Chapter 9 - Enthalpy Flashcards
What is enthalpy?
A measure of the heat energy in a chemical system
What is the equation for enthalpy change?
Enthalpy change = H(products) - H(reactants)
What is the law of conservation of energy?
Energy cannot be created or destroyed, only transferred.
What happens to energy during an exothermic reaction, and what is ΔH for an exothermic reaction?
Energy is transferred from the system to the surroundings
ΔH is therefore negative
What happens to energy during an endothermic reaction and what is ΔH for an endothermic reaction?
Energy is transferred from the surroundings to the system
ΔH is therefore positive
What is activation energy?
The energy input required to break bonds, or the minimum energy required for a reaction to take place
What are the standard conditions?
Standard pressure is 100kPa
Stndard temperature is 298K or 25 degrees
Standard conc is 1 mol dm-3
Standard state is the physical state of a substance under standard conditions
What is the standard enthalpy change of formation?
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 the standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
What is the enthalpy change of formation of an element?
0 kJ mol-1
What is the 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 the standard enthalpy change of neutralisation?
The energy change that accompanies the reaction of an acid by a base to form one mole of H2O (l) under standard conditions, with all reactants and products in their standard states
What is specific heat capacity?
The energy required to raise the temperature of 1g of a substance by 1K
What is the equation for energy change?
q = mcΔT
How can you determine an enthalpy change of combustion experimentally?
Heat a specified volume of water in a copper calorimeter using a small spirit burner with liquid fuel (e.g. methanol). Record the maximum temperature reached by the water, and the change in mass of the spirit burner, then calculate the enthalpy change
What are the reasons for experiemental enthalpy change of combustion data being somewhat inaccurate?
- heat loss to surroundings other than the water
- incomplete combustion of methanol (soot)
- evaporation of methanol from the wick
- non standard conditions
How can you determine an enthalpy change of reaction experimentally?
Add an excess of one of either the solid or solution to an exact amount of the other in a polystyrene cup. Stir the mixture and record the maximum temperature it reaches, then use this data to work out the enthalpy change of reaction.
How can you determine an enthalpy change of neutralisation experimentally?
React 2 solutions together (acid and alkali) and record the exact quantity. Record the temperature rise, and use this to work out the energy change of the solution, and therefore the energy change. Remember that enthalpy change of neutralisation is the change required to form 1 mol H2O.
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 limitations of average bond enthalpies?
Actual bond enthalpy can vary depending on chemical environment of the bond, and this means average bond enthalpy is not precise for each specific individual bond
What is ΔH for an exothermic reaction?
Energy is released when bonds form
Bond making
So ΔH is negative
What is ΔH for an endothermic reaction?
Energy is required to break bonds
Bond breaking
ΔH is positive
How can we find the enthalpy change of reaction from the average bond enthalpies?
ΔrH = Σ (bond enthalpies in reactants) - Σ(bond enthalpies in products)
What is Hess’s Law?
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
What is the rule for using enthalpy changes of formation to work out ΔrH?
ΔrH = Σ ΔfH products - Σ ΔfH reactants
What is the rule for using enthalpy changes of combustion to work out ΔrH?
ΔrH = Σ ΔcH reactants - Σ ΔcH products
