Flashcards in Physical Chemistry Deck (109)
What is enthalpy change, ΔH?
The heat energy transferred in a reaction at constant pressure
What are the units of ΔH (enthalpy change)?
What are standard conditions?
100kPa (1 atm) pressure and a temperature of 298K (25°C)
What is standard state?
A substance's physical state in standard conditions
What are the conditions needed to measure standard enthalpy changes?
Standard state and standard conditions (as enthalpy change is affected by temperature and pressure)
What is standard enthalpy change of reaction?
The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states
What is standard enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in standard states
What is standard enthalpy of neutralisation?
The enthalpy change when solutions of an acid and an alkali react together to form 1 mole of water under standard conditions
What are exothermic reactions?
Reactions that give out energy to the environment
Describe exothermic reactions in terms of energy of products and reactants
The products have less energy than the reactants (as energy is released to the environment)
Will ΔH be +ve or -ve for exothermic reactions?
Will ΔH be +ve or -ve for endothermic reactions?
Give an example of an exothermic reaction?
What are endothermic reactions?
Reactions that take in energy from their surroundings
Describe endothermic reactions in terms of energy of products and reactants
Products have more energy than reactants
Give an example of an endothermic reaction?
What is an enthalpy profile diagram?
A graph showing how the enthalpy changes during a reaction
What is the activation energy?
The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction
What is the activation energy shown by in enthalpy profile diagrams?
The difference between the highest point on the graph and the reactants
When is a substance most stable, in terms of energy?
When it's lost all its internal energy
Lower positions on an enthalpy profile diagram will be...
… more stable than higher positions
What is bond enthalpy?
The energy required to break a bond between 2 atoms
What is average bond enthalpy?
The energy needed to break one mole of a bond in the gas phase, averaged over the different compounds that the bond is found in
Is bond breaking endothermic or exothermic?
Endothermic, because it needs energy to break the bonds
Is bond forming endothermic or exothermic?
Exothermic, as it releases energy
Stronger bonds release..... energy when they are formed
Stronger bonds take..... energy to breaks
What equation links enthalpy change of reaction, total energy absorbed and total energy released?
Enthalpy change of reaction = Total energy absorbed - Total energy released