Two consecutive reactions are performed in the same open calorimeter so that the…

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Two consecutive reactions are performed in the same open calorimeter so that the products of the first reaction become the reactants for the second. The calorimeter contains

500.0\,\text{g}

of water whose temperature change is used to monitor the energy exchanged with the surroundings. Step 1:

A \to B

\Delta H_1 = +85.0\,\text{kJ mol}^{-1}

Step 2:

2B \to C

\Delta H_2 = -210\,\text{kJ mol}^{-1}

Exactly

0.600\,\text{mol}

A

is converted completely to

C

through this two-step sequence. (a) Calculate the total heat

q

(in kJ) exchanged with the surroundings for the complete conversion of

0.600\,\text{mol}

A

C

under the given conditions. Indicate the sign of

q

. (b) Determine the temperature change

\Delta T

(in

^\circ\text{C}

) experienced by the

500.0\,\text{g}

of water in the calorimeter. Assume the specific heat capacity of water is

4.18\,\text{J g}^{-1}\,^\circ\text{C}^{-1}

and that no other materials absorb heat appreciably. (c) Suppose an alternative pathway converts

0.600\,\text{mol}

A

directly to

C

in a single reaction having

\Delta H = -100\,\text{kJ mol}^{-1}

. Compare the magnitude of the calorimetric temperature change for this pathway with that calculated in part (b). Provide quantitative support for your answer. (d) Explain, in the context of energy conservation, why the two different pathways yield different temperature changes even though they produce the same overall chemical transformation from

A

C

Related theory

Browse the problem library