2. Chemical changes B Flashcards
What are the diatomic elements? (Hint: FONBICH)
Oxygen, Hydrogen, Nitrogen, Fluorine, Iodine, Chlorine and Bromine.
Diatomic refers to e.g. Hydrogen Hâ‚‚
What are the general characteristics of physical changes?
Physical changes (e.g. a change in state) are easily reversible as no new chemical substance is formed.
What are the general characteristics of chemical changes?
New products which are chemically different from the reactants are formed.
It is very difficult to reverse this change.
How is a physical change different from a chemical change?
A physical change doesn’t result in the formation of new chemical substances. It is easy to reverse.
A chemical change results in the formation of new chemical substances. It is very difficult to reverse.
Is dissolving sodium chloride in water a physical or chemical change?
Physical. The change can be easily reversed by removing the water through evaporation to leave the dry salt.
What happens during a chemical change?
Bonds between the atoms of elements break.
Then, new bonds form between different atoms, bonding them together in new arrangements.
Resulting in the formation of new chemical substances.
The new chemical substances have different structures to the original chemicals.
Chemicals with different bonds and structures have different physical properties.
What are the 3 types of chemical change? (Hint: COT)
Combustion, oxidation, thermal decomposition.
What is combustion?
A chemical reaction in which a substance is heated in the presence of oxygen to form new substances.
Heat and light are given out during combustion.
E.g. petrol undergoing combustion in engine of moving car: CPE in petrol converted to KE
What is thermal decomposition?
The process by which a single substance breaks down into two or more simpler substances upon heating.
E.g. calcium carbonate decomposes upon heating to produce calcium oxide and carbon dioxide
What is oxidation?
A chemical change that takes place when a chemical gains oxygen.
E.g. rusting: iron reacts slowly with water and oxygen to produce rust.
How to obtain the formula of a compound?
Swap the valencies of the elements/polyatomic ions.
What is an acid?
Chemicals that ionise when dissolved in water to produce hydrogen ions as the only positive ion.
What is a base?
Metal oxides and hydroxides.
What is an alkali?
A base that is soluble in water.
What is a monobasic/dibasic/tribasic acid?
Monobasic: ionises to produce max of 1 hydrogen ion per molecule of acid in water
Dibasic: 2 hydrogen ions
Tribasic: 3 hydrogen ions
Name the 3 common acids, their formulae and uses.
Hydrochloride acid | Hcl | found in stomach, used to digest food
Nitric acid | HNO3 | used to make fertilisers and explosives
Sulfuric acid | H2SO4 | found in batteries of vehicles
Name the 3 acid reactions and what they produce
Acid and metal: salt and hydrogen
Acid and metal oxide: salt and water
Acid and carbonate: salt, water and carbon dioxide
answer in question: to produce ___ as the only reaction products
What is a salt?
An ionic compound that has cation and anion.
Strong acids (hCl, HNO3, H2SO4) vs weak acids (H3PO4, CH3COOH)
Strong acid fully ionises when dissolved in water to produce hydrogen ions.
Weak acid partially ionises.
What metals will not react with dilute acids?
Those that are less reactive than hydrogen.
Metals that react with acids are always more reactive than hydrogen.
What is neutralisation?
Acid + base or acid + alkali
The process when the hydrogen ions from an acid react with the hydroxide ions from base/alkali to form water.
Test for hydrogen gas?
Insert lighted splint into test tube. Splint extinguishes with a pop sound if hydrogen gas is present.
This is because hydrogen gas is highly flammable.
Test for carbon dioxide gas?
Bubble gas through limewater. If carbon dioxide is present, gas forms a white precipitate in limewater.
Test for ammonia gas?
Place piece of moist red litmus at the mouth of test tube. Red litmus turns blue if ammonia gas is present.
This is because ammonia gas is alkaline.
