Question

Why do you not measure delta H of a target reaction directly? In what siutiations i Hess's Law used? Be sure to define state fuunction as you address this point.

What is the purpose of the calibration of the calorimeter? If we did not calibrate the calorimeter, what type of error would this introduce? A negative valuefor the heat capacity should be set to zero. Why? What would a negative value mean, if possible?

Explain the difference between heat capacity and specific heat. Are these intensive or extensive properties?

Answer 1

It is difficult to measure the delta H directly. If we talk about enthalpy of hydration then it is very difficult to measure enthalpy change of hydration directly because the hydration process can't be controlled directly. Similarly we canmot calculate the enthalpy of allotropic transformation directly because conversion of allotropes is a very slow process.

Hess's law allow the enthalpy change for a reaction to be calculated even when it cannot be measured directly. Hess's law folloWS from the fact that enthalpy is a state function. The total amount of heat change in a reaction depends only upon the nature of the initial reactant and final product and is independent of the path by which this change has been brought about.

Calibration of calorimeter gives a calibration factor which us different on each calorimeter, so we need to calibrate it each time so that we can measure the amount of heat gain or lost in reaction accurately.

Instrumental error will occur if we don't calibrate the calorimeter.

A negative value of heat capacity means that if we put energy into a system then its temperature decreases or vice versa. Eg is black hole.

Heat capacity of a system is the amount of heat required to raise the temperature of the system through 1℃.

Specific heat capacity of a substance is the amount of heat required to raise the temperature of 1 gram of the substance through 1℃. Specific heat capacity is an intensive property.