enthalpy changes Flashcards
What does the 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 be transferred from one form to another
What energy change is breaking bonds associated with?
Endothermic reaction
What energy change is making bonds associated with?
Exothermic reaction
What is an endothermic reaction
A reaction with an overall positive enthalpy change
What is an exothermic reaction
A reaction with an overall negative enthalpy change
What does an exothermic reaction graph look like
Reactants are higher than products
What does an endothermic reaction graph look like
Reacts are lower than products
What does activation energy mean
The minimum energy required for a reaction to take place
What are the standard conditions
100 kPa
298 K
Define enthalpy change of formation
The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions
Define enthalpy change of combustion
The energy change that takes place when 1 mole of a substance is completely combusted
Define enthalpy change of neutralisation
The energy change that takes place when 1 mole of water is formed from a neutralisation reaction
What does enthalpy change of reaction mean?
The energy change associated with a given reaction
Why might the experimental method for enthalpy determination not be accurate
- Heat s 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 not be as accurate as using standard enthalpy of combustion/formation
- Bond enthalpies area a mean for the same bond across different molecules
- Standard enthalpy of combustion and formation just apply to that molecule so are more accurate
How to calculate the enthalpy change of a reaction using average bond enthalpies?
(bond enthalpies of reaction) - (bond enthalpies of products)
How do you calculate the heat energy released by a reaction?
q = m c ∆T
What does “ q = m c ∆T “ mean
- q = energy change of water (J)
- m = mass of water (g)
- c = specific heat capacity of water (4.18 Jg^-1K^-1)
- ∆T = temperature change of the water (C)
How do you calculate the enthalpy change of combustion?
Thermal energy (kJ) / number of moles
