Unit 5 periodicity Flashcards
Which element has the highest melting point and why?
Silicon because it is giant covalent and so consists of many covalent bonds
Why has aluminium got a higher melting point than sodium?
It has 3 delocalised electrons per ion
It is the largest charged ion of +3
It is the smallest ion in terms of atomic radius
What is the Trent in shielding along period 3?
Sheilding stays the same
State how a catalyst speeds up a chemical reaction. (2)
Alternative route
Lower activation energy
State the characteristic property of transition metals that enables them to act as
catalysts in redox reactions. (1)
Variable oxidation states
Give one reason, other than cost, why the platinum electrodes are made by coating a
porous ceramic material with platinum rather than by using platinum rods. (1)
Increases the Surface area
Explain why the bond enthalpy of a Cl–Cl bond is greater than that of a Br–Br bond (2)
bonding pair closer to nucleus
So attraction to bond pair is greater
Suggest why the electron affinity of chlorine is an exothermic change. (1)
Net attraction between the chlorine nucleus and the extra electron
Explain the meaning of the term perfect ionic model. (1)
Ions can be regarded as point charges
State whether you would expect the value of the theoretical enthalpy of lattice
dissociation for silver chloride to be greater than, equal to or less than that for
silver bromide. Explain your answer. (3)
Greater
Chloride ions are smaller than bromide
They are attracted more strongly to the silver ions
State why the entropy value for ammonia is equal to zero at 0 K. (1)
No disorder
State and explain the trend in electronegativities across Period 3 from sodium to sulfur. (4)
Electronegativity increases
Proton number increases (increase in nuclear charge)
Same number of electron shells/levels
Attraction of bond pair to nucleus increases
Explain why the oxides of the Period 3 elements sodium and phosphorus have different
melting points. In your answer you should discuss the structure of and bonding in
these oxides, and the link between electronegativity and the type of bonding. (6)
Big difference in electronegativity leads to ionic bonding, smaller
covalent
Sodium oxide ionic lattice
Strong forces of attraction between ions
P4O10 covalent molecular
Weak (intermolecular) forces between molecules
melting point Na2O greater than for P4O10
A chemical company has a waste tank of volume 25 000 dm3
. The tank is full of
phosphoric acid (H3PO4) solution formed by adding some unwanted phosphorus(V)
oxide to water in the tank.
A 25.0 cm3 sample of this solution required 21.2 cm3 of 0.500 mol dm–3 sodium
hydroxide solution for complete reaction.
Calculate the mass, in kg, of phosphorus(V) oxide that must have been added to the
water in the waste tank (5)
Moles NaOH = 0.0212 × 0.5 = 0.0106
Moles of H3PO4 = 1/3 moles of NaOH (= 0.00353)
Moles of P in 25000 l = 0.00353 × 10(little6) = 3.53× 10(little3)
Moles of P4O10 = 3.53 × 10(little3)/4
Mass of P4O10 = 3.53 × 10(little3)/4 × 284 = 0.251 × 10(little6) g
= 251 kg
Define the term standard enthalpy of formation. (3)
Enthalpy change for formation of 1 mol of substance
From its elements
Reactants and products/all substances in their standard states
