3.2 Flashcards
what is standard enthalpy change of formation?
The enthalpy change when one mole of a compound is formed from its elements in their standard states.
what is enthalpy change of combustion?
The enthalpy change for the complete combustion of one mole of a substance.
what is enthalpy change of neutralisation?
the enthalpy change for the formation of 1 mol of water from neutralisation.
what is activation energy?
the minimum energy required for a reaction to take place.
what is a homogenous catalyst?
A catalyst in the same state as the reactants.
what is a heterogenous catalyst?
- A catalyst in a different state to the reactants.
what is dynamic equilibrium?
When the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of reactants and products do not change in a closed system.
Why is experimental value different from ΔHc values?
1) not standard conditions
2) heat lost to surroundings
3) incomplete combustion
Why can’t you measure enthalpy changes directly ?
1) Ea too high
2) other products may form
3) (reaction happens too slowly/ not at all )
how do you experimentally find enthalpy change of combustion?
1) weigh fuel
2) measure volume of water
3) burn fuel in a spirit burner to heat water
4) measure temperature change using thermometer
5) reweigh fuel
6) use q=mcΔT and ΔH=-q/n
what is the setup for the experiment measuring enthalpy change of combustion and improvements?
beaker on a clamp stand-improvement: use calorimetry cup with lid, use insulating jacket
water in beaker- if asked 50g or 100g
spirit burner containing fuel being tested
how do you measure enthalpy change of neutralisation?
1) measure out acid+ alkali of known concentration
2) measure temp change
3) use q=mcΔT for total volume and ΔH=-q/n. use n=cxv for limiting agent.
how do you measure enthalpy change of reaction?
1) add 50cm3 solution1/acid to cup
2) record temp of solution
3) add zinc and start timer. stir with thermometer
4) record temperature at regular intervals
5) plot graph and extrapolate
6) find ΔT. use q=mcΔt and -q/n.
WHAT DO YOU NEED TO REMEMBER ABOUT THE BOLTZMANN
IT DOESN’T CROSS THE X AXIS AT THE HIGH ENERGY
why does the use of a catalyst reduce energy demand?
less HEAT/THERMAL energy is required for the reaction. less FOSSIL FUELS are burnt leading to less CO2 emissions.
in q=mcΔt, what should you use the moles of and what should you use the volume of?
moles of solid (unless the solid’s specifically in excess)
volume of liquid
how does decreasing pressure affect rate of reaction?
less particles per unit volume
less frequent collisions.
how can gases react with a solid catalyst?
1) adsorption of gases- gases attach to surface
2) bonds weaken in reactants-bonds break-new bonds form in products-activation energy decreases
3) desorption of products-released from surface
why is it not possible to measure enthalpy change directly?
- reaction gives mixture of products
- Ea too high
- rate of reaction too slow
- reactants don’t react
what is activation energy
the MINIMUM energy needed for reactants to react.
what is enthalpy change of reaction?
when the number of moles of reactants as specified in the equation react together.
what is bond enthalpy?
enthalpy change when 1 mole of gaseous covalent bonds are broken.
how can energy be used to lower the cost of a certain reaction?
used to heat reactants
used to heat rest of factory
sold to national grid
why shouldn’t you use a low temperature if it shifts equilibrium to the products?
rate of reaction would be too slow.
energy is …. to break bonds
required
energy is… making bonds
released
WHY MIGHT STANDARD ENTHALPY CHANGE OF COMBUSTION BE DIFFERENT TO CALCULATED VALUES FROM AVERAGE BOND ENTHALPIES?
actual bond enthalpies may be different from average values
conditions are not standard.
WHAT STATE IS BUTANE (C4H10?)
GAS
WHAT IS ENTHALPY CHANGE OF HYDRATION
ENTHALPY CHANGE WHEN 1 MOLE OF GASEOUS IONS FULLY DISSOLVE IN WATER.
WHAT IS AVERAGE BOND ENTHALPY
ENTHALPY CHANGE WHEN 1 MOLE OF BONDS IN GASEOUS MOLECULES ARE BROKEN. HAVE TO SAY MOLECULES, DONT SPECIFY THE TYPE OF BOND.
