A student investigates the enthalpy changes of four different reactions ( $P$ , $Q$ , $R$ , and $S$ ) using a simple calorimeter. In each experiment, the reactant is added to a known mass of water, and the temperature change is recorded. The results are shown in the table below:

Reaction	Mass of water / $g\text{g}$	Initial temperature / $∘C^{\circ}\text{C}$	Final temperature / $∘C^{\circ}\text{C}$	Moles of reactant / $mol\text{mol}$
$P$	50.0	20.2	17.4	0.025
$Q$	100.0	19.5	22.1	0.050
$R$	50.0	18.5	23.8	0.020
$S$	100.0	21.0	16.2	0.040

Which reaction is exothermic and has a molar enthalpy change ( $ΔH\Delta H$ ) of approximately -55.4 kJ mol^-1? (Assume the specific heat capacity of the aqueous solution is 4.18 J g^-1 K^-1 and its density is 1.00 g cm^-3)

Reaction	Mass of water / $g\text{g}$	Initial temperature / $∘C^{\circ}\text{C}$	Final temperature / $∘C^{\circ}\text{C}$	Moles of reactant / $mol\text{mol}$
$P$	50.0	20.2	17.4	0.025
$Q$	100.0	19.5	22.1	0.050
$R$	50.0	18.5	23.8	0.020
$S$	100.0	21.0	16.2	0.040

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