Energetics I Flashcards
What is enthalpy, ΔE?
A measure of energy content of the substance
What does lower enthalpy indicate?
Greater energetic stability
What is enthalpy change, ΔE
Change in energy content at constant pressure
Is ΔH positive or negative for exothermic reaction and do products or reactants have higher energy?
- products have a lower energy than reactants
- ΔH = H(products) - H(reactants), ΔH < 0
What happens to the temperature of the surroundings in an exothermic reaction?
- heat is released to the surroundings
- temperature of surroundings rises
Is ΔH positive or negative for endothermic reaction and do products or reactants have higher energy?
- products have higher energy than reactants
- ΔH = H(products) - H(reactants), ΔH > 0
What happens to the temperature of the surroundings in an endothermic reaction?
- heat is absorbed from the surroundings
- temperature of the surrounding drop
What is the system and what is the surroundings for calorimeter?
System: reaction
Surroundings: aqueous medium and calorimeter
What happens in calorimeter in exothermic reaction?
- heat evolved by reaction = heat gained by calorimeter + heat gained by aqueous medium
- thus temperature of reaction mixture increases
What happens in calorimeter in endothermic reaction?
- heat absorbed by the reaction = heat lost by calorimeter + heat loss by aqueous medium
- temperature of reaction mixture decreases
What is the formula for heat evolved/absorbed?
Q = mcΔT
For dilute solutions, what can the density and the specific heat capacity of the solution be assumed as?
- density = density of pure water = 1.00g cm⁻³
- specific heat capacity = 4.18 J g⁻¹ K⁻¹
What are the assumptions made in calorimetry experiments?
- heat capacity of the calorimeter vessel is negligible, thus there is no heat gained/loss to the calorimeter
- no heat exchange with the surroundings
What is specific heat capacity, c?
amount of heat required to raise the temperature of 1g of the substance by 1K
What is heat capacity, C?
heat required to raise the temperature of a given quantity of the substance by 1K
What is standard conditions°?
- temperature: 298K or 25 degree celsius
- 1 bar or 1.00 × 10⁵ Pa
- most stable physical state (s, l or g) at 298K and 1 bar
- 1 mol dm⁻³
What is standard enthalpy change of reaction, ΔHᵣ°?
Standard enthalpy change of reaction is the enthalpy change when molar quantities of reactants, as specified by the balanced chemical equation, react to form products under standard conditions
What does ‘per mole’ in the unit of ΔH° imply?
per mole of equation
What does ΔHᵣ° depend on?
- magnitude of ΔHᵣ is directly proportional to number of reactants consumed (eg. if 2a + 2b → c, ΔHᵣ×2)
- enthalpy change is equal in magnitude but opposite in sign for the reverse reaction
- enthalpy change for reaction depends on physical state of reactants and products
What is a thermochemical equation?
A balanced stoichiometric equation, including state symbols for the reaction with ΔH values
What is standard enthalphy change of atomisation, ΔHₐₜ°?
The standard enthalpy change of atomisation of an element is the enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
What is the sign of ΔHₐₜ° values?
Always positive because atomisation involves bond breaking which takes in energy
What does greater ΔHₐₜ° indicate?
The higher the ΔHₐₜ° value, the greater the strength of the bond broken
What does ΔHₐₜ include for liquids?
enthalpy change of vapourisation, ΔHᵥₐₚ
What does ΔHₐₜ include for solids?
enthalpy change of fusion, ΔH(fus) + ΔHᵥₐₚ
What is the standard enthalpy change of formation, ΔHf°?
Enthalpy change when one mole of the substance is formed from its constituent elements in their standard states under standard conditions
What is ΔHf° of an element in its standard state?
zero
What is ΔHf° a measure of?
Measure of its energetic stability relative to its constituent elements
What does ΔHf° < 0 mean?
- Reaction is more exothermic
- substance is more stable relative to its constituent elements
What is standard enthalpy change of combustion, ΔHc°?
Enthalpy change when one mole of the substance is completely burnt in oxygen under standard conditions
What is the sign of ΔHc°?
Always negative because combustion is an exothermic reaction
What can ΔHc° values be used for?
- It can be used to compare the energy values of fuels and foods
- the more exothermic the combustion reaction, the higher the energy value of the fuel
What are the improvements/ modifications you can suggest for calorimetry experiments?
- minimise heat loss by using thermal lagging on the calorimeter
- use wind shield to reduce draught
- use a lid to prevent heat loss by convection
What is first ionisation energy?
First ionisation energy of an element is the enthalpy change when one mole of electrons is removed from one mole of gaseous atoms
What is second ionisation energy?
Second ionisation energy of an element is the enthalpy change when one mole of electrons is removed from one mole of gaseous, singly charged cations of the element
What is the sign for ionisation energy?
- Always positive because energy is required to overcome the forces of attraction between the nucleus and the electron to be removed
- reaction always endothermic
What is first electron affinity?
Enthalpy change when one mole of gaseous atoms accepts one mole of electrons
What is second electron affinity?
Enthalpy change when one mole of gaseous, singly charged anions accepts one mole of electrons
What is electron affinity a measure of?
- Attraction between the incoming electron and the nucleus
- the stronger the attraction, the more energy is released
What is the sign of the first electron affinity of electronegative elements?
- negative
- the more electronegative the element, the more negative the value of first electron affinity
What is the sign of second and successive electron affinities?
- Usually positive as energy is required to overcome the repulsion between anion and the electron to be added, both of which is negatively charged
- endothermic
What is lattice energy?
Enthalpy change when one mole of solid ionic compound is formed from its constituent gaseous ions under standard conditions
What is the sign of lattice energy?
- Always negative
- process is exothermic as energy is evolved when oppositely charged ions come together to form ionic bonds
What is lattice energy a measure of?
- strength of ionic bonds in an ionic compound
- the more negative the enthalpy change, the more exothermic the reaction, stronger the ionic bond
What is the formula for lattice energy?
LE ∝ |Z⁺ × Z⁻ / r⁺ + r⁻|
Z: charges on ions
r: ionic radius
What is bond energy?
Average enthalpy change when one mole of covalent bonds between atoms in gaseous molecules is broken
What is bond dissociation energy?
Enthalpy change when one mole of covalent bonds between atoms in gaseous molecules is broken
(not average when compared to bond dissociation energy)
What is the sign of bond dissociation energy?
- Always positive
- Bond dissociation is always endothermic because energy is required to break bonds
What does more positive bond energy mean?
The more positive the bond energy, the stronger the bond