(c) Chemical energy Flashcards
What is enthalpy?
A measure of the chemical energy in a substance.
What is the enthalpy change? How is it denoted?
It is written as delta H
It is the difference in potential energy between reactants and products.
What unit is used when calculating enthalpy?
Kilojoules per mole.
What is the formula for enthalpy change?
Enthalpy of products- Enthalpy of reactants.
What is delta H always in exothermic reaction?
Negative.
Why is delta H always negative in exothermic reaction?
In exothermic reactions the energy content of the products is less than that of the reactants.
What is delta H always in endothermic reactions?
Positive.
Why is delta H always positive for endothermic reactions?
The energy content of the reactants is less than that of the products.
What is the enthalpy of combustion?
The energy released when one mole of a substance burns completely in oxygen.
How is enthalpy of combustion denoted?
Delta H.
What is the method used to measure enthalpy of combustion?
Burn a substance and use the heat produced to heat water.
What is the formula to work out enthalpy change?
Eh=c m DELTA T
What does Eh represent in the formula for enthalpy change?
Energy in kJ.
What does c represent in the formula for enthalpy change?
The specific heat capacity of water.
What is the specific heat capacity of water?
4.18 kg-1 degreesC-1.
What does m represent in the formula for enthalpy change?
Mass of water in kg.
What does delta T represent in the formula for enthalpy change?
Temperature rise.
What does Hess’s law state?
The enthalpy change for a reaction is the same irrespective of the route taken.
What is the use of Hess’s Law
It can be used to calculate enthalpy changes which are difficult or impossible to determine by experiment.
What is molar bond enthalpy?
The energy required to break one mole of bonds in a diatomic molecule.
What is mean bond enthalpy?
The average energy required to break one mole of bonds, for a bond which occur in a number of compounds.
Why do mean bond enthalpies have to be calculated?
The value of bonds will vary depending on what environment they are in.
What can bond enthalpies be used for?
To estimate the enthalpy change occurring in a gas phase reaction.
What type of process is bond making?
Exothermic.
What type of process is bond breaking?
Endothermic.
What will all values of bond breaking therefore be?
Positive.
What will all values of bond making therefore be?
Negative.
Why is the value of enthalpy of combustion in the data booklet much higher than the one obtained when in the lab?
- Heat loss to surroundings.
- Incomplete combustion.
- Evaporation of alcohol.
What are the products of incomplete combustion?
carbon monoxide and water.
What are the products of complete combustion?
Carbon dioxide plus water.
Why is the copper tube coiled when performing and enthalpy of combustion experiment?
To redistribute heat.
What are the 5 values that need to be taken in an enthalpy of combustion experiment?
Mass of water Mass of ethanol before burning Mass of ethanol after burning Temperature of water before Temperature of water after.
