Enthalpy Changes (OCR A Level Chemistry A): Flashcards
Exam code: H432
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Define enthalpy change
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Define enthalpy change
An enthalpy change is the heat energy change during a reaction at constant pressure, represented by ΔH, measured in kJ mol−1.
True or False?
In an exothermic reaction, ΔH is positive.
False.
In an exothermic reaction the products have lower energy than the reactants, so ΔH is negative.
What happens to the temperature of the surroundings during an endothermic reaction?
The surroundings decrease in temperature because heat energy is absorbed by the reaction from the surroundings.
Standard conditions for enthalpy measurements are a pressure of .......... and a temperature of .........., with all species in their .......... states.
Standard conditions are a pressure of 100 kPa and a temperature of 298 K, with all species in their standard states.
Define standard enthalpy change of combustion, ΔHθc
The standard enthalpy change of combustion, ΔHθc is the enthalpy change when one mole of a substance in its standard state is burnt in excess oxygen; it is always exothermic.
True or False?
The standard enthalpy change of formation of O2 (g) is zero.
True.
The standard enthalpy change of formation of any element in its standard state is zero.
On an enthalpy profile diagram, what does activation energy (Ea) represent?
The minimum energy required for reactant molecules to collide successfully and start the reaction, shown as the energy difference from reactants to the transition state.
What is calorimetry?
Calorimetry is the experimental technique used to measure enthalpy changes in chemical reactions by monitoring temperature changes in a known mass of liquid.
The equation for heat transferred in a calorimeter is q = .........., where m is mass in grams, c is the .......... and ΔT is the .......... .
The equation is q = m × c × ΔT, where m is mass in grams, c is the specific heat capacity and ΔT is the temperature change.
True or False?
The specific heat capacity of water is 4.18 J g−1 K−1.
True.
This value is used to calculate heat transferred during reactions carried out in aqueous solution.
In a calorimetry experiment, the temperature of the solution rises. Is the reaction exothermic or endothermic?
The reaction is exothermic: heat is released to the surroundings, raising the solution temperature, and ΔH is negative.
To find the enthalpy change per mole, divide the heat transferred q by the .......... of moles of the limiting reactant, giving ΔH = .......... .
Divide by the number of moles of the limiting reactant, giving ΔH = q/n.
Why is polystyrene used as a calorimeter in simple experiments?
Polystyrene is a good thermal insulator, minimising heat loss to the surroundings and improving the accuracy of the temperature measurement.
Define bond enthalpy
A bond enthalpy is the energy required to break one mole of a specific covalent bond in the gaseous phase, measured in kJ mol−1.
True or False?
Bond breaking is an exothermic process.
False.
Bond breaking requires energy input and is endothermic (ΔH is positive); bond forming releases energy and is exothermic.
Why do chemists use average bond enthalpies rather than exact bond enthalpies?
The enthalpy of a bond is affected by neighbouring atoms in the molecule, so an average across many environments is used to give a reliable general value.
To calculate ΔH using bond enthalpies: ΔH = energy to break bonds in .......... minus energy released forming bonds in .......... .
ΔH = energy to break bonds in reactants minus energy released forming bonds in products.
Define average bond enthalpy
An average bond enthalpy is the average energy required to break one mole of a given type of covalent bond across a range of different molecular environments, in the gas phase.
In a bond enthalpy calculation, when is the overall reaction exothermic?
When the energy released by bond forming (products) exceeds the energy needed for bond breaking (reactants), giving a negative ΔH.
True or False?
Bond enthalpy calculations using average values give exact ΔH values.
False.
Average bond enthalpies are approximations, so calculated ΔH values may differ from experimentally measured values.
Define Hess' Law
Hess' Law states that the total enthalpy change of a reaction is independent of the route taken, provided initial and final conditions are the same.
True or False?
Hess' Law can be used to calculate enthalpy changes that cannot be measured directly by experiment.
True.
Hess' Law allows indirect calculation of ΔH values using alternative thermochemical routes via enthalpy cycles.
Using enthalpy changes of formation: ΔrH = ∑ΔHf (.......... ) − ∑ΔHf (.......... ).
ΔrH = ∑ΔHf (products) − ∑ΔHf (reactants).
When using enthalpy changes of combustion in a Hess' Law cycle, what is the formula for ΔrH?
ΔrH = ∑ΔHc (reactants) − ∑ΔHc (products), with reactants and products in the opposite order to the formation route formula.
True or False?
The standard enthalpy change of formation of an element in its standard state is zero.
True.
Elements in their standard states are already in their reference form, so no enthalpy change occurs on formation.
In a Hess' Law enthalpy cycle, what must you do when an arrow points in the opposite direction to your route?
You must reverse the sign of the enthalpy value, because travelling against the arrow direction reverses the direction of the reaction.
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