Unit 4 - Lesson 3: Rates of Reaction Flashcards
What does ‘the rate of the reaction’ mean?
How fast a reaction occurs.
What is collision frequency?
The more particles collide, the faster the reaction.
If there is more energy in the collisions, will the reaction be faster or slower?
Faster
What is a successful collision?
Collisions only result in a reaction if a product is formed. We say it is a successful collision.
What is activation energy?
Particles need to collide with a minimum amount of energy to react. This is called the activation energy and can be shown on a reaction profile.
What factors affect the rate of reaction?
Temperature - the higher the temperature, the greater the rate of the reaction.
Surface area to volume ratio - the greater the s:v ratio, the greater the rate of the reaction because there’s more surface for reactants to collide with.
Concentration or pressure
Presence of a catalyst - Catalysts speed up chemical reactions by lowering the activation energy.
Explain how temperature affects the rate of reaction.
All particles move. They have kinetic energy. The more a particle is heated, the more it moves (we say it has more kinetic energy). If a particle moves more, it has a greater chance of colliding with other particles it can react with. Therefore, the greater the rate of the reaction.
Explain how surface area to volume ratio affects the rate of reaction.
The greater the s:v ratio, the greater the rate of the reaction because there’s more surface for reactants to collide with.
The catalytic converter inside a car is an example of this in action. It turns dangerous carbon monoxide into safe carbon dioxide. It has a honeycomb structure coated with platinum, palladium and rhodium to maximise surface area so as the exhaust gases pass, they react very quickly.
Explain how concentration or pressure affects the rate of reaction.
Increasing the concentration in a liquid means a higher collision frequency for the same volume as there’re more particles in a fixed volume.
Increasing the pressure means taking a fixed mass of gas and squeezing it into a smaller volume. This means particles are forced closer together and collide more frequently.
In your exam, what should you write instead of writing ‘there are more collisions’?
Write there are more collisions per second or collisions occur more frequently.
When discussing concentration, what state are we referring to?
Liquids
When discussing pressure, what state are we referring to?
Gases
In your exam, you may see a graph that plots how the reactants in a reaction fall. When the graph becomes horizontal, what does that mean for the reaction?
The reaction has stopped.
Explain how presence of a catalyst affects the rate of reaction.
Catalysts speed up chemical reactions by lowering the activation energy.
In a graph, what does the steepness of the line mean in terms of the rate of reaction?
The steeper the line, the faster the rate of reaction.
When the line in a graph becomes flat (horizontal), what has happened to the reaction?
The reaction has finished.
What does the height that the line flattens at show?
The height the line flattens shows the amount of product formed.
When drawing rates of reaction graphs, what will the y-axis and x-axis always be?
The y-axis will always be the amount of product formed. This could be given in concentration, volume or mass.
The x-axis will always be time.
Drawing any graph in an exam is worth 4-6 marks. How do you get these marks?
1 mark = Labelled axes
1 mark = Units given to labels
1 mark = Good scale (It must take up at least half the space available)
1 mark = Correct plot
1 mark = Instructions followed (This will normally be ‘draw a line of best fit’ or ‘join points with a straight line’)
What is another word for steepness and what can steepness tell us?
Another word for steepness is gradient. The steepness of the line tells us the rate of the reaction.
How do you calculate gradient if the line is straight?
You divide the change in y by the change in x.
Gradient = Change in y / Change in x
Another way of writing this formula is:
Rate of reaction = Change in concentration, volume or mass / Time
Method:
1. Find 2 points on the straight line
2. Make sure they’re easy to read the values off. (Whole numbers are best)
3. Draw a triangle down from the higher point and across from the lower.
4. Calculate the change in y (height of triangle) and the change in x (base of triangle).
5. (Example) Gradient = 10 / 1.5 = 6.67 (2dp)
How do you calculate gradient if the line is curved?
Draw a tangent line on your curved line at the point you’re interested in. A tangent line should touch the graph line and have equal spacing on either side. Then calculate the gradient on the tangent using the method to find the gradient for a straight line.
Draw a tangent line on a curved line in a graph.
https://i.pinimg.com/736x/73/f6/26/73f6260cebd7bba501a0795ff864b37f.jpg
In what ways can the amount of product formed be measured in?
Concentration, volume or mass.
What is the rate of reaction calculated measured in?
It’s measured in whatever the unit of the y axis is divided by the unit of the x axis. For example, if the y axis is given in g for grams and the x is given in s for seconds, the rate of reaction will be measured in g/s (grams per second).
