3.4 - Enthalpy changes Flashcards
Define the term ‘Enthalpy’ ?
Enthalpy - The measure of heat energy in a chemical system
Define the term ‘Activation energy’ ?
Activation energy - The minimum amount of energy required for a chemical reaction to occur
What is an exothermic reaction ?
A reaction where energy is given out from the system into the surroundings
What is an endothermic reaction ?
A reaction here energy is absorbed from the surroundings into the system
What is considered the system and the surroundings ?
- The system refers to atoms, molecules and ions that make up chemicals
- The surroundings refer to everything beside the system
Draw an enthalpy profile for an exothermic reaction ?
Draw an enthalpy profile for an endothermic reaction ?
What standard conditions are standard enthalpy changes measured under ?
- Standard temperature of 298K or 25oC
- Standard pressure of 100 kPa
- Standard concentration of 1moldm^-3
- Standard state which is the physical state of a substance under standard conditions
Define the term ‘Enthalpy change of formation’ ?
Enthalpy change of formation: The enthalpy change when one mole of any substance is formed from its element in their natural state, measured at STP
Define the term ‘Enthalpy change of combustion’ ?
Enthalpy change of combustion : The enthalpy change when one mole of a substance is completely combusted ( a plentiful supply of oxygen ) at STP
Define the term ‘Enthalpy change of neutralisation’ ?
Enthalpy change of neutralisation : The enthalpy change when an acid and base react to form one mole of water measured at STP
Define the term ‘Enthalpy change of reaction’ ?
Enthalpy change of reaction : The enthalpy change when molar quantities ( moldm^-3 ) of reactants , as written in balanced equation, react under STP
How can enthalpy changes be calculated ?
- It can be calculated by observing the temperature change of some water placed above or around the reaction
- It can be calculated by observing the temperature change of the solution
Why may the calculated enthalpy change of combustion for a given experiment be inaccurate ?
- Heat loss to the surroundings other than the water.
- Incomplete combustion of reactant
- Evaporation of reactant from the wick.
- Non-standard conditions ( The data book value is a standard value. The conditions for this experiment are unlikely to be identical to standard conditions )
How can an experiment measuring the enthalpy change of combustion be improved ?
Use of draught screens and an input of oxygen gas could minimise errors from heat loss and incomplete combustion
what are the equations to calculate the enthalpy change ?
and 1/q x n when scaling up for one mole
Do you know how to calculate the enthalpy change of a reaction ?
Yes
What are the units for enthalpy change ?
kJ/mol or kJmol^-1
Define the term ‘average bond enthalpy’ ?
Average bond enthalpy : The energy required to break one mole of a specified type of bond in a gaseous molecule
How is energy involved in bond breaking/ making ?
- Energy is required to break bonds, bond breaking is endothermic and results in a positive enthalpy change
- Energy is released when bonds form, bond making is exothermic and results in a negative enthalpy change
Explain what an exothermic reaction is in terms of energy involved in breaking/ making bonds ?
During an exothermic reaction, less energy is required to break bonds in reactants than is used to make bonds in products
Explain what an endothermic reaction is in terms of energy involved in breaking/ making bonds ?
During an endothermic reaction, more energy is required to break bonds in reactants than to make bonds bonds in the products
Draw an enthalpy profile of bond breaking/ making during an exothermic reaction ?
Draw an enthalpy profile of bond breaking/ making during an endothermic reaction ?
Do you know how to calculate enthalpy changes of reactions using average bond enthalpies ?
Yes
What are the limitations of average bond enthalpies ?
An average bond enthalpy is calculated using bond enthalpies from different chemical environments, not the actual bond enthalpy of the bond
Do you know how to calculate enthalpy changes from Hess cycles ?
Yes
Do you know how to use Hess cycles to find the enthalpy change of a reaction from enthalpy changes of combustion ?
Yes
Do you know how to use Hess cycles to find the enthalpy change of a reaction from enthalpy changes of formation ?
Yes
