Enthalpy and Entropy - Chemical Ideas 4.1 and 4.3
Enthalpy
Enthalpy is a measure of the total energy given off or taken in when a reaction takes place. In enthalpy, H is the symbol for the total enthalpy and ΔH is the symbol for total enthalpy change. There are two main types of enthalpy reactions;
· An ENDOTHERMIC reaction takes in energy from the surroundings. The temperature in the surroundings decreases. ΔH is positive.
· An EXOTHERMIC reaction gives off energy to the surroundings. The temperature of the surroundings therefore increases. ΔH is negative.
Reactions can also be categorised into the two following categories;
·Standard Enthalpy Change of Combustion – ΔHcᶲ is the energy change when 1 mole of a substance burns completely in oxygen under standard conditions.
CH4 (g) + 2O2(g) ---------> CO2 (g) + 2H2O (l) : ΔHcᶲ =-890kJmol-1
·Standard Enthalpy Change of Formation- ΔHfᶲ is the energy change when 1 mole of a substance is formed
from the reactants under standard conditions.
H2 (g) + 0.5 O2 (g) ---------> H2O (l) ; ΔHfᶲ =-286kJmol-1
· Other Reactions can be given the formula under standard conditions of ΔHrᶲ with r being the type of reaction.
Measuring ΔHcᶲ in the laboratory
Measuring the change in enthalpy of combustion can be easy with the correct equiptment. The picture below shows the equiptment needed and how to set up.
To calculate the enthalpy change of combustion the equiptment must be set up like above. The mass of water must be recorded and the temperature of the water. The liquid fuel can then be burned until the temperature has risen between 15-20K or Celcius. When the temperature has risen by this degree measure the mass of the remaining water. We can then use the following equation (m cat) to work out the total energy transferred to the water.
Energy transferred = M x C x ΔT
M= mass of water
C= heat capacity of water constant = 4.18
ΔT= change in temperature
So if the mass of water to start was 100g to start and the starting temperature was 293K. The final temperature was 312K. The calculation would therefore be E=100 x 4.18 x 19 = 7942J.
Entropy
Entropy is a quantity which relates to the disorder or randomness of particles. In a solid the partcles are held in a lattice so entropy is low. Entropy is higher in liquid because the particles are free to move slightly, and entropy is highest in gases which have the freedom of moving and dispersing. It is also known that mixtures/solutions have a higher entropy than unmixed constituents.
Energy transferred = M x C x ΔT
M= mass of water
C= heat capacity of water constant = 4.18
ΔT= change in temperature
So if the mass of water to start was 100g to start and the starting temperature was 293K. The final temperature was 312K. The calculation would therefore be E=100 x 4.18 x 19 = 7942J.
Entropy
Entropy is a quantity which relates to the disorder or randomness of particles. In a solid the partcles are held in a lattice so entropy is low. Entropy is higher in liquid because the particles are free to move slightly, and entropy is highest in gases which have the freedom of moving and dispersing. It is also known that mixtures/solutions have a higher entropy than unmixed constituents.