A nuclear reaction occurs when a uranium-235 nucleus absorbs a neutron. The reaction may be represented by the equation

Question 18

(a) A nuclear reaction occurs when a uranium-235 nucleus absorbs a neutron. The reaction may be represented by the equation:

²³⁵₉₂U   +          ^W_X n     - - - >  ⁹³₃₇Rb +         ¹⁴¹_ZCs +          Y^W_X 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]

Reference: Past Exam Paper – June 2012 Paper 22 Q7

Solution:

(a)

W = 1 and X = 0                    

{^W_X 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.