5 ENEGETICS/ THERMOCHEMISTRY
5.1 MEASURING ENERGY CHANGES
HEAT AND TEMPERATURE
HEAT - is a form of energy that flows from something at a higher temperature to something
at a lower temperature.
TEMPERATURE - is a measure of the average kinetic energy of particles.
Internal energy is the name given to the total amount of energy (kinetic and potential) in a
sample of a substance
heat will flow from the hotter block to the colder one until they are at the same
temperature
THE ‘SYSTEM’ IS OUR CHEMICAL REACTION AND THE ‘SURROUNDINGS’ IS EVERYTHING
ELSE IN THE UNIVERSE
EXOTHERMIC AND ENDOTHERMIC REACTIONS AND ENTHALPY CHANGES
IN AN EXOTHERMIC REACTION - heat energy is transferred from a system (chemical
reaction) to the surroundings – the surroundings get hotter.
IN AN ENDOTHERMIC REACTION - a system (chemical reaction) takes in heat energy from
the surroundings – the surroundings get cooler
,a) exothermic reaction decrease in b) endothermic reaction
internal energy of the system creation of converted to internal energy,
chemical bonds breaking of chemical bonds
enthalpy change is the heat energy exchanged with the surroundings at constant
pressure.
the amount of heat energy taken in/given out in a chemical reaction
∆H FOR AN EXOTHERMIC REACTION IS NEGATIVE.
∆H FOR AN ENDOTHERMIC REACTION IS POSITIVE
If the reaction is exothermic :
- enthalpy of the products is less than that of the reactants
- negative sign for the enthalpy change indicates a decrease in enthalpy. -
products are more stable than the reactants
• total energy of the reactants=total energy of the products+heat given out •
ΔH = enthalpy of products − enthalpy of reactants
, Total energy - is conserved in a chemical reaction
Stability - used to describe the relative energies of reactants and products in a chemical
reaction. If the products have less energy than the reactants then they are more stable
At this level, the enthalpy of a system can be regarded as essentially the total energy stored
in a substance – i.e. basically the same as the internal energy
If the reaction is endothermic:
- ΔH is positive indicating increase in enthalpy
- The products have higher energy (enthalpy) than reactants
- Products less stable than the reactant
• total energy of products = total energy of reactants + heat taken in
• ΔH = enthalpy of products − enthalpy of reactants
SOME DEFINITIONS
Enthalpy changes have different values, depending on the conditions. To make them
transferable, they are all quoted for the same set of conditions, which is called standard
conditions
An enthalpy change under standard conditions is called a standard enthalpy change and has
the symbol ΔH , where the symbol means ‘under standard conditions’
→ standard conditions : pressure=100kPa (1.00×105Pa)
This can then be written as ΔH298 or ΔH (298K). Where no temperature is stated we will
assume that the temperature is 298K.
Standard enthalpy change of reaction (∆𝑯𝒓 ) is the enthalpy change (heat given out or
taken in) when molar amounts of reactants, react together under standard conditions to
give products.
Standard enthalpy change of combustion (∆𝑯𝒄 ) is the enthalpy change when one mole of
a substance is completely burnt in oxygen under standard conditions. combustion is always
negative because combustion is always an exothermic process.
5.1 MEASURING ENERGY CHANGES
HEAT AND TEMPERATURE
HEAT - is a form of energy that flows from something at a higher temperature to something
at a lower temperature.
TEMPERATURE - is a measure of the average kinetic energy of particles.
Internal energy is the name given to the total amount of energy (kinetic and potential) in a
sample of a substance
heat will flow from the hotter block to the colder one until they are at the same
temperature
THE ‘SYSTEM’ IS OUR CHEMICAL REACTION AND THE ‘SURROUNDINGS’ IS EVERYTHING
ELSE IN THE UNIVERSE
EXOTHERMIC AND ENDOTHERMIC REACTIONS AND ENTHALPY CHANGES
IN AN EXOTHERMIC REACTION - heat energy is transferred from a system (chemical
reaction) to the surroundings – the surroundings get hotter.
IN AN ENDOTHERMIC REACTION - a system (chemical reaction) takes in heat energy from
the surroundings – the surroundings get cooler
,a) exothermic reaction decrease in b) endothermic reaction
internal energy of the system creation of converted to internal energy,
chemical bonds breaking of chemical bonds
enthalpy change is the heat energy exchanged with the surroundings at constant
pressure.
the amount of heat energy taken in/given out in a chemical reaction
∆H FOR AN EXOTHERMIC REACTION IS NEGATIVE.
∆H FOR AN ENDOTHERMIC REACTION IS POSITIVE
If the reaction is exothermic :
- enthalpy of the products is less than that of the reactants
- negative sign for the enthalpy change indicates a decrease in enthalpy. -
products are more stable than the reactants
• total energy of the reactants=total energy of the products+heat given out •
ΔH = enthalpy of products − enthalpy of reactants
, Total energy - is conserved in a chemical reaction
Stability - used to describe the relative energies of reactants and products in a chemical
reaction. If the products have less energy than the reactants then they are more stable
At this level, the enthalpy of a system can be regarded as essentially the total energy stored
in a substance – i.e. basically the same as the internal energy
If the reaction is endothermic:
- ΔH is positive indicating increase in enthalpy
- The products have higher energy (enthalpy) than reactants
- Products less stable than the reactant
• total energy of products = total energy of reactants + heat taken in
• ΔH = enthalpy of products − enthalpy of reactants
SOME DEFINITIONS
Enthalpy changes have different values, depending on the conditions. To make them
transferable, they are all quoted for the same set of conditions, which is called standard
conditions
An enthalpy change under standard conditions is called a standard enthalpy change and has
the symbol ΔH , where the symbol means ‘under standard conditions’
→ standard conditions : pressure=100kPa (1.00×105Pa)
This can then be written as ΔH298 or ΔH (298K). Where no temperature is stated we will
assume that the temperature is 298K.
Standard enthalpy change of reaction (∆𝑯𝒓 ) is the enthalpy change (heat given out or
taken in) when molar amounts of reactants, react together under standard conditions to
give products.
Standard enthalpy change of combustion (∆𝑯𝒄 ) is the enthalpy change when one mole of
a substance is completely burnt in oxygen under standard conditions. combustion is always
negative because combustion is always an exothermic process.
