Chemical Reactions

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  • Created by: Zammi10
  • Created on: 30-05-13 19:26

Exothermic and Endothermic Reactions

What is an Exothermic Reaction?

These are reactions that transfer energy into the surrounding- normally as heat energy therefore the reaction mixture and surronding become hotter.

Examples of exothermic reactions are:                                                                                                                                                                        Combustion.                                                                                                                                                  Neutralisation                                                                                                                                 Explosions

Endothermic Reactions

These are reactions that take in energy from their surroundings- usually as heat energy therefore the surroundings and the reacting mixture become colder. Examples are:

Electrolysis, Photosynthesis, Thermal decomposition of calcium carbonate in a blast furnace.

Energy is absorbed to break bonds. Bond-breaking is an endothermic process.

Energy is released when new bonds form. Bond-making is an exothermic process. 

 

 

 

 

 

 

 

 

 

 

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Physical and chemical changes

Chemical Changes:

Exo and Endothermic reactions, Effavecance, Precipitation, They are Irreversable, They are usually compounds.

Examples of Chemical changes are: Fossil Fuels- combustion. Baking ie baking bread- from dough to bread.

Physical Changes:

Are always REVERSIBLE eg Mixtures, change of state- solid,liquid gas

Examples of Physical changes are: Melting of things eg Ice cubes melting into water or freezing- water freezing into ice cubes. Also, if you make a mixture from two elements

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Percipitation

Percipitation

Percipitation is the formation of a solid in a solution during a chemical reaction. (a percipitation reaction) When the chemical reaction occurs the solid formed is called the precipitate. This usually happens when an insoluble substance (the perticipate) is formed as a result of a reaction. 

If we want to make an insoluble salt, we can react together two soluble salts in a precipitation reaction.

Silver Chloride is an insoluble salt. We need a soluble silver salt and soluble chloride to make Silver Chloride. ie Silver Nitrate and Sodium chloride. When you mix them together you get insoluble silver chloride and soluble Sodium Nitrate.

The silver chloride appears as tiny particles suspended in the reaction mixture: it forms a precipitate. The precipitate can be filtered, washed with water on the filter paper and then dried in an oven.

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Displacement

If a metal which is more reactive than the metal in a compound, and it reacts with it, the more reactive metal will displace the less reactivemetal from the compound.

eg, Zinc+Iron Sulfate= Zinc Sulfate+Iron

Zinc is higher in the reactivity series than Iron so it can displace the sulfate. It takes the sulfate of Iron so you get Zinc Sulfate. Iron is then left on it's own so you get Zunc Sulfate+Iron

If the metal which is added is less reactive than the metal in the compound, no reaction occurs

eg, Copper+Magnesium Sulfate- No reaction

this is due to the fact that copper is lower in the reactivty series than magnesium so it cannot displace the sulfate.

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What is Produced when a fuel burns?

Combustion is an exothermic reaction which procides energy to heat our homes etc.

Combustion is always exothermic and involves the reaction of a fuel with oxygen producing flames. Due to the fact that fuel reacts with Oxygen, we know that combustion can also be classified as oxidisation reactions.

Most comon fuels are hydrocarbons- they are only made up of carbon and hydrogen

Word equation for combustion of a candle:

Hydrocarbon fuel+Oxygen=Carbon Dioxide+Water

-in an experiment, you can test for carbon dioxide by using limewater and you can test for water by using cobalt chloride paper- it turns from blue (being dry) to white (being wet)

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Fruit Batteries

Acids make Ions (charged particles ) when put into a solution like water.- they act the same way as the current in an electrical circuit so a fruit which is most acidic will act best as a fruit battery

The Metals Used:

The further apart the two metals are in the reactivity series, the higher the voltage.

The electrode is the metal that conducts the electricity

The electrolyte is the substance broken down and in a fruit battery, the juice inside the fruit

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Polymerisation

This type of reaction is when small molecules join together to form a long vhain. The product has new and different properties

Polymers aere usually plastucs and nylon. Polymers are long chains of monomers

Glucose/Starch:

Glucose=monomer

Starch=polymer

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Reactions of metals with metal oxides

Zinc+Copper Oxide=Zinc Oxide+Copper

-(A reaction where oxygen is gained=oxidation reaction)

-(A reaction where oxygen is lost- a reduction reaction)

Zinc becomes oxidised and Copper gets reduced.

A substance that reduces oxygen is calleda reducing agent (something that makes something else be reduced)

A substance that adds oxygen to something else is called an oxidising agent (somehting that makes something else beocme oxidised)

In the reaction above:

the oxidising agent- Copper

the reducing agent= Zinc

(Zinc can't react with Zinc Oxide, same way Iron cant react ith Iron Oxide and so on)

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Law of conservation of mass

Reaction:

strip- of magnesium in crucible with lid on. Heated.

Mass at start/= 1.95g

Mass at end= 1.95

the mass of the reactants is always the same as the mass of the product

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