Enthalpy Flashcards
What is enthalpy?
The measure of heat energy within a chemical system
How do you calculate enthalpy change?
Enthalpy of products - Enthalpy of reactants
Where is energy transferred in an exothermic change?
Energy is transferred from system to the surroundings
Where is energy transferred in an endothermic change?
Energy is transferred from the surroundings to the system
What are 4 key features of an exothermic reaction?
Enthalpy change is negative
Chemical system loses energy
Surroundings gain energy
Temperature of surroundings increases
What are 4 key features of an endothermic reaction?
Enthalpy change is positive
Chemical system gains energy
Surroundings lose energy
Temperature of surroundings decreases
What is activation energy?
The minimum energy required for a reaction to take place
What are standard conditions?
25 C or 298K
100kPa
1 mol dm^3
Define standard enthalpy change of formation
Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states
What is the enthalpy change of formation for all elements?
0
Define standard enthalpy change of combustion
Enthalpy change when one mole of a substance undergoes complete combustion in oxygen with all substances in standard states
Define standard enthalpy change of neutralisation
Enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions
What is 0 degrees C in K?
273K
Define specific heat capacity
The energy required to raise the temperature of 1g of a substance by 1K
What is the specific heat capacity of water?
4.18
What is the equation for calculating heat energy?
Q= mc delta T
Give reasons for inaccuracy in heat energy of combustion practical
Heat energy lost to the surroundings
Incomplete combustion
Evaporation of alcohol from the wick
Non standard conditions
How would you minimise effect of heat loss or incomplete combustion?
Draught screens
Input of oxygen gas
What piece of equipment is used to measure enthalpy change of reaction between two solutions?
Polystyrene cups
Define ‘Average Bond Enthalpy’
Energy required to break one mole of a specified type of bond
Are bond enthalpies exothermic or endothermic?
Endothermic (always have a positive value)
What is an average bond enthalpy calculated from?
Actual bond enthalpies in different environments
Bond breaking- exothermic or endothermic?
Endothermic
Bond making- exothermic or endothermic?
Exothermic
What determines whether a reaction is overall exothermic or endothermic?
The difference between the value for making the bond and value for breaking bonds
How is enthalpy change calculated from bond enthalpies?
Bond enthalpies in reactants - Bond enthalpies in products
What are the limitations for using average bond enthalpies?
The bond enthalpies are slightly different in each environment, however not a major difference as an average bond enthalpy is used.
Need all molecules to be gaseous
What does ‘Hess’ Law’ state?
If a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route
