Module 3.2.1 - Enthalpy changes
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What is enthalpy change?
The amount of heat released (or absorbed) by a chemical reaction, carried out at constant pressure. Units = kJ mol^-1
What is an exothermic reaction?
A reaction where heat energy is released to the surroundings. Enthalpy change is negative
What is an endothermic reaction?
A reaction where heat energy is absorbed from the surroundings. Enthalpy change is positive
What is average bond enthalpy?
The average enthalpy change for the breaking of 1 mole of bonds in gaseous molecules (by homolytic fission)
What are the standard conditions?
• Temperature = 25°C (298K) • Pressure = 1atm (100kPa) • Solutions must have a concentration of 1.00 moldm^-3
What is standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states
What is standard enthalpy of neutralisation?
The enthalpy change that accompanies the formation of one mole of H2O(l) from neutralisation, under standard conditions
What is standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions
What is standard enthalpy change of combustion?
The enthalpy change for complete combustion of one mole of a substance under standard conditions, all reactants and products being in their standard states
What does Hess’ Law state?
The enthalpy change of a reaction depends only on the initial and final states and is independent of the route taken
What is activation energy?
The minimum energy required to start a reaction by the breaking of bonds.
What does the first law of thermodynamics state?
Energy can be converted from one form to another and cannot be created or destroyed
How do you measure enthalpy?
The enthalpy change is measured when heat is either added to, or released from a system. **It is not possible to directly measure enthalpy **
When reading the temperature using a thermometer, what temperature is being measured?
The temperature of the surroundings
What is the enthalpy change of a reaction equal to?
The enthalpy of products - enthalpy of reactants
How is an exothermic reaction shown on a enthalpy profile diagram?
• The enthalpy of products lie below the enthalpy of reactants • Enthalpy change is negative and points downwards
What are examples of exothermic processes?
• Combustion • Respiration
How are endothermic processes shown on an enthalpy profile diagram?
• Enthalpy of products lies above the enthalpy of reactants • Enthalpy change is positive, and the arrow points upwards
What are examples of endothermic processes?
• Photosynthesis • Thermal decomposition
What is bond fission?
The breaking of a chemical bond
What is homolytic fission?
Where a covalent bond breaks equally, hence one electron goes to each atom shown by half arrows - forms two radicals (a species with an unpaired electron)
What is heterolytic fission?
Where a covalent bond breaks unequally, hence both electrons go to one atom shown by a full arrow - forms a positive and a negative ion
What does a molecular chemical reaction involve?
The breaking of covalent bonds in reactant molecules and forming new covalent bonds in the product molecules
What process are breaking bonds?
Endothermic ; require energy to be absorbed from surroundings to the system to break bonds