Question

define:

Calorie

Specific heat

Enthalphy change

Answer 1

Calorie :

This is unit used for energy.The energy required to raise the temperature of 1 g of water by on degree is called as 1 calorie.

It is always denoted as Cal. and 1 Cal = 4.184 J

Specific heat :

The amount of heat that is absorbed by any substance per unit mass per degree C is called as specific heat.

Lets look at specific heat of water : 4.184 J /( g deg C )

Specific heat of Ice = 2.03 J/(g deg C )

Specific heat is used to measure how much heat is absorbed when theere is some temperature change by using following formula.

q = m X c X Delta T

Here q is heat absorbed of evolved , m is mass , C is specific heat and delta T is change in T.

When we use above equation with given specific heat , mass and change in temperature we can calculate amount of heat is absorbed or evolved.

Enthalpy change :

Lets look at first law statement :

Delta E = q + w

Change in enery depends on both work done and heat absorbed/ evolved.

Many process in the univese occur at constant pressure and that is atmoshperic pressure. Enthalpy is used then show the reaction at atmospheric pressure.

Delta H is enthalpy change. If we substituted Delta H for q then we get .

When pressrue is constant the w = -pdelta T

delta H = delta E + pDelta V

We apply this concept for the reaction.

Formation of water :

H2 +1/2 O2 ----> H2O

When water is formed then heat is given out. And the heat (Delta H) is shown by negative sign.

That also indicates the reaction is exothermic.

Exothermic : The reaction which gives heat is called as exothermic reaction.

Endothermic : The reaction which absorbes heat is called as endothermic rection.

for endothermic delta H is positive.

We calculate delta H by using experiment for those reaction that are possible to conduct.

Some reaction are not possible to conduct in lab. We can calculate delta H for them by using Hess's law.