Chapter 9 - Enthalpy Flashcards
(24 cards)
What is an exothermic reaction?
A chemical reaction where the products have less energy than the reactants
Energy is released into the surroundings
Temperature on the thermometer will go up
Energy change is negative
Chemical potential energy is converted into heat energy
What is an endothermic reaction?
The products have more energy than the reactants
Energy is absorbed from the surroundings
Temperature on the thermometer will decrease
Energy change in positive
Heat energy is converted into chemical energy
What is activation energy?
The minimum energy required for particles to collide and react successfully
What is enthalpy change?
Heat energy given out/ taken in by a chemical reaction
What is standard enthalpy change?
Enthalpy change under standard conditions
What are the standard conditions?
Pressure - 100kPa
Temperature - 298K
Concentration - 1mol/dm^3
What is standard state?
The physical state of a substance under standard conditions
What is the standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities shown in the equation under standard conditions, with everything in its standard state
What is the standard enthalpy change of formation?
The enthalpy change associated with the formation of 1 moles of a compound from its elements under standard conditions, with everything in its standard state.
What is the standard enthalpy change of combustion?
The enthalpy change with occurs when 1 moles of a compound is burned completely in oxygen under standard conditions, at standard state
What is the standard enthalpy change of neutralisation?
The enthalpy change associated with the formation of 1 moles of water from neutralisation under standard conditions, at standard state.
Why is enthalpy change not always 100% accurate?
Heat energy can be lost to the surroundings
Specific heat capacity is not exactly 4.18kg/K
Density of solution is not exactly 1.00g/cm^3
How can enthalpy changes be measured?
From a calorimetry experiment
Using bond energies
Indirectly using enthalpy/Hess cycles
Why can enthalpy changes not always be determined experimentally?
High activation energies
Slow reaction rates
More than one reaction taking place
What is Hess’ law?
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same
What is the equation for enthalpy change?
- q = mcΔT
- (q/1000)/n
What is average bond enthalpy?
Average bond enthalpy is the energy required to break one mole of a specified type of bond in a gaseous molecule
Bond enthalpies are always positive and endothermic
What is enthalpy?
Measure of heat energy in a chemical system
Cannot be measured but enthalpy changes can be
What is the conservation of energy?
Law that states energy cannot be created or destroyed
Energy is transferred between the system and the surroundings
What could make an experiment seem less exothermic?
Heat loss to surroundings
Incomplete combustion
Evaporation of alcohol from wick of spirit burner
Non-standard conditions
What are the energy changes in bond making and bond breaking?
Bond breaking is endothermic as energy is required to break bonds
Bond making is exothermic as energy is released when making bonds
Which way do the arrows point in a Hess cycle with bond enthalpies?
Down - bonds are being broken
Which way do the arrows point in a Hess cycle with enthalpy change of formation?
Up - bonds are being made
Which way do the arrows point in a Hess cycles with enthalpy change of combustion?
Down - bonds are being broken
