OCR Chemistry A - Enthalpy

Chemical energy is a special form of potential energy within chemical bonds.

Chemical bonds are the forces of attraction that bind together atoms in compounds.

When chemicals react to form new substances, bonds break and new bonds are formed.

This changes the chemical energy of the atoms.

Enthalpy is the heat content stored within a chemical system.

It is impossible to measure the enthalpy of reactants or products, BUT, it is possible to measure the enthalpy change experimentally, by measuring the energy absorbed or released to the surroundings during a chemical change.

Can be light, electrical, but it is easiest to measure heat.

Conservation of energy: No heat energy is lost, it is just transferred from one place to another.

This means:

• heat loss in a chemical system = the heat gain to the surroundings
• heat gain in a chemical system = the heat loss from teh surroundings

An enthalpy change,delta (triangle) H is:

• heat change with the surroundings during a reaction, at constant pressure
• difference between the enthalpy of teh products and the enthalpy of the reactants

Exothermic reactions:

• enthalpy of the products is smaller than the enthalpy of reactants
• heat loss from teh chemical system to the surroundings
• delta H is negative

Endothermic reactions:

• enthalpy of the products is greater than that of the reactants
• heat gain to the chemical system from the surroundings
• delta H is positive

Shows what happens during the course of a reaction, whether the reaction is exo- or endothermic.

In an exothermic reaction:

• H products < H reactants
• delta H is negative

In an endothermic reaction:

• H products > H reactants
• delta H is positive

Activation energy: minimum energy required to start a reaction by the breaking of bonds.

Show the activation energy and enthalpy change using arrows in the CORRECT DIRECTION:

• activation energy arrow is positive and points up
• if the reactions is exothermic, heat energy is being lost, arrow points down.
• if the reaction is endothermic, heat energy is gained, arrow points up.

The activation energy is always the input of energy from reactants to the top of the energy barrier.

Chemists use standard enthalpy changes, meaning under standard conditions : close to the normal conditions found in a lab.

Standard conditions:

• 100Kpa
• 298K
• Concentration of 1 moldm-3 for reactions with aqueous solutions

Standard states:

for a standard enthalpy change, any substance must be in its standard state. A standard state is the physical state of a substance udner standard conditions.

The enthalpy change that accompanies a reaction in the molar quantities expressedin a chemical reaction under standard conditions, all reactants and products being in their standard states.

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states.

... of a compound is the enthalpy change that takes place when one mole of a compound is formed from its constituent elements under standard conditions and in their standard states.

The enthalpy change of formation of an element is 0.

• m- the mass of the surroundings
• c-the specific heat capacity of teh surroundings
• delta T - the temperatrue change of the surroundings

The heat exchange with the surrounsings, Q :

Q= m c delta T in joules

The specific heat capacity, c, is the energy required to raise the temperature of 1g of a substance by 1 degrees c.

You can carry out a reaction where two chemicals in solution are mixed together in a calorimeter.

Simplest calorimeter is a plastic cup.

The explanded polystyrene insulates the solution inside the cup.

In an exothermic reaction, the heat produced is trapped in the calorimeter, increasing the temperature of the solution.

In an exothermic reaction, the temperature of teh solution decreases.

All you need to do is:

• burn a known mass of substance in air
• to heat a known mass of water
• and measure the temperature change in the water

So:

• you measure a volume of water into teh beaker.
• burner containing the fuel is weighed
• initial temperature of the water is taken
• the burner is lit and the water heated until the temperature has risen by a reasonable amount. The maximum temperature is taken and the temperature change can be determined.
• the flame is extinguished and the burner re-weighed to find the mass of the fuel that ahs to be burnt.

The calculated value may be less exotermic than the true value because:

• there may have been incomplete combustion
• heta loss to surroundings

more sophisticated apparatus is needed to:

• cut down on heat loss
• ensure complete combustion

A bomb calorimeter allows accurate measurements of energy changes:

• fuel burns using oxygen to ensure complete combustion
• heat is transferred to teh water which is well insulated to reduce heat losses to teh surroundings

Bond enthalpy: is the enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species.

Average bond enthalpy: is teh average change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species.

Not always possible to measure the enthalpy change directly:

• may be a high activation energy
• slow reaction rate
• more than one reaction taking place

hess's law: if a reaction can take place by more than one route and the initial and final conditions are the same, teh total enthalpy change is the same for each route.

using enthalpy changes of combustion:

delta H = delta H C reactants - delta H C products

using enthalpy changes of formation:

delta H = delta H F products - delta H F reactants