Rate and Extent of Change Flashcards
(22 cards)
What is the rate of reaction? What is the equation and what are its units?
The rate of reaction is the quantity of reactants used up over time, or the quantity of produces formed over time
mean rate of reaction = quantity of reactants used / time
mean rate of reaciton = quanitty of products formed / time
Units: g/s or cm3/s, or mol/s
What are the different factors that affect the rate of reaction?
Concentration of reactions in a solution
Temperature
Surface Area of solid reactans
Pressure of reacting gases
Catalysts (presence of)
How does concentration of reactants affect the rate of reaction?
Concentration is the measurement of the number of particles dissolved in a given solvent.
Increasing the concentration of reactants, increases the rate of reaction. Because:
- there are more particles of the reactants moving around in a given volume of solution
- this increases the frequency of collisions between particles
- therefore there are more successful collisions between particles
- This results in a faster rate of reaction
How does pressure affect the rate of reaction (basically concentration but for gases)?
Pressure is a measurement of the number of gas particles in a given volume of gas.
Increasing the pressure increases the rate of reaction because:
- There are more particles in a given volume because particles are closer together
- This leads to more frequent and successful collisiosn whichc leads to a faster rate of reaction
How does temperature affect the rate of reaction?
Increasing the temperature increases the rate of reaction because:
- particles have greater kinetic energy (makes particles more energetic)
- therefore they move around faster
- causing more frequent and successful collisions
- which increases the rate of reaction
How does surface area affect the rate of reaction?
The larger the surface area to volume ratio, the faster the rate of reaction because:
- greater SA:V means more particles are exposed to the other reactant
- therefore collisions are more likely so more frequent and sucessful
- increases ROR
How does the presence of catalysts affect the rate of reaction?
A catalyst is a substance that speeds up a chemical reaction without being used up.
The presence of a catalyst increases the rate of reaction because:
- A catalyst provides an alternative reaction pathway for the reactants, with a lower activation energy
- as there is a lower activation energy, a higher proportion of reactants have sufficient energy to collide with other particles
- This increases the frequency of successful (effective) collisions, increasing RoR
What is collision theory - what does it state?
Collision theory explains how different factors affect the rate of reaction. It states that for chemical reactions to occur reacting particles need to collide and collide with sufficient energy. The minimum amount of energy particles need to react is called activation energy.
Give an example of a catalyst in a biological system.
Enzyme
Draw an energy profile for an exothermic reaction with no catalyst, then with catalyst added
What is a catalyst?
A catalyst is a substance that changes the rate of a chemical reaction but is not used up during the reaction. Different reactions need different catalysts.
What are reversible reactions?
In some chemical reactions, the products of the reaction can react to produce the original reactants
A different arrow is used when showing the equation
Do energy changes change in a reversible reaction?
Yes
If the forward reaction is exothermic, then the reverse reaction is endothermic
and vice versa
What is equilibrium and how is it reached?
When reversible reactions occur in apparatus which prevents the escape of products or reactants (ie a closed system), then equilibrium is reached when the forward and reverse reactions are occurring at exactly the same rate.
What is dynamic equilibrium?
When the reactants react to form the products, simultaneoulsy and at the same rate as the products react to form the reactants
What is the 1st law of thermodynamis?
Energy cannot be created or destroyed - it is merely transferred
What happens if a change is made to a system at equilibrium?
The system responds to counteract that change.
The relative amounts of all the reactants and products at equilibrium depend on the conditions of the reaction.
What is Le Chatelier’s principle?
If the conditions of a system at equilibrium are changed, then the position of the equilibrium mvoes to oppose that change.
The effect of changing concentration on equilibrium
If the concentration of one of the reactants or products is changed, the system is no longer at equilibrium and therefore the concentrations of all substances will change until equilibrium is reached again.
If the concentration of the reactants is increased, more products will be formed until equilibrium is reached (to oppose the change in concentration and equilibrium will shift to the right).
If the concentration of a product is decreased, more reactants will react (to form the products) until equilibrium is reached again.
The effect of temperature on equilibrium
If the temperature of a system at equilibrium increases:
The relative amount of products at equilibrium increases for an endothermic reaction
The relative amount of products at equilibrium decreases for an exothermic reaction
If the temperature of a system at equilibrium decreases:
The relative amount of products at equilibrium decreases for an endothermic reaction
The relative amount of products increases for an exothermic reaction (yield increases)
The effect of pressure changes on equilbrium
For gaseous reactions at equilibrium:
- An increase in pressure causes the equilibrium position to shift towards the side with the smaller number of molecules (fewer moles) as shown by the symbol equation for the reaction
- A decrease in pressure causes the equilibrium position to shift towards the side with the larger number of molecules as shown by the symbol equation for that reaction.
Effect of catalyst on equilbrium
No effect - catalyst just increases the rate of reaction of both the reactants and the products sides by the same amount, so equilibrium doesn’t change, only rate of reaction.
