Energetics Flashcards
What is enthalpy?
The measure of the heat content of a substance.
What are standard conditions? What is the symbol for standard enthalpy change? What are the units?
100kPa pressure, 1mol dm^3 concentration, standard state at a stated temperature (usually 298K/25 degrees).
ΔH°
units = kJmol^-1
What is enthalpy change? What is the symbol for this?
The change in heat content at constant pressure.
ΔH
What are the symbols for Activation Energy and Catalysts?
Ea and Ec
What is the standard enthalpy of reaction (“enthalpy of reaction”)? What is the symbol?
The enthalpy change for a reaction with the molar quantities shown in the chemical equation.
ΔH°r
What happens to the standard enthalpy change of reaction when the molar quantities in an equation is halved? And doubled?
It halves
It doubles
What is standard enthalpy change of formation (“enthalpy of formation”)? What is the symbol?
The enthalpy change that takes place when 1 mol of a compound is formed from it’s elements under standard conditions.
ΔH°f
e.g. for CH4(g):
C(s) + 2H2(g) –> CH4(g)
What is standard enthalpy change of combustion (“enthalpy of combustion”)? What is the symbol?
The enthalpy change that takes place when 1 mol of a substance reacts completely with O2 under standard conditions.
ΔH°c
e.g. for CH4(g):
CH4(g) + 2O2 –> CO2(g) + 2H2O(l)
What is standard enthalpy change of neutralisation (“enthalpy of neutralisation”)? What is the symbol?
The enthalpy change when 1 mol of water is formed in a reaction between an acid and an alkali under standard conditions.
ΔneutH°
e.g.
1/2 H2SO4(aq) + NaOH(aq) –> H2O(l) + 1/2 Na2SO4
What is the equation for heat energy change?
q = mcΔT
q (heat energy in J)
m (mass in g)
c (specific heat capacity in J g^-1 K^-1)
ΔT ( temperature rise in K)
What is the equation for enthalpy change (per mol)?
q (heat energy change) / number of moles reacting
what is the average bond enthalpy?
average enthalpy change when 1 mole of bonds in gaseous molecules are broken.
Bond enthalpies are always ______. Average bond enthalpies are not always accurate because…
positive
they are only averages - actual enthalpies change depending on the molecule.
Are exothermic and endothermic making or breaking bonds?
MEXO BENDO - or think of magnets
Bond making = exothermic
Bond breaking = endothermic
When more energy goes into making bonds than breaking bonds, the reaction is exothermic (and vise-versa).
how do you calculate enthalpy change from average bond enthalpies?
Sum of bonds broken - Sum of bonds made
What is Hess’s Law? (do not need to remember this definition)
The enthalpy change for a reaction is independent of the route taken.
What is the diagram used to calculate enthalpy change when given the enthalpies of formation?
Reactants —∆H–> products
^ ^
| ∆fH | ∆fH
elements
What is the diagram used to calculate enthalpy change when given the enthalpies of combustion?
Reactants —∆H–> products
↓ ∆cH ↓ ∆cH
oxides
Why can the enthalpy changes of some reactions not be measured directly?
Because elements can react to form many different products.
What is needed for a successful collision to occur?
Correct orientation and Activation energy
What 5 factors impact the rate of reaction?
- concentration
- pressure of gases
- temperature
- surface area
- catalyst
What effect does concentration have on rate of reaction and why?
Increased concentration increases rate of reaction, because…
- more particles per unit volume
- so more collisions per second (more frequent collisions)
What effect does temperature have on rate of reaction and why?
The higher the temperature the faster the rate of reaction because…
- particles have more kinetic energy
- more particles have activation energy
- so more successful collisions
- also move faster, colliding more often
- so more frequent collisions
What size of particle allows for large SA? What effect does Surface Area have on rate of reaction and why?
The smaller the particle the larger the SA and the faster the reaction because…
- larger SA allows more collisions per second (more frequent collisions) at surface
What effect does Pressure have on rate of reaction and why?
Increasing pressure increases rate of reaction because…
- more particles per unit volume
- more collisions per second (more frequent collisions)
If no gases reacting, pressure has no impact on rate.
What effect does catalyst have on rate of reaction and why?
Catalyst increases rate of reaction by lowering activation energy. So…
- a larger proportion of molecules will have sufficient energy for a successful collision
- so more frequent successful collisions
What does the Boltzmann distribution curve look like?
And with a catalyst?
Note: Line starts at (0,0) and does not touch the x-axis at highest energy - because there is no maximum energy.
How does the Boltzmann Distribution curve change with change in temperature?
Catalysts lower ________ energy by allowing the reaction to take place by a different _____. catalysts speed up reactions without being ____ __.
activation
route
used up
What is a homogeneous catalyst?
Give an example.
A catalyst that is the same physical state as the reactants.
e.g. Ozone breakdown by Cl radicals (both gases)
What is a heterogeneous catalyst?
Give an example.
A catalyst that is in a different physical state to the reactants.
e.g. Haber process - reactants = H2 and N2 (gases), iron catalyst (solid)
What are the advantages of using a catalyst in industry?
Reactions can occur at a lower temperature.
Less energy so less electricity / fossil fuels needed.
So lower CO2 emissions.
Allow reactions with higher atom economies / less toxic by-products to be used in industry. Makes reactions more specific.
What happens at dynamic equilibrium?
Rate of forward reaction is equal to rate of backwards reaction so concentrations of reactants and products remain constant.
Dynamic equilibrium is a _____ system.
A catalyst _____ the rate of both the forwards and reverse reactions _______. It does not change the ______ of equilibrium, but reaches equilibrium _____.
closed
increases
equally
position
faster
How does equilibrium position shift with change in temperature?
If temperature is increased, equilibirum shifts in the endothermic direction in order to oppose the increase in temperature.
If temperature is decreased, equilibirum shifts in the exothermic direction in order to oppose the decrease in temperature.
What is Le Chatelier’s Principle?
If the conditions of a reaction at equilibrium are changed, then the position of equilibrium moves to oppose that change.
How does equilibrium position shift with change in pressure?
If pressure is increased, equilibirum shifts to the side with fewer gas moles in order to oppose the pressure increase.
If pressure is decreased, equilibirum shifts to the side with more moles of gas in order to oppose thee pressure decrease.
How does equilibrium position shift with change in concentration?
If concentration of a substance is increased, equilibrium shifts to the opposite side to oppose this increase.
If concentration of a substance is decreased, equilibrium shifts to that side, to oppose the derease.
What is the equilibrium constant (Kc) expression / Eqm law?
Note: Solids do not appear in the Kc expression (as you cannot have a concentration)
What does it mean when:
- Kc = 1
- Kc > 1
- Kc < 1
?
Kc = 1
Equilibrium is exactly in the middle
Kc >1
Equilibrium lies to the right (top > bottom)
Kc < 1
Equilibrium lies to the left (top < bottom)
What is activation energy?
The minimum amount of energy required for a reaction to take place.
