Topic 5 - Chemical Energetics Flashcards
What is the enthalpy change of a reaction?
The heat change in a reaction at constant pressure.
What are the units for enthalpy change?
kJmol^-1
What are Standard Conditions?
100kPa
298K
What are endothermic reactions?
Reactions that absorb energy from the surroundings.
What are exothermic reactions?
Reactions that release energy into the surroundings.
What is activation energy?
The minimum amount of energy required for a reaction to occur.
What is delta H for endothermic reactions?
Positive
What is delta H for exothermic reactions?
Negative
What is delta Hr?
The standard enthalpy change of a reaction.
What is delta Hneut?
The standard enthalpy change of neutralization.
What is delta Hf?
The standard enthalpy change of formation.
What is delta Hc?
The standard enthalpy change of combustion.
What does Standard Enthalpy change of Neutralization mean?
The enthalpy change when an acid and alkali react to form 1 mole of water, under standard conditions.
What does Standard Enthalpy change of Formation mean?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states, under standard conditions.
What does Standard Enthalpy change of Combustion mean?
The enthalpy change when 1 mole of a substance is completely burned in oxygen to make CO2 and H2O, under standard conditions.
Is bond breaking endothermic or exothermic?
Endothermic. +ve
Is bond making endothermic or exothermic?
Exothermic. -ve
What is bond enthalpy?
The amount of energy needed to break 1 mole of a bond type in a molecule in the gaseous state.
What is calorimetry used for?
To work out the enthalpy change of combustion.
What is the formula for energy change?
q = mc^T
What do all the letters stand for in the energy change equation?
q = energy lost or gained
m = mass of water
c = specific heat capacity of water
T = change in temperature
How do you turn an energy change into an enthalpy change?
Divide the energy change by the no. of moles.
What is the formula for enthalpy change?
enthalpy change = q/moles
What is Hess’s Law?
The total enthalpy change of a reaction is independent of the route taken.
What are the two Hess Cycles?
Formation and Combustion.
Do arrows in a Hess Formation Cycle point towards the reactants and products or away from the reactants and products?
Towards.
Do arrows in a Hess Combustion Cycle point towards the reactants and products or away from the reactants and products?
Away.
What are the rules for +/- signs and the direction of the arrow in Hess Cycles?
When the arrow goes with the cycle, the sign stays the same. When the arrow goes against the cycle, the sign flips.
What can also be used to calculate enthalpy changes using a Hess Cycle?
Mean bond enthalpies.
