Energetics 3.1.4 Flashcards
(22 cards)
What is Hess’ law?
The enthalpy change for a chemical reaction is independent of the route taken
What directions do the arrows point in for enthalpy of combustion?
Down
What directions do the arrows point in for enthalpy of formation ?
Up
What is the enthalpy change of a reaction definition
The heat change in a reaction at constant pressure
What is an endothermic reaction
Reactions that absorb energy from the surroundings
What does the energy profile of endothermic reactions look like
Products are higher in energy than the reactants
Delta H is positive
What is an exothermic reaction
Reactions that release energy to the surroundings
What does the energy profile of an exothermic reaction look like
Products are lower in energy than reactants .
Delta H is negative
What type of process / reaction involves breaking a bond
To break a bond energy needs to be absorbed
Bonds are broken in the reactants and this is an endothermic process so delta H is positive
What process / reaction is needed to make bonds
When bonds are formed energy is released
Bonds are made when products are being produced and this is an exothermic process so delta H is negative
what is the equation needed to work out enthalpy change in mean bond enthalpy?
Enthalpy change = Total energy to break bonds - Total energy released forming bonds
What is the standard enthalpy change of formation ?
Enthalpy change when 1 mole of the compound is formed from its elements under standard conditions , all reactants and products being in their standard states
What is the standard enthalpy change of combustion ?
The enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions , all reactants and products being in their standard states .
What are the standard conditions ?
100kPa (pressure)
298k ( room temp)
Solutions at 1mol dm^-3
What is the equation to measure the enthalpy change for a reaction experimentally ?
q=mc deltaT
Why do we use a polystyrene cup?
For insulation and support
Why do we stir the mixture
ensures that all of the solution is at the same temperature
What is the mean bond enthalpy
The enthalpy needed to break one mole of covalent bond into gaseous atoms , averaged over a range of compounds
What are some errors that might occur when measuring enthalpy change
Incomplete combustion
Heat loss
How to improve the method and analysis of the result in finding enthalpy change experimentally (6)
No reference to precision of measuring equipment
•Use polystyrene cup or lid to reduce heat loss
• Record temperature values at regular intervals to plot the temperature result against time on a graph
• Extrapolate the cooling back to the point of addition to establish a temperature change
EQ : State one condition necessary for enthalpies of formation to be quoted as standard values at a specified temperature of 298k (1)
100KPa
EQ : Experiment can be repeated by using a different volume of water that would result in a more accurate value for enthalpy of combustion because there would be a reduction in heat loss . State a change in volume of water that would cause a reduction in heat loss and explain your answer (2)
Increase in volume
- Smaller heat loss by evaporation
