Exam (elaborations)
AQA Chemistry Energetics Q&A
- Module
- Unit 3.1.4 - Energetics
- Institution
- AQA
Exam questions and answers for the topic of Energetics in Chemistry AQA
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What is the enthalpy change? Enthalpy change is the amount of heat energy that is taken in orgiven out during any change in the system provided PRESSURE IS CONSTANT
What is the enthalpy change in the EXOTHERMIC reaction? The enthalpy change is
negative because energy is being transferred from the system to the surroundings
What is the enthalpy change in an ENDOTHERMIC reaction? In an ENDOthermic
reaction, the enthalpy change is positive because energy is transferred from the surroundings
to the system. E.g Calcium Carbonate
What is the standard enthalpy of Formation? The standard enthalpy of formation is the
enthalpy change that occurs when one mole of a compound is formed from its elements under
standard conditions, all reactants and products in their standard states
What is the standard enthalpy change of Combustion? The standard enthalpy change of
combustion is the enthalpy change that occurs when one mole of a substance is combusted
completely in oxygen under standard conditions, all reactants and products in their standard
states
What are the Standard conditions? 100 kPa pressure and 298K
Solutions at 1 moldm-3
What is incomplete combustion lead to? Soot, carbon monoxide particles and water. It
will be less exothermic than complete combustion.
What is the equation for energy change? Q= mc (del·ta )T
What is the calorimetric method of calculating enthalpy change? substances mixed in an
insulated container and the temperature change is measured.
What are the problems with the calorimetric method of measuring enthalpy change? 1.
If the reaction is slow, the temperature rise can be difficult to obtain, as cooling occurs
simultaneously with the reaction. To counter this, we take readings at regular time intervals
and extrapolate the temperature curve.
How can a better average temperature be recorded? To find a better average temperature,
measure the temperature of the reactants a few minutes before they are mixed together
List 4 common errors in the caloirmetric method? 1. Energy transfer from the surroundings
(usually loss)
2. Approximation in specific heat capacity
3. Neglecting the specific heat capacity of the calorimeter - we ignore any energy absorbed
by the apparatus
4. Reaction may be incomplete
What are the 4 steps in calculating the enthalpy change of a reaction from experimental data?
1. Q= mc (del·ta )T
2. Work out the moles of REACTANT used
3. divide Q by the number of moles
4. Add a sign and a unit (divide by 1000 to go from Jmol-1 to kJmol-1)
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