Energetics Flashcards
What is enthalpy?
A measure of the heat content of a substance
What letter represents enthalpy?
H
What is enthalpy change?
The heat energy transferred in a reaction at a constant pressure in kJmol-1
What is enthalpy change represented by?
Delta H
What are standard conditions?
100kPa and a given temperature
What is the delta H value for exothermic reactions and why?
Negative because energy is given out to the surroundings
What is the delta H value for exothermic reactions and why?
Positive because energy is being absorbed by the surroundings
What is a reaction profile?
A diagram of enthalpy levels of the reactants and products in a chemical reaction.
What is the activation energy with reference to the reaction pathway?
The minimum input of energy required by reaction pathways to break the bonds in the reactants.
Where is the activation on an energy profile diagram?
The peak of the pathway
What is bond enthalpy?
The energy needed to break a bond
Is breaking bonds endo or exo?
Endothermic
Is making bonds endo or exo?
Exothermic
What is the mean bond enthalpy?
The average energy needed to break a bond over a range of compounds
What is the equation to calculate enthalpy change of reaction?
Total energy absorbed - total energy released
What are standard enthalpy values?
The delta H values for enthalpy changes of specific reactions measured under standard conditions
What is the standard enthalpy 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 the standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions while all the reactants and products are in their standard states
What is the standard enthalpy change of reaction?
The enthalpy change of a reaction when it occurs in the molar quantities shown in the equation under standard conditions with all the reactants and products in their standard state
What is a calorimetry experiment?
Experiments which measure the heat given out by reactions
Describe how to conduct a calorimetry experiment for reactants in solution?
Dump reactants in a polystyrene cup, mix them and then measure the temp. change with a thermometer
What is the enthalpy change of neutralisation?
The energy change when one mole of water is formed by the reaction of an acid and alkali
What are the problems with calorimetry in general?
Heat absorbed by container, heat lost to surroundings
What are the problems for flammable liquid calorimetry?
Some incomplete combustion so less energy given out
Some flammable liquid may evaporate
What does Hess’s Law state?
The total enthalpy change for a reaction is independent of the route taken
How do you calculate enthalpy change?
Enthalpy of products - enthalpy of reactants
What are some examples of exothermic reactions?
Combustion and respiration
What are some examples of endothermic reactions?
Photosynthesis and thermal decomposition
What is the enthalpy change of reaction?
The enthalpy change associated with a stated reaction
What is the enthalpy change of combustion?
The enthalpy change when one mole of a substance undergoes complete combustion
What is enthalpy change of formation?
When one mole of a substance is formed from its elements
