Question

A student mixes a 10.0 mL sample of 1.0 M NaOH with a 10.0 mL sample of 1.0 M HCl in a polystyrene container. The temperature of the solutions before mixing is was `20.0^@ C`; the final temperature is `26.0^@ C`. What is the value of `DeltaH_(rxn)`?

Assume that the solution has a specific heat of 4.2 /g*K and a density of 1.0 g/mL.

Answer 1

The polystyrene container is a calorimeter for undergraduates!

Given,

`C_"s" = (4.2"J")/("g"*°"C")`

`DeltaT = 6.0°"C"`

`m = 20.0"mL" * (1.0"g")/"mL" approx 20.0"g"`

Recall,

`q = mC_"s"DeltaT`

Hence,

`DeltaH_"rxn" = -q_"cal" = mC_"s"DeltaT approx -500"J"`

Consider,

`HCl + NaOH rightleftharpoons H_2O + NaCl`

In this scenario, we're only given `1*10^-2"mol"` of each reactant. In the answer, we need it in dimensions of per mole. Hence,

`(-500"J")/(1.0*10^-2"mol") approx (-50"kJ")/"mol"`