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Thermodynamics 11 29
3. Draw a simple diagram to visualise the problem:

4. Write down the appropriate equations:
Mass (in g) m = nM; Cm,p = qp/(nAT)
5. Substitute the values:
Au: qp = n(Au)Cm,,(Au)AC H20: qp = n(H20)Cm,p(H20)AT,
wheren = m/M.
6. Manipulate the equations: heat lost by Au = heat gained by H20
i.e. n(Au)Cm,p(Au)AT = n(H20)Cm,p(H20) AT
3 - [(m/197) x 25.4 x (286 - 298)] = + [(12/18) x (75.3) x
(286 - 281)]
Note the signs on both sides: heat loss is - ve and heat gain is -k ve.
r~m [(75.3 x 5 x 197 x 12)/(25.4 x 12 x 18)] = 162.2g.
=
Answer: Mass = 162.2g

ENTROPY, S, AND CHANGE IN ENTROPY, AS

Entropy, S, is a measure of the disorder of a system. In Chapter 1, the
three states of matter-solid, liquid and gas-were introduced. As you
change from the highly regular and ordered solid state to the disor-
dered gaseous state, the disorder or entropy, S, increases (Figure 3.2).

(a) Solid (s) (b) Liquid (1) (c) Gas (g)

Increasing disorder or entropy, S

Figure 3.2 Entropy, S