CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
Two different metallic samples are examined.
Sample A is an alloy composed of 70.0 % nickel atoms and 30.0 % copper atoms (percentages are atomic %). The metallic radii are 124 pm for Ni and 128 pm for Cu. A verbal description of a computer rendering shows Ni and Cu atoms randomly occupying the same lattice sites in a face-centered cubic array.
Sample B is steel in which carbon atoms are incorporated into body-centered cubic iron. A textual schematic states that 6.0 % of the available octahedral interstitial sites are occupied by carbon atoms whose covalent radius is 70 pm. For the purposes of this question, assume there is one octahedral interstitial site per iron atom.
Answer the following.
(a) Predict whether Sample A forms a substitutional or an interstitial alloy. State one piece of evidence from the information provided that supports your prediction.
(b) Explain, with reference to atomic size and lattice positions, why carbon in Sample B occupies interstitial sites rather than substituting for iron atoms.
(c) Calculate the number of carbon atoms present per 100 iron atoms in Sample B.
(d) Briefly discuss one way in which the chosen positions of the carbon atoms are expected to affect the hardness of steel compared with pure iron.