Reactions And Rate - Seneca Flashcards
What is collision Theory?
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 with which particles must collide in order to cause a chemical reaction is known as the activation energy.
How to increase rate of reaction using named factors.
We can increase the rate of a reaction by increasing the number of collisions and/or the energy of reactant particles.We can do this by changing the four main factors affecting reaction rates.
-Temperature
-Concentration
-Pressure (for gases)
-Surface area
How to plot reaction rates on a graph
Plotting Reaction Rates
Rates of reactions at specific times can be calculated using graphs:Draw a tangent to the curve (straight line that touches the curve at just 1 point).Work out the gradient of the tangent:Rate of reaction = change in y / change in
According to collision Theory, how can we increase rates of reaction ?
1- increase the frequency ofcollisions
2- increase theenergyof reacting particles
Factors affecting rate.
Rates of Reaction & Energy Changes
Rates of Reaction
Factors Affecting Reaction Rate
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Rate of Reaction - Key Factors
The main factors that affect the rate of chemical reactions are:
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Rate of Reaction - Key Factors
The main factors that affect the rate of chemical reactions are:
Concentration of dissolved reactants
Increasing the concentration increases the collision frequency. This increases the rate of reaction.
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.
Temperature
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.
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.
Catalysts
Substances that increase a chemical reaction’s rate without reducing in quantity during the reaction are called catalysts.
How can we increase the S.A of solid lumps
By breaking them down - exposing more particles.
How can we maintain catalysts ?
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
Examples of catalysts
Different catalysts are appropriate for different reactions:
-Iron is used in the process that makes ammonia.
-Platinum and palladium are used in catalytic converters in cars.
-Enzymes catalyse reactions in biological systems.
What do catalysts usually come as ?
Catalysts often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas.
How do catalysts increase the rate of reactions ?
Catalysts increase the rate of chemical reactions by lowering the activation energy. They do this by changing the reaction pathway.
Why may we not be able to produce high rate reactions in industry
Faster reactions are often better as they yield more product in a given amount of time.
Drawbacks
It may be expensive to generate the conditions for very fast reactions.There can be safety concerns associated with very fast reactions.Therefore, reaction rates used in industry are the result of a trade-off between speed, cost and safety.
How do reactions operate financially ?
In industry, making the required amount of product as cheaply as possible maximises all-important profit.
Experiment to measure gas production. (Rate of reaction).
We can see how changing the surface area affects rate of reaction by measuring how much gas is produced when marble chips (calcium carbonate) of different surface areas react with hydrochloric acid.Set up equipment as shown. Make sure the measuring cylinder is full with water.Place in a known mass of marble chips. The surface area is the independent variable. Pour in a known concentration and excess volume of acid. Start the timer. Record volume of gas produced every 5 seconds. This is the dependent variable. Plot a graph of time against volume of gas. Draw a curve of best fit. Calculate the gradient to find rate at different times.
