Enthalpy, equilibrium Flashcards
when does an enthalpy change occur?
when there is a chemical reaction
define enthalpy change?
Enthalpy change is the change in heat energy at a constant pressure
enthalpy change is denoted as?
ΔH (delta H)
what is an exothermic reaction?
in Enthalpy..?
energy is transferred to surroundings - so there is a rise in temperature
products have less energy than reactants
ΔH = negative
the more negative ΔH the more…
exothermic
what is an endothermic reaction?
in Enthalpy..?
energy is taken in from surroundings - so there is a fall in temperature
products have more energy than reactants
ΔH = positive
are most reactions endo or exothermic?
most are exothermic
examples of endothermic reactions?
- thermal decomposition
- melting and boiling
- dissolving of some substances e.g ammonium nitrate
Enthalpy change under standard conditions?
∆HƟ
what are the standard conditions?
- 100kpa (1 bar)
- a stated temperature, usually 298k
- all substances in their standard state at room temperature ∆H298Ɵ
- units kJ mol -1
What is the standard enthalpy of formation?
∆HfƟ
enthalpy change when 1 mole of a substance is formed from its elements under standard conditions with all elements in their standard state at 100kpa
standard enthalpy of combustion
∆HcƟ
Enthalpy change when 1 mole of a substance is completely combusted in oxygen under standard conditions with all elements in their standard state at 100kpa
write equations for the standard enthalpy of formation for:
CH4, H20, NH3
write equations for the standard enthalpy of combustion for:
c, h2, C3H6
what is calorimetry?
is an experimental method used to determine the enthalpy change
- The experiments involved usually involve measuring the temperature change when energy is given out or taken in to a known mass of water.
what are the two equations which are used to calculate calorimetry?
q = m x c x ∆t and
∆H = -q / moles of substance reacted per mole
Calorimetry questions. (add in)
what is Hess’s law?
the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs
the enthalphy of formation cannot be carried out in a lab but calculated using…?
data of the enthalpies of combustion
so the enthalpy changes in the… direction equal the enthalpy changes in the… direction
clockwise and
anticlockwise
what does the enthalpy cycle look like when given combustion data
- combustion products at the bottom
- arrows pointing down
what does the enthalpy cycle look like when given formation data?
- elements in their standard state
- arrows pointing upwards
the enthalpy of combustion can be calculated using the…?
data of enthalpies of formation
enthalpy questions.
what is the mean bond enthalpy?
mean bond enthalpy is the enthapy change when a certain type of bond is broken averaged over many compounds
breaking bonds is …?
endothermic - takes (heat) energy to break bond
making/forming bonds is..?
exothermic - gives out energy
what is the equation for calculating the mean bond enthalpy?
ΔH = Σ bond enthalpy of reactant - Σ bond enthalpy of products
questions.
what is the collision theory?
in order for particles to react, particless must collide with energy above the activation energy
why do most reactions not lead to a reaction?
because most collisions have energy below the activation energy
what is the activation energy?
the minimum amount of energy required for a reaction to start/occur
what are the factors affecting the rate of reaction
- temperature
- concentration
- surface area
- pressure
- catalyst
explain how temperature affects the rate of reaction
- at a higher temperature increases the rate
- more particles have energy above the activation energy
- so more successful collisions per unit of time
explain how concentration affects the rate of reaction?
- increase in concentration increases the rate
- as there are more reactant particles per unit of volume and
- so there are more successful collisions per unit of time
explain how surface area affects the rate of reaction?
using smaller pieces of solid reactant increases the rate as there is a greater surface area so more successful collisions per unit of time
explain how pressure affects the rate of reaction? (for gases only)
there are more reactant particles per unit of volume so there are more successful collisions for unit of time
what is a catalyst?
a catalayst is a substance that speeds up the rate of a chemical reaction without being used up
how does a catalyst work?
a catalyst provides an alternative reaction pathway that has a lower activation energy
explain how a catalyst affecta the rarte of reaction?
by providing an alternative reaction pathway with a lower activation energy. therfore more collisions have energy above the lower catalysed activation energy
what are some examples of catalyst?
- cataytic converters - platinum/rhodium
- iron catalyst - for ammonia
- phosphoric acid catalyst - makes ethanol
- chromium (III) oxide catalyst - makes methanol
What is the Maxwell-Boltzman distribution graph?
how energy is distributed amongst particles in a gas
what are the 4 key ideas?
- no particles have zero energy
- particles can gain or loose energy through collisions
- most collisions do not lead to a reaction because most collisions have energy below the activation energy
- a small proportion of collisions have energy above the activation energy, so result in a successful reaction
what is the effect of increasing the temperature on the curve?
increasing the temperature increases the rate as more particles have energy above the activation energy 9 the activation energy stays the same), so more successful collisions per unit of time.
However:
- total number of particles are the same
- average energy has increased
- fewer particles have this average energy
- curve shifts more to the right
explain the effects of adding a catalyst on the curve?
adding a catalyst has no effect on the shape of the curve. a catalyst provides an alternate pathway with a lower activation energy
many reactions go to completion as the reactants are totally used up but many reactions do not go to completion. what is this reaction called?
Equilibria or Reversible
what is dynamic equilibria?
the rate of the forward reaction equals the rate of the backwards reaction. the concentration of all substances remains constant
what is homogenous dynamic equilibria?
the rate of the forward reaction equals the rate of the backwards reaction. the concentration of all substances remains constant. homogeneous means that all substances are in the same state.
what is le Chatelier’s principle?
a system in equilibrium will shift to oppose the change made to it
what does le chateliers’s principle tell us about temperature?
- increasing the temperature will shift equilibrium to the endothermic side to oppose the temperature rise
- decreasing the temperature will shift equilibrium to the exothermic side to oppose the temperature decrease
what does le chatelier’s principle tell us about pressure?
increasing the pressure shifts equilibrium to the side with the fewest moles of gas to oppose the pressure rise
- decreasing the pressure will shift equilibrium to the side with the most moles of gas to oppose the pressure decrease
what does le Chatelier’s principle tell us about concentration?
- increasing the concentration of a substance will shift equilibrium to the other side of the reaction to use up some of the extra substances added
- decreasing the concentration of a substance will shift the reaction to the other side to replace some of the extra substances whose concentration has been decreased
what does le Chatelier’s principle tell us about catalyst?
- adding a catalyst has no effect on the position of the equilibrium as it will increase the rate of the forward and backward reactions equally
- but will increase the amount of the product formed in a given time
in industrial processes why are compromised conditions used?
to obtain the optimum conditions in order to maximise profit
write the equation for making methanol.
CO(g) + 2H2(g) ⇌ CH3OH(g)
what are the reaction conditions for making methanol?
400 degrees c
5MPa
Cr2O3 catalyst
enthalpy change = -90 kjmol-1
for making methanol explain why a compromised temperature of 400°c used?
- at a higher temperature there is a lower % yield of methanol.
- as the forward reaction is exothermic
- but at a lower temperature the reaction is too slow
- so a balance between moderate yield and fast rate is used
for making methanol explain why a compromised pressure of 5MPa is used?
- at a higher pressure, there is a higher percentage yield of methanol
- as there are fewer moles of gaseous product
- but a higher pressure requires more expensive machinery,
- so a balance between higher yield and cost is used
for making methanol explain why a compromised Cr2O3 catalyst is used?
- catalyst increases the rate of reaction
- so more product produced per unit of time
- as it provides an alternative pathway
- with a lower activation energy
write an equation for making ethanol.
C2H4 + H20 ⇌ C2H5OH
What are the reaction conditions for making ethanol?
300 degrees celsius
6.5 MPa
phosphoric acid catalyst (H3PO4)
enthalpy change = -46 jkmol-1
what practical can we use to show Equilibrium?
Chromate in equilibrium with dichromate
What is the equilibria equation for this reaction?
2CrO42- (aq) + 2H+ ⇌ Cr2O72- (aq) + H2O
(Chromate ions + H+ ⇌ Dichromate ions
suggest how we would carry out this practical?
- measure 1cm depth of potassium dichromate into a test tube. - yellow solution
- add a few drops of sulphuric acid to the test tube.
- colour of solution changes from yellow –> orange
- then add sodium hydroxide - colour changes from orange back to yellow
what does this show?
- when we add the acid to the chromate ions equilibrium shifts to the right making dichromate which is orange
- when we add sodium hydroxide it removes the H+ ions as bases and acids react together to make water as well as shifting equilibrium to the left
- (optional) adding barium nitrate, forms a solid with chromate ions but no dichromate. so to replace the missing chromate ions we push equilibrium to the left so appear yellow
what is the equation to show this reaction?
2CrO4 2- (aq) + 2H+ ⇌ Cr2O7 2- (aq) + H2O(l)
Equilibrium constant
most reactions do not go to completion, what does this mean?
this means that at least one of the reactants will be totally used up
many reaction do not go to completion what are these reactions called?
equilibria/ reversible reactions
what is dynamic equilibria?
the rate of the forward reaction equals the rate of the backwards reaction. the concentration of all substances remains constant
what is homogenous dynamic equilibria?
the rate of the forward reaction equals the rate of the backwards reaction. the concentration of all substances remains constant. but with all substances in the same state
what does the Kc constant tell us?
the position of equilibrium
if Kc = 1, the equilibrium mix is..?
reactant = 50%
products = 50%
the larger the Kc constant, the greater the …?
the greater the yield of product
How do we calculate the Kc constant?
Kc = [C]^ [D]^ / [A]^ [B]^
what’s special about the Kc constant?
has no set units, they have to be calculated everytime
factors affecting the Equilibrium constant
the Kc constant can only be changed by changing the …?
temperature
if the temperature is changed , then…?
le Chatalier’s principle can help us predict how the equilibrium constant will change
describe how the Kc will change if the forward reaction was exothermic?
if the forward reaction was exothermic, when the temperature of the system is increased, equilibrium will move to the left to oppose the temperature rise, meaning a lower yield of product, meaning the value of Kc decreases
what effect does changing the concentration have on the value of Kc?
no effect
why does changing the concentration have no effect on equilibrium?
if the concentration of any substance changes, the concentration of all the other substances changes to oppose this change, keeping the value of Kc the same
what does using a catalyst have on the Kc value? why?
Catalysts have no effect on the position of equilibrium, or the value of Kc, as catalysts increase the rate of the forward and backward reactions equally
DONE!!!
