Summary sheets - Where do all our raw materials come from.
What do chemists do with raw materials?
Chemists use chemical reactions to turn raw materials into new and more useful substances. During chemical reactions all that occurs is a rearrangement of atoms. Each new arrangement of atoms represents a new substance.
Chemical reactions can sometimes seem like magic because a new substance can seem to appear out of nowhere. When this happens it might be that for example; two colourless gases (that you can’t see) have reacted together to form a liquid (which you can see).
At other times a substance may seem to have disappeared. This could be when a solid has reacted with another substance to produce a colourless gas.
To repeat; during a chemical reaction nothing is lost or gained! A chemical reaction is simply a rearrangement of atoms. The reactants on the left hand side of the equation will be the same mass as the products on the right hand side of the equation. In Chemistry we refer to this as the CONSERVATION OF MASS.
The Law of Conservation of Mass states that in a chemical reaction no mass is lost or gained.
Take for example the reaction of hydrogen and oxygen gases to produce water.
You can see that exactly the same atoms appear on the left hand side as on the right hand. During the reaction there has simply been a rearrangement of atoms. The reaction would be represented as:
2H2 (g) + O2(g) 2H2O(l)
This means that for every two hydrogen molecules there will be one oxygen molecule and that the two will react together to form two water molecules.
If there was a mass of 20g hydrogen reacting with 30g of oxygen then the amount of water produced would be 50g.
Energy changes during chemical reactions
In physics you will have found out that there are a number of different types of energy. All reactants contain a certain amount of CHEMICAL ENERGY. During a chemical reaction particles collide together. Bonds may be broken and new bonds made forming new products. As the reaction occurs it will be accompanied by an energy change.
As an example, titanium metal is put into fireworks because when it reacts with oxygen it creates silver sparks. This is because chemical energy changes to heat and light energy when titanium and oxygen particles collide and then react.
Reactions during which energy is given out to the surroundings are known as EXOTHERMIC reactions. Most reactions are exothermic.
Some reactions take in energy from the surroundings. These reactions are said to be ENDOTHERMIC.
A summary of what can happen to show that a chemical reaction has taken place.
· A colour change occurs
· A gas is produced (bubbles are seen)
· The substance seems to have disappeared (usually if gases have formed and escaped)
· A new substance seems to have appeared (usually if one of the starting materials was a gas and a solid or liquid has been formed)
· Heat is given out (the reaction vessel heats up this is called an exothermic reaction)
· Heat is taken in (the reaction vessel cools down, this is called an endothermic reaction)
· Sound is given out
· Light is given out
· A precipitate forms (this occurs for example when two liquids react to form an insoluble solid.
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