HC Prelim Flashcards
Electrolysis of magnesium chloride takes place. Suggest why this is carried out at a temperature of 750 degrees
Below 750 degrees, they are solids and electrolysis must take place between liquids
Why is sulphuric acid added to a reaction?
To add hydrogen ions
State the term for the unstable arrangement of atoms formed at the top of the curve on the potential energy diagram
Activated complex
State what is meant by the term electronegativity
Electronegativity is the measure
of attraction an atom has
for the electrons in a bond
Enthalpy
Energy change for a chemical reaction or physical change
Activation energy
Minimum kinetic energy required for a reaction to occur
OR minimum kinetic energy required by colliding molecules to form an activated complex
Enthalpy of combustion
Enthalpy change when one mole of a substance burns completely in oxygen
Enthalpy of solution
Enthalpy change when one mole of a substance dissolve completely in water
Enthalpy of neutralisation
Enthalpy change per mole of water formed when an acid is neutralised by an alkali
Enthalpy of formation
Energy needed to make one mole of a compound from its elements in their standard state
Equilibrium
- rate of forward reaction and reverse reactions are equal
- conc of reactants and products remain constant but not necessarily equal
Explain fully why the boiling points of the halogens increase going down
group 7.
In your answer you should name the intermolecular forces involved
(Intermolecular) forces/bonds
increase (going down the group).
(1)
LDFs are the forces (broken)
between the molecules. (1)
The more electrons the stronger the
LDFs.
Silicon tetrachloride can be used to make silicon nitride (Si3N4), a
compound found in many cutting tools.
(i) Silicon nitride has a melting point of 1900 °C and does not conduct
electricity when molten.
Explain fully, in terms of structure and bonding, why silicon nitride
has a high melting point.
Silicon nitride is a (covalent)
network
(Strong) covalent bonds are broken
Explain why the aluminium foil needs to be heated at the start of
the preparation, despite the reaction being highly exothermic
To provide (initial) activation
energy/(sufficient) energy to form activated (activation) complex.
Suggest a reason why there is a small test tube filled with cold water in
the neck of the tube containing the reaction mixture.
Condense reactants or products/acts
as a condenser/to prevent escape of
(volatile/gaseous) reactants or
products/to prevent the escape of gas(es)
Name a reagent which could be used to distinguish between
3-methylbutanal and a ketone
Acidified dichromate solution
3-Methylbutanal is found in olive oil.
Explain fully what can happen to 3-methylbutanal that will cause the
olive oil to develop an unpleasant taste.
Will react with oxygen/undergo
oxidation. (1)
Forming a carboxylic acid (which has unpleasant taste).
Explain why step 1 is described as a condensation reaction.
Because it has two molecules joining together with the loss of a
small/water molecule
Describe how free radicals are formed.
Bond breaking by UV
State how free radical scavengers prevent further chain reactions
Can react with free radicals forming
stable molecules/free radicals (and
prevent chain reactions).
OR
Donates electron(s).
OR
Acting as a reducing agent.
Ethene is an important feed stock for the production of plastics. Which type of reaction is involved in converting ethane to ethene?
Alkenes made from alkanes from cracking
The chemical indicator in a breathe test kit turns from orange to green when a motorist is over the legal alcohol limit. Which indicator is used:
- acidified potassium dichromate
- benedicts solution
- fehlings solution
- tollens reagent
acidified potassium dichromate
Which one is not a raw material in chemical industry?
- iron ore
- sodium chloride
- water
- ammonia
Ammonia
On descending group 1 from Lithium to Caesium, the electronegativity of the elements decreases. Explain why this happens
Greater shielding effect as more shells as you go down therefore attraction nucleus has for electrons in bond decreases
