Enthalpy Changes Flashcards
What does system mean in a chemical reaction?
The atoms and bonds involved in the chemical reaction
Explain the law of conservation
The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, it can only be transferred form one form to another
What energy change is breaking bonds associated with?
Energy is taken in to break bonds so endothermic
What energy change is making bonds associated with?
Energy is released when making bonds so exothermic
What is an endothermic reactions?
A reaction with an overall +ve delta H
The Enthalpy of products is greater than the Enthalpy of reactants
So temperature of surroundings is lowered as energy is taken in
What is an exothermic reaction?
A reaction with an overall negative Enthalpy change -ve delta H
Enthalpy of products is less than the Enthalpy of reactants
Heat is released to surroundings
Explain an energy level diagram for an exothermic reaction
Products have a lower energy than reactants
Activation energy is from reactants to top of curve
Delta H is from reactants to products
Explain an endothermic reaction?
Reactants is lower than products
Activation energy is from reactants to top of curve
Delta h is from reactants to products
What does activation energy mean?
The minimum energy required for a reaction to take place
Which way does the arrow point for activation energy in an energy profile diagram?
Upwards
What are standard conditions
100 KPa
298 K
What does in standard state mean?
The state an element or compound exists at in standard conditions 100 kPa and 298 K
Define Enthalpy change of formation
The energy change that takes place when 1 mole of a compound is formed under its constituent elements in their standard state under standard conditions
Define Enthalpy change of combustion
The energy when 1 mole of a substance is completely combusted in oxygen in their standard states and under standard conditions 100 kPa and 298 K
Define Enthalpy change of neutralisation
The energy change when 1 mole of water is formed on a neutralisation reaction in standard states under standard conditions 100kPa and 298K
What does Enthalpy change of a reaction mean?
The energy change associated with a given reaction
How can you calculate the Enthalpy change form experimental data ?
Q= mc delta T
M is the mass of water / substance
C is specific heat capacity of water 4.18
Delta T is change in temp initial - max/min
What are the advantages of using a bomb calorimeter?
Minimises heat loss
Pure oxygen used ensures complete combustion
Why might experimental methods for Enthalpy determination not be accurate?
Heat is lost to the surroundings
Not in standard conditions .
Reaction may not go to completion
What does average bond Enthalpy mean?
The mean energy required to break 1 mole of bonds in gaseous molecules
Why will using bond enthalpies be as accurate as using standard Enthalpy of combustion/formation ?
Bond enthalpies are a mean for the same bond across different molecules whereas standard Enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.
How to calculate Enthalpy change using average bond enthalpies?
Delta H= Sum of bind Enthalpy of reaction— sum of bond Enthalpy of products
