5.5 - The rate and extent of chemical change
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59 Terms
What is the rate of reaction?
How quickly reactants are converted into products.
What happens to reactants during a reaction?
Their concentration decreases.
What happens to products during a reaction?
Their concentration increases.
How can reaction rate be calculated?
Rate = quantity of reactant used ÷ time, or quantity of product formed ÷ time.
What units can rate have?
g/s, cm³/s or mol/s, depending on the quantities measured.
What happens to the rate of a reaction as it proceeds?
It usually decreases.
Why does the rate usually decrease during a reaction?
Reactant concentration decreases, so successful collisions become less frequent.
What is collision theory?
Particles must collide with enough energy and the correct orientation for a reaction to occur.
What is a successful collision?
A collision with enough energy to overcome activation energy.
What is activation energy?
The minimum energy required for a reaction to occur.
How does concentration affect reaction rate?
Increasing concentration increases the frequency of collisions, increasing rate.
How does pressure affect the rate of reactions involving gases?
Increasing pressure brings gas particles closer together, increasing collision frequency.
How does surface area affect reaction rate?
Increasing surface area increases the number of particles exposed for collisions.
How does temperature affect reaction rate?
Increasing temperature increases particle kinetic energy and the frequency of successful collisions.
How does a catalyst affect reaction rate?
It increases rate by providing a pathway with lower activation energy.
Why does powdered solid react faster than large lumps?
Powder has a greater surface area exposed to collisions.
What happens to reaction rate as surface area increases?
Reaction rate increases.
Why does increasing surface area increase collisions?
More particles are exposed at the surface and available to collide.
Why does increasing temperature increase reaction rate?
Particles move faster, collide more often and more particles have energy greater than the activation energy.
Why is the increase in successful collisions more important than just collision frequency?
A collision only causes a reaction if particles have enough energy to overcome activation energy.
What is a catalyst?
A substance that increases reaction rate without being used up.
How does a catalyst work?
It provides an alternative reaction pathway with lower activation energy.
Does a catalyst change the amount of product made?
No.
Does a catalyst change the energy change of a reaction?
No.
Why are catalysts important in industry?
They reduce the energy needed and can make processes faster and cheaper.
What is an enzyme?
A biological catalyst.
How can you investigate the rate of a reaction producing gas?
Measure the volume of gas produced at regular time intervals.
How can you investigate a reaction where a solid disappears?
Measure the time taken for the solid to disappear.
How can mass be used to measure reaction rate?
Measure the decrease in mass as gas escapes.
What does the gradient of a reaction-rate graph represent?
The rate of reaction.
What does a steeper gradient mean?
A faster reaction.
What does a horizontal section of a graph mean?
The reaction has stopped.
Why does the graph become less steep as a reaction proceeds?
Reactants are used up, so successful collisions become less frequent.
How can you improve reliability?
Repeat measurements and calculate a mean.
What is a reversible reaction?
A reaction where products can react to form the original reactants.
What symbol represents a reversible reaction?
⇌
What is dynamic equilibrium?
A state where the forward and reverse reactions occur at equal rates.
Can reactions still happen at equilibrium?
Yes, both reactions continue.
What happens to concentrations at equilibrium?
They remain constant.
Does equilibrium mean the amounts of reactants and products are equal?
No, their concentrations are constant but do not have to be equal.
What conditions are needed for dynamic equilibrium?
A closed system and constant conditions.
What happens when the concentration of a reactant is increased?
The equilibrium shifts towards the products.
What happens when the concentration of a product is increased?
The equilibrium shifts towards the reactants.
What happens when pressure is increased in a gaseous equilibrium?
The equilibrium shifts towards the side with fewer gas molecules.
What happens when pressure is decreased?
The equilibrium shifts towards the side with more gas molecules.
What happens when temperature is increased?
Equilibrium shifts in the endothermic direction.
What happens when temperature is decreased?
Equilibrium shifts in the exothermic direction.
What does Le Chatelier's principle state?
An equilibrium shifts to oppose a change made to the system.
Why does increasing concentration increase reaction rate?
More particles are present in the same volume, increasing collision frequency and therefore successful collisions.
Why does increasing pressure increase the rate of a gas reaction?
Gas particles are closer together, increasing collision frequency.
Why does increasing temperature increase rate?
Particles have more kinetic energy, so collisions occur more frequently and a greater proportion have energy above activation energy.
Why does a catalyst increase rate without changing equilibrium position?
It lowers activation energy for both forward and reverse reactions equally.
Why does increasing temperature favour an endothermic reaction at equilibrium?
The equilibrium shifts in the direction that absorbs the added heat.
Why does increasing pressure favour the side with fewer gas molecules?
This reduces the pressure, opposing the change.
Why must an equilibrium system be closed?
Reactants and products must not escape or enter the system.
Why doesn't equilibrium mean the reaction has stopped?
Forward and reverse reactions continue at equal rates.
Why can industry use a compromise temperature rather than the temperature giving the maximum yield?
A higher temperature may increase rate but reduce the equilibrium yield for an exothermic reaction.
Why can industry use a compromise pressure?
Higher pressure may increase yield but requires more expensive equipment and energy.
What is the difference between rate and equilibrium?
Rate describes how quickly a reaction occurs; equilibrium describes the relative amounts of reactants and products once forward and reverse rates are equal.