Chemistry-Chemical Energetics

Topics: Energy, Thermodynamics, Temperature Pages: 7 (1397 words) Published: June 3, 2013
Chemical Energetics

Energy can be defined as the ability/ capacity to do work, and there are several forms of energy which we encounter every day, including: • Kinetic energy
• Potential energy
• Heat energy
• Mechanical energy
• Chemical energy
• Nuclear energy

Within every chemical compound, energy is present in 2 forms: 1. Kinetic energy that the molecules contain due to their movement (whether through vibrations or more free movements) 2. Chemical energy which is locked up in the chemical bonds of the compound

During any chemical reaction 2 processes occur:
Firstly, bonds are broken in the reactants and then bonds are formed in the products.

In order for bonds in the reactants to break, energy has to be absorbed from the surroundings & when the products are formed, energy is released to the surroundings.

REACTANTS → PRODUCTS
Energy absorbed & bonds Energy released & bonds
broken formed

Each of these energy changes occur together, however one normally dominates over the other.

A reaction is ENDOthermic if:
The energy absorbed in breaking bonds is greater than the energy released in forming new bonds

During an endothermic reaction, heat is taken into the reaction & therefore the reaction vessel becomes cooler. Example: dissolving ammonium chloride or potassium nitrate in water.

A reaction is EXOthermic if:
The energy released in forming new bonds is greater than the energy absorbed to break old bonds.

During an exothermic process, heat is given off after the reaction and this results in the reaction vessel getting warm. Example: neutralization reactions, burning fuels.

ENTHALPY CHANGE OF A REACTION

The energy or heat content of a substance is called its enthalpy (H), and changes in the heat content (enthalpy) are given the symbol ∆H (delta H).

The enthalpy change of a rxn = enthalpy of PRODUCTS
- enthalpy of REACTANTS

ie: ∆H = H(products) – H(reactants)

If the rxn is ENDOthermic:

H(products) > H(reactants)
∆H is positive & heat is absorbed from the surroundings

If rxn is EXOthermic

H (reactants) > H (products)
∆H is negative & heat is given off to the surroundings

Energy changes/enthalpy changes are represented by energy profile diagrams: [pic][pic]

When sketching a reaction profile, the activation energy is also included. Recall that: the activation energy is the energy barrier which the reactants must overcome in order to react. Once the activation energy is overcome the reaction can proceed. [pic][pic]

Heat of Neutralization

The heat of neutralization is defined as: the energy change which occurs when 1 mole of water is formed during a neutralization reaction (between an acid & a base).

Heat of neutralization = m x c x ∆T
Where: m= total mass of solution
c= specific heat capacity of water (4.18 Jg-1 oC-1)
∆T= temperature change

Calculating heat of neutralization

Q: 25cm3 of 0.25M KOH are mixed with 25cm3 of 0.25M HCl & the temperature rose by 8.73oC. Calculate the heat of neutralization.

Solution:

STEP 1: Write a balanced equation & deduce the mole ratio

EQN:KOH+ HCl KCl+H20

It is a 1:1 mole ratio right thru the equation

STEP 2: Calculate the # of moles for each reactant

# of moles of KOH: 0.25 moles are in 1000cm3
So x moles are in 25cm3; x= (25 x 0.25) / 1000 = 6.25 x 10-3 mols

# of moles of HCl:0.25 moles are in 1000cm
So X moles are in 25cm3, x= (25 x 0.25) / 1000 = 6.25 x 10-3 mols

STEP 3: Use mole ratio & # of moles to calculate the # of mole of water produced

Based on the mole ratio, 6.25 x 10-3 mols of water are formed in the reaction

STEP 4: Calculate the total mass of the solution & therefore the energy change based on the # of moles of water used

Total volume of solution = 25cm3 + 25cm3 = 50cm3
So Total mass of solution= 50g
(1cm3 has a...
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