4.0 Energetics Flashcards
(25 cards)
What is an energy change at a constant pressure called?
An enthalpy change
What is the symbol for enthalpy change?
∆H
What are the units for enthalpy change?
KJ mol-1
What are the conditions for a standard enthalpy change?
100KPq
298K
What value does ∆H take for an exothermic reaction?
Negative
What value does ∆H take for an endothermic reaction?
Positive
Why is the enthalpy change for an exothermic reaction negative?
The products have less energy than the reactants because energy is lost to the surroundings
Why is the enthalpy change for an endothermic reaction positive?
The products have more energy than the reactants because energy is taken in from the surroundings
What is the standard enthalpy of formation?
The enthalpy change when one mole of a compound is formed from its elements under standard conditions with all reactants and products in their standard states
What is the standard enthalpy of formation of an element in its standard state?
Zero
What is the standard enthalpy of combustion?
The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions with all reactants and products in their standard states
What equation is used to calculate enthalpy change?
Describe each of the components in this equation including units.
q = mc∆T
• q is the heat lost or gained in Joules (J)
• m is mass water (or aqueous solution) in grams (g)
• ∆T is the change in temperature of water (or aqueous
solution) in Kelvin (K)
• c is the specific heat capacity of water in joules per
gram per Kelvin (Jg-1K-1)
What is the specific heat energy of a substance?
The amount of energy needed to raise the temperature 1g of substance by 1K
Describe the experiment to calculate the enthalpy change of a combustion reaction? What is the name given to this experimental technique?
Calorimetry
Burn a known amount of reactant and record the ∆T
of known mass of water
Why is the experimental combustion data of a substance generally lower than the data book value?
Some heat is lost to the surroundings and not all transferred to the water
What could be done to reduce the heat loss in a calorimetry experiment?
Use a draught screen to prevent the heat lost from the burning substance
Describe an experiment to calculate the enthalpy change of a neutralisation or displacement reaction?
• Carry out the reaction in a polystyrene cup
• Measure the temperature every minute starting
before mixing
• Plot time against temperature on a graph to obtain ∆T on mixing by extrapolating
Why is extrapolation used to find an accurate ∆T
when calculating the enthalpy change of an
exothermic or endothermic reaction?
To allow for a heat loss from the polystyrene cup
What is Hess’s law?
The overall enthalpy change for a reaction is the same independent of the route taken
What equation is used to calculate the enthalpy change of reaction if given formation data?
∆Hr = Sum ofHr products – Sum of Hr reactants
What equation is used to calculate enthalpy change of a reaction when given combustion data?
∆Hr = sum of Hr reactants - sum of Hr products
What is the mean bond enthalpy?
The energy needed to break a gaseous covalent bond averaged over a range of componds
Why are mean bond enthalpies always positive?
Energy is required to break bonds
Why might a calculated value for a mean bond
enthalpy be different to the mean bond enthalpy
found in a data book?
Mean bond enthalpies are not exact, they are averaged over a range of compounds
