U3 AOS 2: Rates Of Chemical Equations Flashcards
Define the term activation energy.
Minimum amount of energy required to break bonds of reactants.
3 Marks
Consider the 2 reactions:
A: CH3Cl + NaOH -> CH3OH + NaCl
B: CH3Br + NaOH -> CH3OH + NaBr
Explain which reaction would have lower activation energy.
1) ‘A’ contains a C-Cl bond in its CH3Cl whereas ‘B’ contains a C-Br
2) C-Br has a lower bond enthalpy of 285kJ/mol than C-Cl bond enthalpy of 324kJ/mol.
3) It takes less energy to break the reactant bonds of reactant B hence lower activation energy.
What are the three conditions for successful collisions hence chemical reaction?
1) Sufficient energy to overcome repulsion
2) Sufficient energy to break bonds
3) Correct orientation for reactant bonds to break and product bonds to form.
Define the term transition state.
The state when activation energy is absorbed, and a new arrangement of atoms are formed. (Peak of the enthalpy diagram)
What two factors affect the rate of reaction?
1) Frequency of successful collisions.
2) Proportion of successful collisions.
2 Mark
Explain what increasing the pressure of H2 in a reaction with O2 has on the rate of reaction?
1) Increased frequency of collisions between H2 and O2
2) Increased frequency of successful collisions between H2 and O2
Increased rate of reaction
3 Mark
A chemist uses the Haber process:
N2 and H2 to produce NH3. Initially the reaction was at a rate of 1500M/s. After 3 hours the rate of reaction dropped to 340M/s explain why this happened.
1) Overtime, pressure of N2 and H2 decrease as they are consumed.
2) Frequency of collisions decreases implies frequency of successful collisions decreases.
3) Hence slower rate of reaction
Why does powdered sugar dissolve quicker in the mouth than blocks of sugar.
1) Increased TSA implies more sugar exposed to saliva, hence more readily to dissolve and higher frequency of collisions.
2) Increased frequency of collisions implies higher frequency of successful collisions
Therefore higher rate of reaction.
3 Marks
Ethanol can be formed through the anaerobic fermentation of glucose by bacteria according to
C6H12O6 -> 2C2H5OH + 2CO2
If a chemist wants to have the higher rate of reaction which temperature should be choose, 15C or 30C
1) 30C
2) Higher temperature means increased average kinetic energy which implies higher frequency of collisions.
3) Proportion of glucose with required activation energy increases. Hence increased proportion of successful collisions.
Both effects lead to higher frequency of successful collisions, hence increased rate of reaction.
What is a catalyst?
A catalyst increases the rate of reaction by providing an alternative reaction pathway that has LOWER ACTIVATION ENERGY.
Why do catalysts typically last a long time?
Catalysts are not consumed in the reaction and can be reused multiple times.
A student claims that catalysts can increase the rate of reaction through increasing frequency of collisions. Justify why they are or aren’t correct.
1) False
2) Catalysts provide alternative pathway with lower activation energy. Therefore proportion of successful collisions increases hence increases frequency of SUCCESSFUL collisions and increases rate of reaction.
** Note: Catalysts increase frequency of successful collisions, not frequency of collisions themselves.
2 Marks
Explain one ECONOMICAL downside to catalysts.
1) Catalysts are very expensive as they frequently adsorb impurities from the reactant or product mixture, poisoning them and rendering them useless.
2) Industries must use very high purity catalysts to prevent catalysts poisoning. Higher purity catalysts typically cost more.
What does it mean when a reaction is in equilibrium?
State of reversible chemical equation where rate of forwards reaction = rate of backwards reaction in a closed system.
Give three reasons why a reaction would be irreversible:
1) products are more stable than reactants
2) reverse reaction has very high Ea
3) reaction is not a closed system so products escape and are unavailable.
Differentiate extent of reaction and rate of reaction.
Extent is how much product is formed when system reaches equilibrium, whereas rate is the # of successful collisions in a time period.
5 MARKS (SEPARATOR)
Explain why the conductivity of an aqueous HCl Solution in water is much higher than the conductivity of an aqueous CH3COOH solution in water. Assume water is the excess reagent.
1) Strong acids fully ionise in water whereas weaker acids partially ionise in water.
2) HCl is a strong acid and CH3COOH is a weak acid.
3) When HCl ionises in water, all of it turns into H3O+ ions and Cl. Hence it has a lot of FMCP
4) When CH3COOH ionises in water, it partially ionises, hence some of it remains as CH3COOH whereas some becomes H3O+ and HCOO- ions.
5) As HCl has more FMCP than CH3COOH, it has higher conductivity.
What is partial pressure?
Pressure exerted by an individual has in a mixture of gases.
Consider the following equation:
3H2 + N2 <-> 2NH3
Kc=0.51 at 25C
Write the equilibrium EXPRESSION, Kc
Kc = ((NH3)^2) / ((H2)^2 * (N2))
Explain how adding O2 in the system increases the yield of SO3
1) Le Chatelier’s Principle states that the position of equilibrium will shift to partially oppose (the addition of O2)
2) The position of equilibrium shifts to the (right) to (remove the O2)
3) The foreword/backward reaction is favoured, (increasing yield of SO3)
Define Le Chatelier’s principle
When a system is at equilibrium and the pressure, temperature or concentration is changed, the system will try to PARTIALLY oppose the change.
What 4 things do you need in a concentration time graph?
1) Axes labelled
2) Flat lines at equilibrium (at the same time)
3) Curves correct direction
4) Proportional changes (x)
