3.2.3-6: Enthalpy Flashcards
enthalpy terms, calorimetry, reactions, enthalpy profile diagrams, bond enthalpies, hess' law
What is thermodynamics and what do thermodynamic equations include?
A branch of physical chemistry that focuses on energy in a chemical system, which is the reactants and products.
Thermodynamic equations include a balanced symbol equation and the deltaH value.
What are standard conditions?
Pressure - 100kPa
Temp. - 298K (25 dc)
Conc. - 1.0moldm-3
All substances in their standard states (most stable form)
Define enthalpy change of reaction
The energy change associated with a given reaction in standard conditions where all reactants and products are in their standard states
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
What is the standard enthalpy change of formation for an element in its standard state?
0 kJmol-1 - as there is no change, so no energy is released or taken in
Define enthalpy change of combustion
The energy change that takes place when:
- 1 mole of a substance
- reacts completely with oxygen
- under standard conditions
- all products and reactants being in their standard states
Define enthalpy change of neutralisation
The energy change that takes place when:
- 1 mole of water is formed from a neutralisation reaction
What are the calculation equations associated with calorimetry?
q = mcdeltaT
q: energy change J
m: mass g
c: specific heat capacity of substance heated/cooled Jg-1K-1 (usually 4.18)
deltaT: change in temp. K (dc + 273)
deltaH = q/n
enthalpy change Jmol-1 = energy change J / number of mols mol
0.25g of ethane was used to heat 100cm cubed of water from 19.0dc to 42.2dc. What was the molar enthalpy change for this reaction?
Why may the experimental value not be accurate?
-1212.2 kJmol-1
- experiment was unlikely to have been completed under standard conditions
- incomplete combustion could have occurred
- heat was lost to the surroundings
Define enthalpy
Enthalpy (H) is the thermal energy that is stored in a chemical system. Chemists usually measure energy changes in reactions by monitoring thermal energy
Define enthalpy change and how do we calculate it
Enthalpy change is the heat exchange with the surroundings during a chemical reaction, at constant pressure.
It is the difference between the enthalpy of the products and the enthalpy of the surroundings
Enthalpy change = enthalpy of products - enthalpy of reactants (measured in kJmol-1)
Describe the enthalpy profile diagram of an exothermic reaction?
- enthalpy of products is smaller than enthalpy of reactants (there is heat loss from the chemical system to the surroundings so temp. increases)
- enthalpy change = negative as heat has been lost from the chemical system
Describe the enthalpy profile diagram of an endothermic reaction?
- enthalpy of products is greater than enthalpy of reactants (there is heat gain to the chemical system from the surroundings so temp. decreases)
- enthalpy change = positive as heat has been gained by the chemical system
What is activation energy?
The minimum energy required to start a reaction by breaking bonds in the reactants.
Ea arrow always points up and is shown leading from the reactants to the top of the energy barrier.
Ea is often supplied by a spark or by heating the chemicals.
What is a bomb calorimeter?
A piece of equipment that minimises heat loss as much as possible, it uses pure oxygen to ensure complete combustion is achieved.
Why is it difficult to measure the enthalpy change for an incomplete combustion reaction?
There is often complete and incomplete combustion happening at the same time.
Why is the H-H bond enthalpy always the same?
A H-H bond can only ever exist as a H2 molecule.
What is average bond enthalpy?
Average bond enthalpy is the mean energy needed to break 1 mole of chemical bonds in a gaseous molecule
What happens in a chemical reaction?
- reactant bonds are broken - takes energy from surroundings so is endothermic
- atoms rearrange to form products
- product bonds are formed - releases energy so is exothermic
What does Hess’ law state?
The enthalpy change in a chemical reaction is independent of the route it takes.
What type of enthalpy change is associated with average bond enthalpy?
Endothermic enthalpy change
Describe a limitation of using average bond enthalpies in calculations
- actual energy required to break a bond depends on the chemical environment
- average bond enthalpies are just averages of values taken from across several compounds containing that type of bond
Explain why calculations of enthalpy change of reactions using bond enthalpy values are not standard values
- for calculations using bond enthalpy values, all species must be in the gaseous state
- for many substances, the gaseous state is not their standard state, so the reaction is not in standard conditions so the enthalpy change of reaction is not a standard enthalpy change
Explain, in terms of bond breaking and bond forming, why a reaction can be exothermic
Bond breaking is endothermic. Bond making is exothermic. More energy is released making new bonds than breaking old bonds.
Why do Br2 and I2 not exist in the gaseous state under standard conditions?
Energy is needed to break the LFs between molecules
Explain why the enthalpy change of vaporisation of bromine is endothermic?
Energy is required to overcome LFs
Write an equation for the neutralisation reaction taking place when hydrochloric acid is added to sodium hydroxide
HCl (aq) + NaOH (aq) -> H2O (l) + NaCl (aq)
C = 4.18 Jg-1K-1
What does the symbol c represent in q = mc(change in)T
Specific heat capacity of water
What is an appropriate material for a calorimeter to be made out of and why?
Copper - because it conducts heat well
