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Exothermic and Endothermic



Decomposition:-when a compound breaks down into simpler compounds or elements. This happens when the bonds between the atom weakens by heat or by adding a catalyst (a chemical which can alter the speed of a chemical reaction without getting involved).

e.g. calcium carbonate------(heat)---- calcium oxide + carbon dioxide

When you eat, the enzyme (biological catalyst) called amylase in your saliva decomposes starch into glucose.

starch ----amylase---- glucose

Neutralization:- when an acid cancels the alkalinity a base or vice versa. When an acid reacts with a base, the result is a neutral substance or substances.

metal oxide (base) + acid = salt + water

Because bases and acids always have metals and non-metals, salts are always produced as a result of neutralization.

metal hydroxide + acid = salt + water

Because all bases and acids have oxygen and hydrogen then the result of neutralization is always water.

metal carbonate + acid = salt + water + carbon dioxide

Because all carbonates have carbon and oxygen, the result is carbon dioxide.

e.g. sodium hydroxide + hydrochloric acid ------ sodium chloride + water

NaOH + HCl = NaCL + H20

If you use hydrochloric acid, the salt will always be a chloride, if you use sulphuric acid, the salt will be a sulphate and a salt will be a nitrate if nitric acid is used.

Oxidation:- when any substance reacts with oxygen or releases hydrogen, the result is an oxide. Oxidation may need time to occur (e.g. rusting) or it may need heat (e.g. burning). When metals like iron corrode or rust, the result is iron oxide.

iron + oxygen ----water---- iron oxide

When other metals or substances burn, the result is an oxide.

magnesium + oxygen ----heat---- magnesium oxide

Reduction:- when a substance reacts with hydrogen or releases oxygen. One example of reduction is when iron is produced from iron oxide in a blast furnace.

iron oxide ------ iron + oxygen

We can also call this decomposition.

Redox:- when REDuction and OXidation occur at the same time (REDOX). When hydrogen is passed over hot copper(II) oxide, this reaction takes place:

copper(II) oxide + hydrogen ----- copper + water

CuO                   +       H2    ------ Cu      +    H2O

In this reaction copper has been reduced while hydrogen has oxidized.

Polymerization:- When monomer molecules bond together to form a polymer. There can be 50,000 monomers in one polymer. One example is making polythene from ethene.

Photosynthesis:- when energy from the Sun is stored in a sugar called glucose. Glucose is formed when carbon dioxide reacts with water.

6CO2 + 6H20 -----sunlight and chlorophyll---- 602 + H12C6012

In this reaction, water was split into hydrogen and oxygen. Oxygen was released into the air while the hydrogen reacted with carbon dioxide.

Respiration:- Respiration is the opposite of photosynthesis. Its target is to release energy from glucose. It is very similar to burning or oxidizing.

602 + H12C6012-----sunlight and chlorophyll----6CO2 + 6H20

Exothermic and endothermic:- any reaction which takes in more heat than it produces is an endothermic reaction. Any reaction which produces more heat than taking is an exothermic reaction. Oxidation and respiration are exothermic: they release energy. In an exothermic reaction, energy used up to bond the elements together in the outer-most is released as heat while the reverse happens in endothermic reactions. The mass of a substance increases by 1 trillionth of a gram in endothermic reactions and decreases by 1 trillionth of a gram in an exothermic reaction but you don't notice it.

Displacement:- One substance pushes out another to take its place. For example, if an iron nail is placed in copper(II) sulphate solution, some iron dissolves and displaces copper in the solution, because iron is more reactive than copper. A substance becomes more reactive when its outermost shell needs to take or release a few electrons to become full or empty. In other words, an element becomes more reactive as is its group number decreases. The copper is deposited on the nail as brown coating:

iron + copper sulphate -------- copper + iron sulphate

In this reaction, the oxygen in the acid reacted with the metal and the hydrogen was released.

metal + acid ----- metal oxide + hydrogen

Precipitation:- When some solutions are mixed, they react and give a product which is insoluble. It appears as tiny, solid bits called a precipitate.

e.g. silver nitrate (soluble) + sodium chloride (soluble) =

silver chloride (insoluble precipitate) + sodium nitrate (insoluble)