chemcial reactions
TRANSCRIPT
CHEMICAL REACTIONS Physical and chemical changes
ESSENTIAL QUESTIONS
How are new substances made? How can you tell a new substance has been formed?
I. Both living and non-living things are made up of matter (atoms).
II.Matter around us undergoes changes. These changes can be classified into two types
- physical changes - chemical changes
“Those who matter don’t mind…”
Chemistry – study of matter.
1. Physical changes in a substance affect its physical properties, such as
Physical changes
• Shape
• Volume
• Density
• State of matter (solid, liquid, gas)
• Colour
• Mass
1. No new substance is formed during a physical change. No bonds between atoms are broken and no new bonds formed.
2. Physical changes are usually reversible. This means that the physical properties of the substance which has undergone a physical change can be changed back.
Physical changes
Water ice water
3. ExamplesPhysical changes
a) Changes of physical state
• Caused by heating
Steam water ice
Ice water steam
• Caused by cooling
• No new substance formed because H2O remains
as H2O
Physical changes b) Dissolving a solid in water
Sugar + water aqueous sugar
• Reverse changes
Aqueous sugar sugar + steam evaporated
dissolved
Aqueous sugar sugar + water
distillation
Describe the behaviour of molecules in physical changes. Move from one station to the next and record your observations.
Exploding can- demo Inflating balloon Deflating balloon Dry ice Classify each according to the type of
physical change
You do -Task
Experiment
Exploding can demo
Inflating balloon
Dry Ice Heating salt solution
Change of shape, state, colour, size?Molecule gain or lose energy
What happens?
1. Changes of matter that produce new substances. Bonds between atoms are broken or new bonds are formed.
2. The new substances have chemical properties which are different from the original substances.
3. Chemical changes are usually difficult to reverse. This means that the product formed cannot be easily changed back into the reactant.
Chemical Changes
4. Example
Chemical Changes
a) Burning of paper
Paper ash + carbon dioxide Burn
but
Ash + carbon dioxide Cannot be
paper
A chemical change has definitely occurred if there is any of these: –
A permanent colour change A gas is given off A change in temperature A precipitate (solid) forms when two liquids
are mixed Light emitted
Signs of Chemical change
Chemical Changesb) Browning of a peeled apple
Peeled apple browning appleExposed to air
Chemical Changesc) Combustion of methane gas
methane + oxygen carbon dioxide + water vapour
Chemical Changesd) Photosynthesis
Carbon dioxide + water glucose + oxygenlight
(in plant)
Examples in daily life Physical changes in daily life
Chemical changes in daily life
Freezing of water to form ice cube
Boiling an egg to get a hard-boiled egg
Boiling of water Burning of fossil fuel in motor vehicles
Dissolving sugar or salt in water
Respiration in living organism
Melting of wax to form different shapes of candles
Photosynthesis in plants
Melting of chocolate Digestion of food in our body
Evaporation of sweat Baking a cake
Comparison between physical and chemical changes
Physical changes
Chemical changes
SIMILARITIESSubstance in both processes undergo changesBoth changes require energy
Comparison between physical and chemical changes.
Chemists often use word equations as a way of quickly telling us what the reactants and products are.
We just write: 1) the names of the reactants - always on
LEFT 2) an arrow (meaning “reacts to give”) 3) the names of the products – always on
RIGHT
Word Equations
Magnesium reacting with copper oxide to form magnesium oxide and copper
Magnesium + Copper Oxide Magnesium Oxide + Copper
Products are
Reactants are
For example:
Steel(Iron)Wool + copper sulphate-> iron sulphate + copper
Products are
Reactants are
“It turned into Copper” “heat released” “blue disappeared – it went clear (the
CuSO4)” “Some of it split apart’ “it turned red” “it absorbed the CuSO4”
Observations:
Magnesium + hydrochloric acid
Magnesium (s) + Hydrochloric acid (aq) -> Magnesium chloride (aq) + Hydrogen(g)
Observations:- bubbles form and gas is released- temperature increase during reaction- removing tarnish has the effect of making the Mg sink
and react faster.
Chemical states:- (s) – solid- (aq) – aqueous solution- (g) – gas
Sodium Sulphate + Barium Chloride Sodium Chloride + Barium Sulphate.
Evidence of a chemical reaction:
A white precipitate is formed
Sodium Sulphate + Barium Chloride
Magnesium burned brightly reacting with the oxygen in the air to form magnesium oxide.
List Products List Reactants
Write word equations for these reactions:
Hydrochloric acid reacted with the calcium hydroxide to give water and calcium chloride
List Products List Reactants
Electrolysis of aluminium oxide produced aluminium metal and oxygen gas
List Products List Reactants
Write word equations for the following chemical reactions.
◦ Octane gas is burnt with oxygen in a car engine to produce carbon dioxide and water.
◦ Sodium metal reacts with chlorine gas to form sodium chloride.
◦ Hydrogen gas and oxygen gas combine to form water.
◦ Zinc metal dissolves in hydrochloric acid to form hydrogen gas and zinc chloride.
Practice Questions:
Factors that affect the rate of reaction: Surface area Temperature Concentration of reactants Rate of reaction – how fast the alka –seltzer
dissolves – gas released is carbon dioxide.
Grasp task rate of reaction – Alka - Seltzer