Enthalpy changes Flashcards
What is enthalpy?
A measure of the heat energy in a system
What is the equation for enthalpy change?
ΔH = H(products) - H(reactants)
What is the system?
The chemicals - the reactants and products
What is the surroundings?
The apparatus, the lab and everything that is not the system
What does exothermic mean?
The energy is transferred from the system to the surroundings - ΔH is negative
What does endothermic mean?
The energy is transferred from the surroundings to the system - ΔH is positive
What does the energy profile look like for an exothermic reaction?
- The system releases heat energy to surroundings
- Any energy loss by the chemical system is balanced by the same energy gain by by the surroundings
- ΔH is negative
- Temperature of the surroundings increases
What does the energy profile look like for an endothermic reaction?
- The chemical system takes in energy from surroundings
- Any energy gain by the system is balanced by the same energy loss by the surroundings
- ΔH is positive
- Temperature of the surroundings decreases
What is activation energy?
The minimum energy required for a reaction to take place
What does the enthalpy profile, including activation energy, look like for an exothermic reaction?
What does the enthalpy profile, including activation energy, look like for an endothermic reaction?
What is standard pressure?
100kPa
What is standard temperature?
298 K
What is standard concentration?
1 mol dm^-3
What is standard state?
The physical state of a substance under standard conditions
What is standard enthalpy change of reaction, Δr HΘ?
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, will all reactants and products in their standard states
What is standard enthalpy change of formation, Δf HΘ?
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
What is standard enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
What is standard enthalpy change of neutralisation?
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all reactants and products in their standard states
What is the energy and specific heat capacity equation?
q = mcΔT
q= energy J
m = mass g
c = specific heat capacity J g^-1 K^-1
T = temperature K
How do you determine the enthalpy change of combustion?
How do you carry out the spirit burners experiment?
How do you carry out the cooling curves/ polystyrene cup experiment?
What is average bond enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule
What are the rules of bond enthalpies?
- They are always endothermic
- They always have a positive value
What in bond breaking?
Endothermic
What is bond making?
Exothermic
How can you find the enthalpy change of reaction using bond enthalpies?
Δr H = E (bond enthalpies in Reactant - E (bond enthalpies in Product_
What is Hess’ law?
If a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
How can Hess’ law be represented on an enthalpy profile?
How can you find the enthalpy change of reaction from the enthalpy changes of formation?
Δr H = E (Δf H products) - E (Δf H reactants)
How can you find the enthalpy change of reaction from the enthalpy changes of combustion?
Δr H = E(Δc H reactants) - E(Δc H products)
