Rates and equilbiria Flashcards
Describe the relationship between time and rate
When the time increases, the rate of reaction decreases. Time and rate are inversely proportional
time= 1/rate and rate= 1/time
eXothermic
Energy absorbed (taken in) is less than the amount of energy released.
eNdothermic
Energy absorbed (taken in) is greater than the amount of energy released.
How do catalysts increase the rate of chemical reactions?
By lowering the activation energy. Reactants are converted into products in a shorter period of time.
List the advantages of a catalyst
Energy
(economically beneficial)
Electricity
Environmentally friendly (less pollutant gases are released) (eg: CO2 and SO2)
Types of reactions
- Redox
- decomposition
- displacement
- combustion
- combination
- neutrilisation
- precipitation
precipitation
When 2 aqueous solutions chemically combine to form an insoluble solid, a precipitation reaction takes place. A precipitate is produced.
pRecipitATE
High rate of reaction
rate of reaction
The speed at which reactants are converted into products or the speed at which products are formed from reactants.
collision theory
- reacting particles must have sufficient ENERGY
- the energy must be sufficient so that the particles collide successfully
- the energy needed must be sufficient for the particles to collide successfully (effectively)
- the rate of the reaction will be high if there are more successful (effective) collisions
Activation energy
It is the minimum energy needed to allow the reactants to be converted into products/ to start a chemical reaction
surface area to volume ratio
- surface area refers to the number of reacting particles for chemical substances
- the greater the surface area, the greater the number of successful collisions- therefore the rate of reaction will increase.
What are reaction rates affected by? (TALC requires No Common Sense)
Temperature
Agitation (stirring)
Light (photosynthesis)
Concentration
Nature of substance
Catalyst
Surface area
Q: How/ using the collision theory explain how temperature changes the rate of reaction
- Temperature is a measure of the average kinetic energy of the particles (temp directly proportional to kinetic energy)
- when the temperature increases, a higher proportion of particles will have an activation energy
= the number of successful collisions will increase- increasing the reaction rate.
Q: How does concentration change the reaction rate? (using collision theory)
- Concentration is the moles (number of reacting particles) divided by volume (per unit volume)
- when the concentration increases, the number of reacting particles increases
- the number of successful collisions will increase, increasing the reaction rate
Successful collisions need
- particles must have activation energy
- particles must collide at the right orientation
Q: How does a catalyst increase the rate of reaction?
- catalysts find an alternate pathway with a lower activation energy without getting used up so a higher proportion of particles will have enough energy to react which increases the frequency of successful collisions, therefore increasing the rate of reaction.
How can we measure the rate of reaction?
- volume of product formed/time
- volume of reactant used up
closed system
- no entry/exit of reactants and products
- no external factor changes (eg: pressure)
Equilibrium
- rate of forward reaction= rate of backwards reaction
- AND the concentration must remain constant.
Haber process equation
N2 + 3H2 –> 2NH3
Ammonia is used for
fertilisers because soil is acidic which is not ideal for plants to grow.
Decrease in temperature- haber process
When the temperature of the equilbrium is decreased, equilibrium will shift to the right (forwards exothermic reaction) to oppose the change and increase the temperature. But a low temperature leads to a low rate of reaction so a compromised temperature of 450 degrees celsius is used. (done in factory).
N2 (g) +3H2 (g) –> 2NH3 (g) pressure increase?
When the pressure of the system is increased, equilbrium will shift to the right where there are fewer moles of gas (2 moles) produced compared to to the backwards reaction which has 4 moles. This opposes the change and decreases the change.
compromises for pressure
A high pressure is dangerous and expensive to produce so a compromised pressure of 200 atm is used.
Catalyst-iron
It increases rate of reaction by finding an alternate pathway with a lower activation energy without getting used up. Increases yield of NH3. Both the rates of the forwards and backwards reaction increase by the same amount.
activation energy
Minimum amount of energy needed to start a reaction
How can the mean rate of reaction be found?
- by measuring the quantity of a reactant used or a product formed over the time taken.
quantity of reactant used/time
quantity of product formed/time
Reaction rate graphs: steeper slope
faster raction
Reactions take place at different rates depending on the identities and conditions. Some are extremely slow eg:
rusting
Other reactions are extremely fast eg:
explosives
How can the rate of reaction be measured?
Either by:
- How fast a reactant is used up
- how fast a product is made
What is usually easier to measure?
The amount of product forming
If the product is a gas…
- cm3
- dm3
for volume
units for rate of reaction
g/s
dm3/cm3/s
If one of the products is a gas given off,
the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant
Pressure of reacting gases
- The greater the pressure of the gas, the closer the reactant particles are together. Therefore they are more likely to undergo frequent, successful collisions which increases the rate of reaction.
surface area of solid reactants
-When the surface area of a solid reactant increases, more particles on the surface are exposed so are more likely to undergo frequent, successful colisions so there will be a higher rate of reaction
Haber process physical conditions
- temperature :450 degrees Celsius
- Pressure: 200 atmospheres
(1 atmosphere = 100 Kpa)
Le Chatelier’s Principle
If a system in equilbrium is subjected to stress caused by a change in temperature, concentration or pressure, the equilibrium position will shift so as to restore equilibrium.
How does the yield change oif the temperature is increased?(END IT)
-An increase in temperature subjects the equilbrium system to stress. According to Le Chatelier’s Principle, an increase in temperature always favours the endothermic reaction (net energy absorbing reaction). The reverse reaction will be favoured and the yield of ammonia will decrease. The rate of reaction however will increase.
How does the yield change if temperature is decreased? (4) (DEXT)
- A decrease in temperature subjects the equilbrium system to stress. According to Le Chatelier’s Principle, a decrease in temperature always favours the exothermic reaction (net energy releasing reaction). Since the forward reaction is exothermic, the equilibrium position will shift to the right; the yield of NH3 will increase. The rate of reaction however will decrease.
How does the yield change is pressure is increased? (IPMF)
- An increase in pressure subjects the equilibrium position to stress. According to Le Chatelier’s Principle, an increase in pressure will shift the equilbrium position from many moles of molecules to few moles of molecules. The forward reaction will be favoured and the yield of ammonia will increase.
How does the yield change is pressure is decreased?(DPFM)
- A decrease in pressure subjects the equilbrium position to stress. According to Le Chatelier’s Principle, a decrease in pressure will shift the equilibrium position from few moles of molecules to many moles of molecules. Since there are 2 moles on the right and 4 moles on the left, the reverse reaction will be favoured and the yield of ammonia will decrease.
Haber Process- What happens to the yield of ammonia if the concentration of hydrogen increases?
- the equilibrium system is subjected to stress
- According to Le Chatelier’s Principle, an increase in the concentration of a substance will shift the equilibrium position so as to remove the excess hydrogen.
- The forward reaction will therefore be favoured
- The yield of ammonia increases.
What happens to the yield if the concentration of N2 decreases?
If the concentration of N2 decreases, the equilbrium system is subjected to stress. According to Le Chatelier’s Principle, a decrease in the concentration of a substance will shift the equilibrium position to produce more of that substance/nitrogen. The reverse reaction is favoured. The yield of ammonia will decrease which won’t be good for industrialists.
(Any unreacted N2 and H2 are recycled back into the reaction vessel).
The effect of catalysts
-Catalysts have no effect on the position of equilibrium. However the catalyst does increase the rate of forward and backward reactions, both by the same amount. This means that the system reaches equilibrium faster and the product is formed faster.
