No change in chemical content and properties

Reversible and temporary

Change of characteristics and physical properties

No new substances produced

Physical change

Examples of physical changes :•The condensation of steam•The evaporation of water•The sublimation of ammonium salt•The change in the shape of a ball when pressed•The melting of ice into water

Example picture of physical change

The change in the shape of a ball when pressed.

The melting of ice into water

The evaporation of water

Irreversible and permanent

New substance(s) is formed

Release or absorption of energy

Change in chemical content and properties

Chemical Change

Example of chemical changes include :•The rusting of iron nails : iron + oxygen + water rusted iron•Photosynthesis : carbon dioxide + water food + oxygen•The burning of magnesium in air : magnesium + oxygen magnesium oxide•The reaction of an acid with an alkali : Sulphuric acid + Calcium hydroxide Calcium sulphate + water

chlorophyll

sunlight

Example of chemical change

Rusting of iron nails Burning of magnesium in air

Similarities and differences between chemical change and physical change

Heat change in chemical reactions• All chemical reactions which take place involve energy change, whether it

is the absorption or the release of energy.• The main energy change is heat energy.• In chemical reactions:

– The energy is absorbed to break the chemical bonds between atoms– The energy is released when chemical bonds between atoms are formed

• If the energy released is more than the energy absorbed in a particular chemical reaction, the overall reaction would release heat energy into the surrounding. The total energy content of the products is lower than the total energy content of the reactants. For example, heat is released when sodium hydroxide and dilute hydrochloric acid solutions undergo neutralisation : Sodium Hydroxide + Hydrochloric acid Sodium Chloride + water

• On the other hand, if the energy absorbed is more than the energy released, the overall reaction would absorb heat energy from the surroundings. For example, heat is absorbed when calcium carbonate decomposes into calcium oxide and carbon dioxide : Calcium carbonate Calcium oxide + Carbon dioxide

Heat

Reactions involving heat loss and heat gain

• Based on the heat change which occurs, chemical reactions can be classified into – Exothermic reactions– Endothermic reactions

• An exothermic reaction is a reaction which releases heat(heat loss) into the surroundings. This type of reaction causes the surrounding temperature to increase.

• An endothermic reaction is a reaction which absorbs heat(heat gain) from the surroundings. This reaction causes the surrounding temperature to decrease.

Comparison between exothermic and endothermic reactions

Energy level diagrams for exothermic and endothermic reactions

Haber Process

More details about Haber Process

The Haber Process.flv

© Chris Goodman (http://www.youtube.com/watch?v=CZDaJ7do6Go)

Contact Process

More details about Contact Process

Sulfuric acid and the Contact process.flv

© Chris Goodman (http://www.youtube.com/watch?v=_zj3bMjFclA)

Reactivity series of metals

• The reactivity of a metal is the activity of a particular metal in a chemical reaction.

• Different metals have different reactions towards water, dilute acids and oxygen.

• There are metals which are very reactive, reactive and not reactive. For example :– Potassium and sodium --- very reactive– Magnesium and zinc --- reactive– God and platinum --- not reactive

Comparing and contrasting the reactivity of metals with water, acids and oxygen

Position of carbon in the reactivity series of metals

• Although carbon is not a metal element, it is also inclined to react with oxygen to produce carbon dioxide. (carbon + oxygen carbon dioxide)

• Carbon is also a reducing agent because of its high position in the reactivity series. Thus, carbon can reduce metal oxide K to metal K and release carbon dioxide when two substances are heated together. (carbon + metal oxide K Carbon dioxide + metal K)

• The position of carbon in the reactivity series can be determined by heating a mixture of carbon and metal oxide. Carbon reduces metal oxide to its metal if carbon is more reactive than the metal. Carbon does not react with metal oxide if carbon is less reactive than the metal.

• Therefore, carbon can only be used to extract metals which are less reactive than carbon itself.

• Carbon is less reactive than potassium, sodium, calcium, magnesium and aluminium. Thus, carbon is unable to reduce the oxides of these metals.

• Carbon is more reactive than zinc, stannum, lead, copper, mercury, silver and gold. Thus, carbon is able to reduce (remove the oxygen) oxides of these metal.

Heat

Two methods of extraction of metals

Electrolysis• Electrolysis is a process of breaking down an electrolyte using electric

current.• In electrolysis, electrical energy is changed into chemical energy.

( electrical energy chemical energy )• The apparatus used in electrolysis as shown in Figure 5.24 is made up of :

– Electrolyte– Electrodes– Electrical source– Ammeter

Anode Cathode

Chloride ions move to the anode Copper ions move to the cathode

Chloride ions release electrons to the anode and form chlorine atoms

Copper ions receive electrons from the cathode and form copper atoms

Chloride ions chloride atoms + electrons

Copper ions + electrons copper atoms

Two chlorine atoms combine to form a chlorine gas molecule

The copper metal is deposited at the cathode

Various Types of Cells and Their Uses

• Chemical energy can be changed into electrical energy through chemical reactions in electrochemical cells such as simple cells, lead- acid accumulators and nickel- cadmium batteries. ( chemical energy electrical energy)

Electrochemical cell

Advantages and disadvantages of various electrochemical cells

Chemical reactions which require light

• Light energy can start a chemical reaction.• Light energy can also be changed into chemical energy through chemical

reactions.• Examples of chemical reactions which are stimulated by using light energy:

– Photosynthesis process– Effect of light on photographic film and sliver chloride

Photosynthesis• Photosynthesis is a process in which green plants produce food using

water, carbon dioxide and light as shown in Figure 5.37.

• Photosynthesis involves two stages :– Light reaction : Chlorophyll in the leaf cells absorbs light energy. The light energy is used

to break down the water molecule absorbed from the roots into its components, hydrogen and oxygen. This reaction is called the photolysis of water. ( Water hydrogen + oxygen)

– Dark reaction : Hydrogen gad produced from the light reaction will react with carbon dioxide which is absorbed through the stomata of the leaf to produce food (Glucose) and oxygen.

Light energy

• Photosynthesis can be summarised by this equation : water + carbon dioxide glucose + oxygen

Light energy

chlorophyll

Effect of light on photosensitive chemicals

• Light plays an important role in photography.• Photographic film is made from plastic material and is wrapped with a thin

layer of emulsion. This layer is made up of gelatin and white sliver compounds like sliver bromide, silver chloride or silver iodine which known as photosensitive chemicals because they change when exposed to light.

• When a picture is taken with a camera, light enters the camera through the camera lens as shown in Figures 5.38.

• Light sparks off a chemical reaction in the photographic film as follows: – Light breaks down sliver bromide into black silver spots and bromine vapour. (silver

bromide silver + bromide)

• The reaction between light and sliver bromide causes the film to change from white to black.

• The photographic film is then processed with sodium thiosulphate solution in a dark room to dissolve any white bromide which did not react with light. The end product is a negative image formed on the photographic film as shown in Photograph 5.6.

• The image on the photographic film is projected onto a piece of white photographic paper. Finally, the paper is processed with chemicals.

light

Storing chemical substances

• Chemical substances like chlorine water , sodium hypochlorite solution and silver salt are very sensitive to light.

• These chemical substances will decompose to other substances if exposed to sunlight.

• As a result, photosensitive chemicals must be stored in dark condition.• Chlorine water and sodium hypochlorite solution must be stored in dark

bottles.• Photographic paper is also stored in a black bag or black box.

Effects of disposing electrochemical cells in the environment

• Disposing electrochemical cells (batteries) in the environment has causes pollution which endangers human health.

• This is because most of the components in these electrochemical cells are poisonous substances.

• For example, the lead-acid accumulator contains toxic metals like lead, nickel, cadmium, mercury and manganese. These toxic substances pollute water and soil.

Disposal of electrochemical cells

• The best way to dispose electrochemical cells is to recycle them. • Used batteries are collected and then reprocessed so that the raw

materials used like zinc and mercury can be reused.• These batteries can also reused after the chemical substances in them are

replaced by the new substances.