P- Energetics Flashcards
What is net energy change?
- △H
- diff. in energy between reactants & products
Energy change during a chemical reaction
- energy is CONSERVED
- Energy of system + energy of surroundings before reaction = energy of system + surroundings after reaction
- chemical bonds (mostly covalent) broken + remade –> products
Why is it important to understand the amount of energy required in a chemical reaction?
- measure efficiency (energy values of fuels)
- calculate energy requirements for an industrial process
- calculate theoretical amount of energy needed for bond breaking + making
- predict whether or not a reaction will take place
What does the quantity of heat taken in/ given out in a chemical reaction depends on?
Quantity of reactants
– unit: kJ/mol
How do you tell from overall energy change whether a reaction is exothermic or endothermic?
- -ve = exothermic
- +ve = endo
Why is a reaction exothermic?
- overall temp ↑
- energy released in making bonds > used to break bonds
Why is a reaction endothermic?
- overall temp ↓
- energy to break bonds > released when making bonds
What is enthalpy change?
Heat energy change under constant pressure
What is mean bond enthalpy?
- av. amount of energy
- needed to break a specific type of bond
- measured over a wide variety of diff. molecules
Standard conditions
∅ - 100kPa
298K
Explain what is happening in the following reactions.
H2 (g) + 0.5O2 (g) –> H2O (l)
Hydrogen GAS reacts with oxygen GAS to form LIQUID water.
Standard enthalpy change of formation (△Hϝ)
- enthalpy change
- when 1 mole of a substance is formed from its constituent elements
- in their standard states under standard conditions (298K, 100kPa, 1moldm-3)
- reactants/ products in standard states
Standard enthalpy change of reaction (△Hͱ)
- enthalpy change
- accompanies a reaction in molar quantities shown in chemical equation
- under standard conditions (298K, 100kPa, 1moldm-3)
- products & reactnats in standard states
(total energy change- every reaction)
Standard enthalpy change of combustion (△Hϲ)
- enthalpy change
- when 1 mole of a substance completely reacts w/ O2
- under standard conditions (298K, 100kPa, 1moldm-3)
- all reactants & products in their standard states
What is calorimetry?
Experiments that measure heat given out by a reaction
What is a simple calorimeter?
- polystyrene cup + thermometer (x lid)
- only used in solutions
Why is bomb calorimeter preferred to simple calorimetry?
- most accurate- measure enthalpy change
- sealed + insulated –> ↓ chance of heat loss
- x have to be in solution
:)
- motorised stirrer- heat distributed uniformly
Why is it hard to obtain accurate calorimeter results?
- some combustion may be incomplete
- flammable liquids can be volatile- some may evaporate
- some heat lost to surroundings- x transferred to water
Explain the difference between heat and temperature.
- Heat: total energy of all particles in given amount of substance
- Temperature: av. ke of particles in a system (independent on no. of particles)
Specific heat capacity def
- amount of heat needed to
- raise temp of 1g of substance
- by 1K
- unit: Jg-1K-1
- Q=mc△T
Law of conservation of energy
Energy x created/ destroyed
(only change form/ be distributed in diff. ways)
What is Hess’s Law?
- energy change from reactants A to products B
- is independent of the pathway taken
- no matter how many stages it involves (intermediate reactions)
The enthalpy change of formation of Gold (III) oxide is +81kJ/mol.
What does it say about the stability of the compund?
- unstable
- ∵ energy is put in to decompose
Explain why values from mean bond enthalpy
calculations differ from those determined using
Hess’s law.
Mean bond enthalpies (enthalpy for reaction)
- mean values
- across a range of compounds containing that bond
Enthalpies of formation
- ↑ accurate
- ∵ specific to the chemical/ molecule
How to find the best estimate of the temperature immediately after mixing?
- draw line of best fit through points
- extrapolate back to time of mixing
Suggest why the value for standard enthalpy of formation of liquid antimony (Group 5 element) given is not zero. (1)
- x in its standard state (should be solid x liquid)
- △Hϝ of an element in standard state = 0 by def.
When is a reaction energetically favourable?
when it releases ↑ energy
