3b Rates of Reaction Flashcards
What is the rate of a reaction?
The rate of a reaction is how quickly the products are formed or how quickly the reactants are used up
Why do we measure reaction rates?
- If a reaction runs too quickly it might get out of control and cause an explosion
- If a reaction runs too slowly this will make the process inefficient and will raise production costs
- It is therefore important to measure reaction rates so that we can learn what influences them and how we cant control them
What is the equation for Reaction Rate?
Reaction Rate = Change in amount of substance/Time
How do Chemical Reactions occur?
- Chemical Reactions occur when particles of a reactant collide with enough energy to react.
What is Collision Energy?
- However not every collision will result in a reaction
- Collisions will only react in a reaction if they have enough energy. This energy is known as the Collision Energy
What is Collision Theory?
- For a reaction to occur between 2 substances, their particles must collide with sufficient energy to allow existing bonds to break and new bonds to form
- A reaction rate may increase if:
- The probability (or chance) of 2 particles colliding is increased
- The energy with which the particles collide is increased
- Increasing the probability of 2 particles colliding is increased by changing the conditions of:
- Concentration (of a solution)
- Pressure (of a gas)
- Surface area (of a solid)
- Increasing the energy with which particles collide is increased by changing:
- Temperature at which the reaction occurs
Why does the rate of a reaction slow down over time and then stop?
- At the start of the reaction there are the maximum number of reactant particles and so the most frequent collisions leading to the fastest rate
- As the reaction progresses there are less reactant particles as they have been converted to products. Therefore there are less frequent collisions and a slower rate
- The graph flattens out when the reaction is complete as there are no more reactant particles to collide
Concentration: Key points
- Increasing the concentration of an acid whilst keeping all other conditions the same will increase the rate of reaction.
- The total amount of product will be the same as long as the acid is in excess
- The total amount of product will change in line with concentration if the solution is the controlling factor
Is there a correlation between temperature and rate of reaction?
- To be sure that there is a correlation you could take more measurements across a wider range of temperatures.
- Measurements at each temperature should be taken more than once to ensure they are repeatable
- You could also see the effect of temperature on a different reaction (eg. Magnesium + acid) to see if the same correlation exists for other reactions
How does Increasing temperature increase the reaction rate?
The higher the temperature:
- The more kinetic energy the particles have and so the faster the particles are moving
- The harder the collision will be
Both of these will result in more frequent collisions
What are some sources of error when conducting the temperature experiment?
- Judgement being used and not necessarily producing the same amount of iodine each time if volumes not accurately measured
- A colorimeter which will measure light absorbed can be used to give a more accurate value
What are catalysts?
- Catalysts are substances that change the rate of a reaction without being used up in the reaction
- Catalysts never produce more product - they just produce the same amount more quickly
How do catalysts work?
- Different catalysts work in different ways, but most lower the reaction’s activation energy (Ea)
- They do this by providing an alternative pathway for the reaction
How does concentration affect the rate of a reaction?
Low concentration = less frequent collisions = slower rate of reactions
High concentration = more frequent collisions = faster rate of reaction
There are more particles per unit volume so collisions are more frequent
How does surface area affect the rate of a reaction?
Smaller Surface area = less frequent collisions = slower reaction rate
Larger surface area = more frequent collisions = faster reaction rate
How does temperature affect the rate of a reaction?
At the higher temperature, particles have more KINETIC ENERGY.
There are therefore:
- More frequent collisions
- Collisions are harder
BOTH result in a faster rate of reaction
