Chemistry science a

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  • Created by: leena
  • Created on: 12-06-12 18:59

Forming Bonds

Compounds of metas bonded to non-metals have ionic bonds

the formua of an ionic compound shows the simplest ratio of ions 

compounds of non-metals have a covaent bonds

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chemical equations

the mass of the products is equal to the mass of reactants

symbol equation should always be balanced

(learn how to balance  an equation)

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limestone and its uses

many uses

blocks of limestone can be used for building 

limestone is used to make calcium oxide and cement

we make concrete by mixing cement with sand, aggregate and water

limestone is mainly calcium carbonate CaCO3

calcium carbonate decomposes when heated to make calcium oxide and carbon dioxide    

this type of reaction is called thermal decompostion

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reactions of carbonates

metal carbonates decompose to the metal oxide and carbon dioxide when heated strongly enough

a Bunsen burner flame cannot get hot enough to decompose sodium carbonate or potassium carbonate

all carbonates react with acids to produce a salt, water carbon dioxide gas

limestone is damaged by acid rain because the calcium reacts with calcium carbonate in the limestone reacts with acids in the rain

calcium hydroxide solution is called limewater

liimewater is used to test for carbon dioxide 

limewater turns cloudy because it reacts with carbon dioxide to produce insoluble calcium carbonate 

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fuels from crude oil

crude oil contains many different compounds that boil at different tempretures

these burn under different conditions and so crude oi needs to be separated to make useful fuels

we can separate a mixture of liquids by distillation

simple distillation of crude oil of crude oil can produce liquids that boil within different temperature ranges. these liquids are called fractions.

most of compounds in crude oi are hydrocarbons (hydrogen and carbon)

many of these hydrogen's are called alkanes (                   )

alkanes contain as many hydrogen atoms as possible in each molecule and so we call them saturated hydrocarbons

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fractional distillation

crude oil is separated into fractions using fractional distillation

the properties of each fraction depends on the size of the hydrocarbon molecules

-inside the column there are many trays with holes to allow gasses through

-the vapours move up the column getting cooler as they go up

-the hydrocarbons condense to liquids when they reach the level that is at their boiling point

-different liquids collect on the trays at different levels and there are outlets to collect the fractions

hydrocarbons with the smallest molecules have the lowest boiling points and so are collected at the top of the column . the fractions collected at the bottom of the column contain hydrocarbons with the highest boiling points

fractions with low boiling ranges have low viscosity so they are runny liquids

they are very flammable so they ignite easily

they also burn with clean flames,producing little smoke.

this makes them very useful as fuels

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burning fuels

when pure hydrocarbons burn completely they are oxidised to carbon dioxide and water. 

in a limited supply of air incomplete combustion may produce carbon monoxide

carbon may also produced and some of the hydrocarbons may not burn

this produces solid particles that contain soot and unburnt hydrocarbons called particles

most fossil fuels contain sulfur compounds

when the fuels burn these sulfur compounds produce sulfur dioxide

sulfur dioxide produces acid rain

at high tempertures produced when fuels burn oxygen and nitrogen in the air may combine to form nitrogen oxides 

nitrogen oxides also produce acid rain

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cleaner fuels

burning any fuel that contains carbon produces  carbon dioxide

carbon dioxide is a greenhouse gas that many scientists believe is the cause of global warming

incomplete combustion of these fuels produces poisonous gas carbon monoxide 

it can also produce  tiny solid particles that reflect sunlight and so causes global dimming

exhaust system of cars are fitted with catalytic converters to remove carbon monoxide and nitrogen oxide 

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alternative fuels

biofuels are made from plant or animal products and are renewable 

biodiesel can be made from vegetable oils extracted from plants

biosdiesel advantages: little contribution to carbon dioxide levels because carbon dioxide given off when it burns was taken from the atmosphere by plants as they grew

there are also disadvantages: plants that are grown from biodiesel use large area of farmlands 

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