CHAPTER 5ENERGY AND CHEMICAL CHANGES

5.1 PHYSICAL AND CHEMICAL CHANGES Have you noticed that a matter can

change?

There are two types of changes, physical changes chemical changes

5.1 PHYSICAL AND CHEMICAL CHANGES a) Physical change only involves a

change of physical properties (shape or appearance)

Physical change

No new substances

No change in chemical properties

Reversible

Small amount of energy required

EXAMPLE OF PHYSICAL CHANGE boiling water, dissolving sugar in water, mashing potato water cycle.

b) Chemical change is a change which produces substances which have different chemical properties from the original substances

Chemical change

Change in chemical

properties

New substances are formed

Irreversible

A lot of energy

required

EXAMPLE OF CHEMICAL CHANGE rusting of iron burning of kerosene frying egg photosynthesis raising cake batter souring milk

INQUIRY EXPERIMENT Section A: potassium iodide and

lead(II)nitrate solution

Section B: Copper(II)sulphate crystals and water

5.2 HEAT CHANGE IN CHEMICAL REACTIONS Endothermic reactions

Exothermic reaction

PEKA: EXOTHERMIC AND ENDOTHERMIC REACTIONS

Aim: to study exothermic and endothermic reactions

Hypothesis: different types of chemical reactions cause different changes in temperature

Variables:constant variables: volume of distilled watermanipulated variable: type of substanceresponding variable: final temperature

Materials and apparatus:sodium hydroxide,Ammonium chloride, distilled water, test tube, thermometer, spatula

Steps:1. 5cm³ of distilled water was filled into a test tube2. the initial temperature of distilled water was

recorded.3. a few sodium hydroxide were added into the test

tube using a spatula.4. the mixture was stirred until the sodium hydroxide

dissolved.5. the final temperature was recorded in the table.6. the outer wall of the test tube was touched.7. the experiment was repeated using ammonium

chloride powder.

5.3 THE REACTIVITY SERIES OF METALS

Reaction of metals with water

- the reaction metals with water will produce hydrogen gas and alkaline solution

- metal + water hydrogen gas + alkaline solution

- for example,

sodium + water hydrogen gas + sodium hydroxide

- different metals, give different reaction with water

Reaction of metals with acids- metals react with acids to produce salt

and hydrogen gas- metal + acid salt + hydrogen gas- for example,

magnesium + hydrochloric acidmagnesium chloride + hydogen gas

Results:

Analysis: 1. sodium hydroxide is the substance that increase the

water temperature because it released heat (exothermic

reaction)

2. ammonium chloride is the substance that decrease the water

temperature and absorbed heat (endothermic reaction) Conclusion: Hypothesis is accepted. Different types of

chemical

reactions cause different change in temperature.

Type of substances

Initial temperature/ºC

Final temperature/ºC

Change in temperature/ºC

Outer wall of test tube

Sodium hydroxide crystals with distilled water

29 48 19 Hot

Ammonium chloride powder with distilled water

29 28 -1 Cold

Reaction of metals with oxygen- Metals produce metal oxide when

heated in oxygen gas- Metal + oxygen metal oxide- For example,

aluminium + oxygen aluminium oxide

5.7 REACTION BETWEEN METALS AND OXYGEN

To provide oxygen

To fix the position of the potassium manganate (VII)

crystals at its place

Results:a) Aluminium b) Zinc

c) Copper d) Magnesium

5.8 POSITION OF CARBON IN THE REACTIVITY SERIES OF METALS If carbon can remove oxygen from its metal

oxide, so carbon is more reactive than that metal

Results:

Analysis: Conclusion: Reflection:

Mixture Observation Reaction Comparison of reactivity

Carbon + lead(II)oxide

Mixture glows(menyala)

yes

Carbon + aluminium oxide

No change no

Carbon + zinc oxide

Mixture glows slowly(menyala perlahan)

yes

REACTIVITY SERIES OF METAL

5.4 APPLYING THE CONCEPTS OF REACTIVITY SERIES OF METALS Importance of the reactivity series of

metals1. Reaction of a metal

- predict reactions which involve metals2. Displacement of metal from a solution3. Displacement of metal from an oxide4. Extraction of metal using carbon5. Terminal of a simple cell

Positions of Metals in the Reactivity Series and Methods of Extractions

Metal Method of extractionPotassium Sodium Calcium Magnesium Aluminium

Electrolysis of the molten ore•Metals that are more reactive than carbon •Extracted by flowing of electric current through the molten metal oxide

CARBON

ZincIron Tin Lead

Heating mixtures of ore and carbon•Metals that are less reactive than carbon•Heating metal ores with carbon (coke)

Copper Mercury

Heating ore (metal sulphides) in air

Silver Gold

Exist as free elements in the ground

Increase in

reactivity

Extraction of Metals from Their Ores

(Carbon dioxide)•Coke (type of

carbon)

• Limestone (eliminate

impurities to form slag)

Metal ore (example: tin and iron)

•Tin oxide + carbon tin + carbon dioxide

•Iron oxide + carbon iron + carbon dioxide

5.5 ELECTROLYSIS The process of using electrical energy to

decompose a molten or an aqueous (watery) solution of an ionic compound.

Electrolysis involves the movement of ions.

Positive carbon electrode(+)

Cation(positive ion)

Anion(negative ion)

Negative carbon electrode(-)

5.11 INQUIRY EXP: THE PROCESS OF ELECTROLYSIS

•Lead (+) : grey•Bromine (-) : brown

USES OF ELECTROLYSIS IN INDUSTRY

1. Metals extraction- Metals that are more reactive

than carbon are extracted from the ores.

2. Metal purification- Impure metal is used as the anode while

pure metal used as cathod- Electrolyte is the salt solution

3. Metal electroplating- An object can be electroplated with a

metal.- The metal is used as the anode while

the object is used as the cathode.

(Object to be plated/CATHODE)

(ANODE)

5.12 PROCESS OF ELECTROPLATING METAL OBJECTS USING ELECTROLYSIS

Aim: to study the electroplating of objects made of iron with copper

Hypothesis: copper deposites as a thin layer on the iron nail

Variables:a) Constant variable:b) Manipulated variable:c) Responding variable: Materials and apparatus:

Steps:1. A beaker was filled with copper(II)sulphate

solution.2. The apparatus for the electrolysis was set up

as shown in diagram.3. The copper plate was used as the anode

whereas the iron nail was used as the cathode.4. The switch was turned on and a current of

0.5A flowed through the circuit for 10 minutes.5. The switch was turned off and the iron nail was

taken out.6. Observation of the products at both electrodes

were recorded.

(Iron nail)

Results: Analysis: Conclusion:

Metal Observation

Copper The copper plate becomes thinner

Iron nail A brown solid is deposited

5.6 PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICAL REACTIONS

Simple Cell- Consists of two electrodes

(different metals)- The electrolyte can be a salt

solution, a diluted acid or a diluted alkali.

- The more reactive metal electrode acts as the negative terminal

- The less reactive metal electrode acts as the positive terminal

- Chemical energy to electrical energy

V

5.13 SIMPLE CELL Result:a) Zinc and Copper b) Copper and

copper

CHEMICAL CELL Dry cell- Easy to carry and supply constant

current- Cannot be recharged and not long

lasting

Lead-acid accumulator- 6 lead-acid cells connected in series- 12 V (high voltage)- Car battery (heavy & electrolyte is easy

to spills over )

Alkaline battery- Different electrolyte with dry cell- Long-lasting & higher current than dry

cell- Cannot be recharged & expensive

Mercury cell/ zinc-silver oxide battery

- 1.2 V- Long-lasting & supply constant current- Cannot be recharged

Nickle-cadmium battery- Same principle as an accumulator but

uses different substances- Long-lasting, can be recharged

5.7 LIGHT AND CHEMICAL REACTIONS

Example of chemical reactions which require light is photosynthesis

Change light energy to chemical energy

LIGHT ENERGY AND PHOTOGRAPHIC FILM Photographic film contains photosensitive

coatings made up of silver bromide. Photosensitive substances are substances that

undergo chemical change when exposed to light.

The silver bromide decomposes to black silver metal and bromine gas when exposed.

The exposed area produces opaque area.

Silver bromide silver + bromide

5.15 EFFECT OF LIGHT ON CHEMICAL REACTIONS

Covered with thumb Exposed to light

SPM KEY TO SUCCES/ PAGE 104-105

SPM ENHANCEMENT CORNER 5/ PAGE 106-109