Q#18 (Past Exam Paper – June 2012 Paper 22 Q7)
(a) A nuclear reaction occurs when a uranium-235 nucleus absorbs a neutron. The reaction may be represented by the equation:
23592U + WX n - - - > 9337Rb + 141ZCs + YWX n
State the number represented by the letter
W .............................................................
X ..............................................................
Y ..............................................................
Z ...............................................................
[3]
(b) The sum of the masses on the left-hand side of the equation in (a) is not the same as
the sum of the masses on the right-hand side.
Explain why mass seems not to be conserved. [2]
Solution:
(a)
W = 1 and X = 0
{WX n is a neutron. So, W = 1 and X = 0}
Y = 2
Z = 55
{On both sides, the total proton number and nucleon number should be the same.
Proton numbers:
92 + 0 = 37 + Z + (y×0)
Giving Z = 55
Nucleon numbers:
235 + 1 = 93 + 141 + (y×1)
236 = 234 + y
y = 236 – 234 = 2}
(b)
Mass–energy is conserved.
{Some of the mass is converted to energy, such that energy E = Δmc2 where Δm is the difference in mass. So, the mass is actually conserved, but it is now on the form of energy.}
Energy is released as gamma or photons or kinetic energy of products or EM radiation.
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