Reactivity 1. 1: Energetics Flashcards
a) Bond enthalpy
b) write an bond enthalpy in CH3Cl
a) the energy needed to break one mole of bonds in gaseous molecules under standard conditions
b) Cl3Cl (g) ➯ CH3 (g) + Cl (g), HBR (g) ➯ H(g) + BR (g)
The average bond enthalpy
the average bond enthalpy is the energy needed to break one mole of bonds in gaseous molecules under standard conditions averaged over similar compounds
Exothermic
the energy released from the system to the surrounding loosing free energy with no external heat therefore, products are more stable than reactants . The surrounding becomes hotter (∆H is negative)
- eg: Gas -> Liquid -> Solid, combustion, bond formation
- Σ𝐸(𝑏𝑜𝑛𝑑𝑠 𝑏𝑟𝑜𝑘𝑒𝑛) > Σ𝐸(𝑏𝑜𝑛𝑑𝑠 𝑓𝑜𝑟𝑚𝑒𝑑)
Endothermic
energy gained to the system from the surrounding. The surrounding becomes cooler, to form products less stable thatn reactants. (∆H is positive)
- eg, bond breaking, photosyntehsis, solid->liquid->gas
- Σ𝐸(𝑏𝑜𝑛𝑑𝑠 𝑏𝑟𝑜𝑘𝑒𝑛) < Σ𝐸(𝑏𝑜𝑛𝑑𝑠 𝑓𝑜𝑟𝑚𝑒𝑑)
Temperature
Is the measure of average kinetic energy (motion Ek) of the particles in the substance
Heat
transfer of energy (Ep) as result of temperature difference, increasing its temperature/ quantity of energy transfer
energy
ability to do work
standard conditions for enthalpy changes ∆H𝜃
𝜃: the pressure of 100 kPa, concentrations of 1 moldm^3 for all solutions, and all the substances in their standard states at 298K (25celsius)
consistent intermolecular forces of the reactants and product
open system
energy and matter
a) Specific heat capacity
b) specific heat capacity of water
- The energy required to raise the temperature of 1g of substance by 1K
- 4.18 J-g-1k-1
heat capacity depends on the number of particles presemt in unit mass sample, therefore mass of the indiviaul particles
* the specific heat capacity of water is 4.18 J-g-1k-1
* the unit for specific heat capacity can be Jg-1k-1 or Jg-1C-1
* highest temperature happens at the lowest specific heat capacity
Closed system
only energy is transferred to the surroundings
isolated system
matter nor energy can be transferred
standard bond enthalpy change of combustion ∆Hºc
the enthalpy change that occurs when one mol of a compound reacts with exess oxygen under standard condition of at 298ºK and 1.00 * 10^5 Pa
All combustion reactions are exothermic
What is the reason to difference in experimental and theoretical value of enthalpy change of combustion from the databooklet?
1) Decrease: heat loss form the sides of copper calorimeter
2) Decrease: incomplete combustion
2)Decrease: evaporation of alchol during experiment
3)Increase: the thermometer touches the calorimeter
4)random measurement error
enthalpy
The amount of heat energy contained in a substance. Stored in chemical bonds and intermolecular forces as potential energy.
The standard enthalpy change of formation ∆𝐻𝑓𝜃
the energy change when one mole of a compound is formed from its elemts in their standard states under standard conditions (298 K (25 °C) and 1.00 × 105 Pa)
∆𝐻 𝜃 = ∑ ∆𝐻𝑓𝜃(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − ∑ ∆𝐻𝑓𝜃(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)
- (Elements have a ∆𝐻𝑓𝜃 value of zero) as it dosent take much energy to trun element into element/ most stable
- x(solid) + y (gas) ➟ xy (gas)
Method for calculating the energy released, in kJ, when two reactant react together given the thermodynamic equation and ∆𝐻𝜃
Step 1: Calculate how many moles of reactant there are
Step 2: compare 1 mole of ∆𝐻𝜃 in the thermodynamic equation to moles of reactant.
Step 3: Use ratios to find how many KJ are produced comapred to 1 mol.
What are three factors that temperature change of an object depends on
- the mass of the object
- the heat added
- the nature of the substance
a) Equation for enthalpy change of combustion
b) what are assumption made on the combustion experiment?
c) How can you improve the calorimetry experiment
d) how can you reduce percentage uncertainty of an experiemnt?
1.
* all heat is transferred from the beaker to the water/surrounding → no heat loss (some transferd to metal or in the system)
* all solution has same specific heat capacity as pure water
* experiment not done in standard condition
* incomplete combustion (soot)
* the specific heat capcity of the beaker is negligble
- insulate/ put a lid on
- bomb calorimeter
- accurate balance
- large temperature different
- large quanitity of water/sulfur
- more precise division per degree on the therometer
The unit q: J
The unit of water: Jg-1k-1 or Jg-1c-1
The unit of temperature: kelvin or celsius
* The mass of water in grams can be placed by cm3
1) Define calorimetry
2) what are the 5 steps to finding the enthalpy change from calorimetry?
a) technique used to measure the enthalpy change of a reaction. Used for combustion, neutralization and displacement reaction.
b)
1. inital mass - final mass = m (H2O)
2. Tfinal - Tinitial = ∆T (H2O)
3. exploriate the line, T(recorded maximum temp/maxium temperature with heat loss)- initial temperature
4. calculate Q of water using Q = mc∆T
5. calculate ∆H using ∆H = - Q/n
1) Equation for standard enthalpy of neutralization ∆𝑯𝒏𝒆𝒖𝒕
2) What are assumptions to neutrallization reaction
- no heat loss from the system
- all the heat produced from neturalization reaction passes into the water
- the volume of solution is equal to the volume of water ( solution cm^3 ➟ g ➟ m(H2O) g)
- water has a density of 1gcm-3
- at standard temperature
- neutralization reaction is always exothermic since more stable water is formed
- temperature found from: (maximum tempeature allowing heat loss) - (recorded maximum temperature)
- In neutralization reaction, the sytem is the compound and the surrounding is the water.
1) Define Hesses law
2) Draw the Hesses law reaction for 3C +4H2 +5O2
total change in chemical potential energy (enthalpy change) must be equal to the energy lost or gained by the reaction system due to the law of conservation of energy
must write its states
a) find ∆Hºf and ∆Hºc
b) why is ∆Hºf and ∆Hºc different?
b)
Formation Enthalpy is the energy it takes to form a compound from elements
Bond Enthalpy: an approximation of the energy it takes to form a compound from anything by calculating the bond energies (bond energies of the reactants - bond energies of the products)
1) define average bond enthalpy
2) What is the equation for avergae bond enthalpy?
1) energy needed to break one mol of bonds in gaseous molecules under standard conditions averaged over similar compounds
2) 𝚫𝑯 = 𝚺𝑬 (𝒃𝒐𝒏𝒅𝒔 𝒃𝒓𝒐𝒌𝒆𝒏/endo/reactants)−𝚺𝑬 (𝒃𝒐𝒏𝒅𝒔 𝒇𝒐𝒓𝒎𝒆𝒅/exo/products) = 𝒌𝑱∗𝒎𝒐𝒍−𝟏
- single bonds < double bonds < triple bonds
- bond forming and bond breaking
- ignores intermolecular forces since its in gaseous state
1) What are the products of incomplete combustion and complete combustion?
2) how do you distinguish different carbon chain compounds through combustion analysis
3) Why is carbon monoxide an dangerous pollutant?
1) complete: CO2 +2H2O
incomplete: CO + 4H2O (+c if asked)
2) find carbon percentage content in the hydrocabon chain. The more soities flame has the largest carbon content.
3) CO affects oxygen uptake in the blood.
It is absorbed by the lungs and binds to haemoglobin in red blood cells more effectively than oxygen and si bound permanently. This prevents oxygen from being transported around the body.
Which reaction has higher bond enthalpy?
bond enthalpy of HCl is higher than HBr. Cl is higher in group than Br therefore has higher polarity/electronegativity/reactivity therefore harder to break the bonds
- related to displacement reaction
Draw hesses diagram for
Define born harber cycle
a) the lattice enthalpy of an ionic soid is the energy required to completely seperate one mole of a solid ionic compound into its gaseous ions
- compound can be made from energy change of formation or the born harber cycle