Enthalpy reactions Flashcards
what is enthalpy and how i sit presented
Enthalpy: stored energy in a chemical system
Enthalpy= H
how can enthalpy be measured and by what process
cant be measured experimentally but we can measure enthalpy changes with accompany reactions from temperature change in enthalpy(ΔH)
Calorimetry-process of measuring change.
what is the equation for enthalpy change
∆H = H of products - H of reactants
What is the first law of thermodynamics
energy cannot be created or destroyed; it can only be converted from one form to another
define exothermic. What kind of change does it show in enthalpy and give examples
- release energy to surroundings (negative change in enthalpy)
- recognised b a temp rise i surroundings, eg resprtaion, combustion
define endothermic. What kind of change does it show in enthalpy and give examples
- Absorbs energy from reactions (positive change in enthalpy)
- require an energy input from surroundings eg evaporation, photosynthesis
is more energy required to form bonds or break bonds?
form bonds
Describe an exothermic reaction in terms of a profile diagram
- Heat is given out to the surroundings (temperature rise)
- Chemicals lose energy…products at a lower energy level than reactants
- enthalpy change, ΔH is negative
- products are more stable than reactants
Describe an endothermic reaction in terms of a profile diagram
- Heat is taken in from the surroundings (temperature fall)
- Chemicals gain energy…products at a higher energy level than reactants
- enthalpy change, ΔH, is positive
- reactants are more stable than products
Why are bond enthalpies quoted as average bond enthalpy?
We use values of mean bond enthalpy becauseevery single bond in a compound has a slightly different bond enthalpy. E.g. In CH4 there are 4 C-H bonds. Breaking each one will require a different amount of energy
what is bond enthalpy
Bond enthalpy is the energy required to break a bond between two atoms
define average bond enthalpy
The energy required to break 1 mole of a specified bond in a gaseous molecule
define standard conditions and the symbols for it
Standard conditions (ΔHθ) - allow enthalpy changes for different reactions to be compared.
what ar ethe standard conditions for enthalpy
Standard conditions are:
- Pressure of 100 kPa (1 bar)
- Previously 1 atm = 101.3 kPa
- Temperature of 298 K (25°C)
- Reactants and products must be in their ‘standard’ states, solutions at 1 moldm-3
define standard enthalpy change of reaction and show the symbol
ΔHrθ
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states.
define standard enthalpy change of Combustion and show the symbol
(ΔHcθ)
The enthalpy change that occurs when 1 mole of a substance reacts completely with oxygen under standard conditions.
define standard enthalpy change of formation and show the symbol
ΔHfθ
The enthalpy change that occurs when 1 mole of a substance is formed from its elements under standard conditions.
define standard enthalpy change of neutralisation and show the symbol
ΔHneutθ
The enthalpy change that occurs when 1 mole of water is formed from its ions in solution under standard conditions.
how is enthalpy change measured
- Calorimeters are used to measure the temperature rise or fall of water when a fuel is burned or reaction takes place in solution
- Temperature change is proportional to enthalpy change
what is specific heat capacity and what is the SHC of water
Specific Heat Capacity: The energy required to raise the temperature of 1 g of a substance by 1K
waters specific heat capacity: 4.18Jg-1K-1
how do you convert celcius to kelvin
K = °C + 273
what is the equation to find how much energy is transferred to surroundings
Q=mcΔT
Q- Energy transferred to surroundings (water) (J)
M- Mass of surroundings (g)
C- specific heat capacity
ΔT- Temperature ris eof surroundings (K)
What is Hess’ law in general
- Enthalpy changes cannot always be measured directly, due to:
- high activation energy,
- slow rate of reaction,
- more than one reaction occurring.
- Hess’ law is an extension of the law of conservation of energy.
- It can be used to measure enthalpy changes indirectly.
What is the actual definition of hess’ law
“The total enthalpy change accompanying a chemical change is independent of the route by which the chemical change takes place, provided the initial and final conditions are the same”
how does enthalpy triangles work
A= B+C
Clockwise = Anticlockwise
- how alternative routes between reactants and products, to allow indirect determination of enthalpy changes.
