Rates of reaction Flashcards
How can we show the rate of a reaction?
Use a graph with amount of product formed on the y axis and time on the x
Alternatively, the graph has reactant used on y axis instead
On a graph with amount of product formed, what shape is the curve and why?
Curve is upwards
Because the reactant molecules become the product molecules thus increase in product formed
On a graph with amount of reactant used, what shape is the curve and why?
Curve is downwards
Because the reactant molecules form product molecules thus there’s a decrease in reactant
On these graphs, what does the curve and its gradient show?
The rate of reaction at that specific point
On a product formed graph, what structure does the curve usually follow and why?
Begins increasing rapidly = product formed quickly = more collisions = more reactant particles to collide
Flattens out because less reactant particles = less collisions = less product made
On a reactant used graph, what structure does the graph usually follow and why?
Rapid decreasing = the most number of reactant particles = more collisions = more product being made so less particles all together
Flattens because less reactant particles = less collisions = less reactant used
When does a reaction stop?
No more reactant molecules
What do you measure a solid product in?
grams (g)
What do you measure a gas product in?
Cm3
Mean rate of reaction
Average rate of reaction over the whole reaction
How to find the mean rate of reaction?
Amount of product formed/ reactant used
———————————————
Total time taken of reaction
What is the rate of reaction measured in?
g/s or cm3/s
What is important about the mean rate when calculating it?
You do not need to use a graph, just the total amount divided by the total time
What do you need when calculating the rate at a specific point?
A tangent on the graph!!!
Why do you need to draw a tangent to find rate at a specific point?
Because the curve constantly changes, so does the rate
So you must estimate it with a tangent
How to draw a tangent to find the rate of reaction at a specific point?
Go to the specific time it asks for
Find that point on the curve and draw a tangent to it
Connect the 2 ends of the tangent to make triangle
Find the difference between x and y values Of the triangle
y/x = estimated rate of reaction
What effects the rate of a reaction?
Concentration
Surface area
Temperature
Catalysts used
Concentration
The amount of specific particles in a given volume
Collision theory
Reactions happen when reacting particles collide with sufficient energy
Therefore the rate of reaction = amount of times there’s a successful collision thus reaction happens faster
How does a higher concentration affect the ROR?
High concentration of particles = more successful collisions = collisions per second increases = higher rate of reaction
How does a lower concentration affect the ROR?
Low concentration of particles = less successful collisions = collisions per sec decreases = lower rate of reaction
How does the concentration correlate to the shape of the curve on a graph?
On a product formed graph, as this begins with lots of reactant to none
Rapid incline at first because higher concentration of reactant particles = more collisions = more product formed
Flattens = lower concentration of reactant particles = less collisions = less product formed
Flat at the end = no reactant particles = no collisions
How does the effect of concentration apply to reacting gases?
More molecules = more collisions has the same effect
Although in this case it is not the concentration but the pressure
What does the rate if reaction depend on?
The number of successful collisions per second
If we have higher concentration of reactant molecules, how does this effect the reaction compared to if we had lower?
When it is higher concentration, the rate of reaction is increased because there are more particles so more successful collisions per second
Also more product is formed because we had more reactant molecules at the start anyway
When does the surface area effect the rate of reaction?
When a solid is reacting with a liquid or gas
How does surface area effect rate of reaction?
In a solid, particles on the surface of the solid can only react with the liquid/gas particles and not the underlying layers
Therefore a larger surface area = more exposed particles = more collisions per second = increased rate of reaction
What experiment shows how surface area effects rate of reaction?
Conical flask with hydrochloric acid
Place marble powder inside
Connect flask to a bung and delivery tube
Put measuring cylinder with water in a water tray above delivery tube
Measure how long it takes for marble to produce carbon dioxide
Calculate rate of reaction with amount produced/ time
Repeat with different sized marble samples and compare
How can we find a more accurate reading of carbon dioxide produced when measuring effect of surface area?
Gas syringe
How else can we measure effect of surface area on rate of reaction?
Find the amount of carbon dioxide lost when marble reacts with HCl
Place conical flask with marble chips in hydrochloric acid on a balance
Block top of flask with cotton wool
Cotton wool let’s co2 out put prevents acid leaving
Therefore gives us an accurate result
Collision theory
Chemical reactions can only take place when reacting particles collide with each other
The collisions must have sufficient energy therefore be successful collisions
Rate of reaction depends upon amount of successful collisions per second
Activation energy
Minimum amount of energy the particles must have to collide successfully
And therefore react
How does temperature effect the rate of reaction?
Increasing temperature = increases energy of particles = move faster, colliding more times per second = increase rate of reaction
Also have more energy to overcome activation energy battier thus collide successfully
Catalysts
Increase rate of reaction so they are quicker
Are not used up in the reaction
Useful as when using them you don’t have to increase temp, saving money
Also because they are reusable
How do catalysts affect rate of reaction
They increase it
Because they provide a different pathway for the reaction
That has lower activation energy
Therefore more reacting particles can cross this activation energy barrier = more successful collisions
Do we include catalysts in a word chemical equation?
No they aren’t because they don’t get used up
Can we use the same catalyst for all reactions?
No different reactions require different catalysts
