH) 3 : a unit of Na 2 SO 4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together a unit of Fe(OH) 3 contains one iron atom, three oxygen atoms and three hydrogen atoms the brackets show that the 3 applies to O and H Chemical equations You should be able to write word equations for the reactions you study in GCSE Science or GCSE Chemistry. If you are taking the Higher Tier, you should also be able to write and balance symbol equations. Copper and oxygen reaction: getting a balanced equation Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react 3/5

4 together to make copper oxide. Take a look at this word equation for the reaction: copper + oxygen copper oxide Copper and oxygen are the reactants because they are on the left of the arrow. Copper oxide is the product because it is on the right of the arrow. If we just replace the words shown above by the correct chemical formulas, we will get an unbalanced equation, as shown here: Cu + O 2 CuO Notice that there are unequal numbers of each type of atom on the left hand side compared with the right hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides. Here is the balanced symbol equation: 2Cu + O 2 2CuO You can see that now there are two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction. Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide 4/5

5 Remember: never change a formula to balance an equation. Conservation of mass No atoms are lost or made during a chemical reaction. This means that the mass is always conserved. In other words, the total mass of products after the reaction is the same as the total mass of the reactants at the start. This fact allows you to work out the mass of one substance in a reaction if the masses of the other substances are known. For example: Carbon reacts with oxygen to form carbon dioxide: C + O 2 CO 2 12 g of carbon will react to form 44 g of carbon dioxide. It must react with = 32 g of oxygen to do this. The animation below shows two more examples of conservation of mass. Now try a Test Bite Back to Revision Bite 5/5