Chapter 6 - The Rate and Extent of Chemical Change Flashcards
What is rate of reaction?
How fast the reactants are turned into products
What is the equation for mean rate of reaction?
Reactant used/product formed divided by time taken
What are the factors that affect rate of reaction?
- Surface area of reactants
- Temperature
- Concentration (for liquids)
- Pressure (for gases)
- Catalysts
What two things must happen for a chemical reaction to occur?
- Reactants must collide
- Reactants must have enough energy
What is the effect of concentration on rate of reaction?
The higher the concentration the more particles there are in a given volume, so it is more likely that particles collide and react - more frequent collisions which results in faster reaction
Explain collision theory (what is a successful collision)
- Two pairs of particles move towards each other
- Pairs collide and reform so each member of the original pair joins with a member of the other pair. This forms two new pairs
- The new pairs move away from each other
What is activation energy?
The minimum energy needed for particles to be able to react
What is the link between rate of reaction and frequency of successful collisions?
Directly proportional
What is the effect of the surface area on rate of reaction?
The larger the surface area the faster the rate of reaction because there are more particles able to react meaning there are more collisions
What is the effect of temperature on rate of reaction?
- Increasing the temperature gives the particles more energy so they move faster meaning there is a higher frequency of collisions
- More particles have energy greater than the activation energy so there are more successful collisions
What are the ways you can measure the formation of product?
- Precipitation (colourless to cloudy) - disappearing cross method
- Change in mass using a balance (gas is formed)
- Volume of gas given off (collect gas in a gas syringe)
Pros and cons of disappearing cross method
Pros: -simple -easy Cons: -subjective -only works for visual change -can't plot rate
Pros and cons of measuring change in mass using balance
Pros: -accurate Cons: -releases gas into room -some gases won't weigh enough to be picked up (would need to increase resolution) -only use when gas is given off
Pros and cons of collecting gas with a gas syringe
Pros;
-sensitive (gives very accurate results)
-gas isn’t released into the room
Cons:
-don’t get bung on quick enough (human error)
-if reaction is too vigorous you could blow plunger out of the syringe
-only use for gases
What is a catalyst?
A substance which speeds up the rate of reaction without being used up. It does this by lowering the activation energy of the reaction.
Many catalysts are … ?
Transition metals
What effect do catalysts have on rate of reaction?
Catalysts will increase the frequency of successful collisions because they lower the activation energy meaning more particles have energy greater than the activation energy
What is a reversible reaction?
The reactant can turn into the products and the products can turn back into the reactants
What do we call the reaction that goes from reactants to products (arrow to the right)
Forward reaction
What do we call the reaction that goes from products to reactants (arrow to the left)
Backward reaction
What factors can determine the direction of the reaction?
- Temperature
- Pressure
- Amount of reactants/products (volume)
- Concentration
How are the energy changes in a reversible reaction balanced?
- energy released in one direction = energy absorbed in other direction
- if forward reaction is exothermic then the backward reaction is endothermic and vice versa
What is dynamic equilibrium?
- Rate of forward reaction = rate of backward reaction
- No observable change but reactions are occuring in both directions
- Both reactants and products present
- Concentration and amount of products and reactants aren’t always equal
What is needed for a dynamic equilibrium?
- Reversible reaction
- Closed system
What is an open system and closed system?
Open system = heat and matter can leave
Closed system = only heat can leave
What do we call it when there are more reactants than products and vice versa?
More reactant: the reaction lies to the left (like the chemical equation)
More product: lies to the right
If the conditions change the equilibrium becomes unstabilised. What are these conditions/stresses?
- Temperature
- Concentration
- Pressure
- Amount of reactant
What is Le Chatelier’s principle?
Whatever you do to a reaction at equilibrium, the equilibrium will move to oppose the change e.g. make it hotter then equilibrium moves to cool it
What happens if you add more reactant (higher concentration) to a reaction in dynamic equilibrium?
The equilibrium will shift to the right so more product will be formed.
What happens if you remove product (lower concentration) from a reaction in dynamic equilibrium?
The equilibrium will shift to the right so there’ll be less reactant as it would be used to produce more of the products
What happens if you cool a reaction in dynamic equilibrium?
The equilibrium will shift to oppose the change and the exothermic would be favoured (shift towards the products of the exothermic reaction).
What happens if you heat a reaction in dynamic equilibrium?
The equilibrium will shift to oppose the change and the endothermic would be favoured (shift towards the products of the endothermic reaction).
What happens if you increase the pressure of a reaction in dynamic equilibrium?
Equilibrium will shift to the side with less gas molecules to reduce the pressure.
What happens if you decrease the pressure of a reaction in dynamic equilibrium?
Equilibrium will shift to the side with more gas molecules to increase the pressure
What happens if you increase the pressure of a reaction in dynamic equilibrium where the reactants and products have equal pressure?
The equilibrium will not be affected but the rate of both the forward and backward reaction would speed up
What happens if you add a catalyst to a reaction in dynamic equilibrium?
- does not affect equilibrium (no shifts)
- increases rate of forward and backward reaction
