Seneca C6 Flashcards
What are the considerations when choosing reaction rates in industry
Speed
Safety
Profit
What are the drawbacks of fast chemical reactions
Cost
Safety Concerns
How do we calculate rate of reaction from a graph
Draw a tangent to the curve
Work out the gradient of the tangent
Collision theory
Collision theory explains how reactions happen and why they happen at different rates.
Particle Collisions
Particles must collide for chemical reactions to happen.
Importantly, these collisions must happen with enough energy.
This minimum amount of energy is known as the activation energy.
Rate of Reaction
We can increase the rate of a reaction by increasing the frequency (number) of collisions and/or the energy of reactant particles.
What does collision theory say the minimum requirements are for a reaction to happen?
Particles must collide
The energy of particles must exceed the activation energy
What does a chemical reaction rate tell us
A chemical reaction’s rate tells us how quickly reactants are converted into products.
What are the main factors that affect the rate of chemical reactions
Concentration of dissolved reactants
Pressure of gas reactants
Temperature
Surface area of solid reactants
Catalysts
Rate of reactions - Concentration of dissolved reactants
Increasing the concentration increases the collision frequency. This increases the rate of reaction.
Rate of reactions - Pressure of gas reactants
Increasing pressure is like increasing the concentration.
It has the same outcome of increasing the collision frequency, which increases the rate of reaction.
Rate of reactions -
Increasing the temperature increases the rate at which collisions happen.
Increasing the temperature increases the energy of reactant particles.
This means that a greater proportion of the particles will have more energy than the activation energy that is needed
Together, these lead to more successful collisions. This increases the rate of reaction.
Rate of reactions - Surface area of solid reactants
Increasing the surface area increases the collision frequency. This increases the rate of reaction.
This is often done by breaking up solids into smaller lumps (e.g. powders).
This increase in surface area to volume ratio makes sure that more particles are exposed to attack
Rate of reactions - Catalysts
Substances that increase a chemical reaction’s rate without reducing in quantity during the reaction are called catalysts.
Are catalysts used up in reactions?
Catalysts are not used up during chemical reactions. This means that:
They can be reused indefinitely.
They are not found in chemical equations.
But, catalysts will often need cleaning or regenerating, which has knock-on effects for energy expenditure and environmental impact.
What are some examples of catalysts?
Iron is used in the process that makes ammonia.
Platinum and palladium are used in catalytic converters in cars.
Enzymes catalyze reactions in biological systems.
What form do catalysts come in?
Catalysts often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas.
Catalysts
Catalysts increase the rate of chemical reactions by lowering the activation energy. They do this by changing the reaction pathway.
What are the advantages of Catalysts in Industry
Cost effective
Reduced burning of fossil fuels
What are the disadvantages of Catalysts in Industry
Toxicity
many catalysts are toxic (e.g. transition metals), and these can escape into the environment and contaminate ecosystems.
Advantages of Catalysts in Industry - Cost effective
a small amount can speed up a reaction by a lot. It may be cheaper to pay for the catalyst at the start and increase the rate of reaction after that.
Paying for a catalyst may be cheaper than paying for the energy needed to increase either temperature or pressure.
Advantages of Catalysts in Industry - Reduced burning of fossil fuels
By reducing the necessary temperatures and/or pressures, fewer fossil fuels need to be burned.
This means that we can reduce our negative environmental impact.
Reversible Reactions
Some chemical reactions can proceed in both forwards and backwards directions. We describe these reactions as reversible.
A reaction which can proceed in both forwards and backwards directions.
What is meant by
A + B ⇌ C + D
The reactants (A + B) can combine to give the products (C + D).
The products (C + D) can combine to give the reactants (A + B).
Exothermic and endothermic reactions
If the forwards reaction is exothermic transfers energy from reactants to the the backwards reaction will be endothermic
Conservation of energy
Energy is conserved during chemical reactions.
The energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backward reaction
Exothermic reaction
A reaction that transfers energy from reactants to the environment.
Endothermic reaction
A reaction that transfers energy from the environment to reactants.
What is Dynamic equilibrium
When reversible reactions happen within a closed system, a dynamic equilibrium will eventually be reached.
Closed system
A closed system is a system where reactants and products can neither be added nor removed.
If a reversible reaction takes place in a closed system, dynamic equilibrium will be reached
Dynamic equilibrium
At dynamic equilibrium, the rates of the forwards and backwards reactions are equal.
The equilibrium is dynamic because both the forward and backward reactions are still taking place.
The concentrations of reactants and products do not change when a reaction is at dynamic equilibrium.
Why is chemical equilibrium described as dynamic?
both forwards and backwards reactions still happen
Changing the conditions in a reversible reaction
The conditions determine the relative quantities of the different reactants and products in a reaction at equilibrium.
Le Chatelier’s principle
Le Chatelier’s principle says that if any of the conditions of a reversible reaction at equilibrium are changed, the closed system will adapt to counteract whatever has changed.
This principle is used to predict the outcome of changes imposed on a system at equilibrium.
Increasing the temperature for a reaction at equilibrium
The position of equilibrium will shift in the endothermic direction.
The amount of products generated by the endothermic reaction will increase.
The amount of products generated by the exothermic reaction will decrease.
Decreasing the temperature for a reaction at equilibrium
The position of equilibrium will shift in the exothermic direction.
The amount of products generated by the exothermic reaction will increase.
The amount of products generated by the endothermic reaction will decrease.
Decreasing pressure for a reaction at equilibrium
The position of equilibrium will shift towards the side of the reaction that produces the most gas molecules.
Increasing pressure for a reaction at equilibrium
The position of equilibrium will shift to favor the reaction that produces the fewest gas molecules.
Increasing the concentration of a reactant at equilibrium
This will shift the position of equilibrium towards the products.
Increasing the concentration of a product at equilibrium
This will shift the position of the equilibrium towards the reactants.
How do we predict the outcome of pressure change
To predict the outcome of a pressure change, we must assess balanced equations to see how many gas molecules are on each side of the equation.
