4.6- The rate and extent of chemical exchange Flashcards
What does the rate of reaction tell you?
The rate of a chemical reaction tells you how fast reactants turn into products.
What are two ways you can calculate rate of reaction?
There are two ways that you can calculate the rate of a chemical reaction. You can find out how quickly:
The reactants are used up as they make products, or
The products of the reaction are made
There are three techniques that you can use to calculate rates of reaction. What are they?
- Measuring the decreasing mass of a reaction mixture:
- Measuring the volume of gas given off:
- Measuring the decreasing light passing through a solution:
Practical: Investingating the rate of reaction by measuring the volume of gas given off:
- Measure 50cm3 of 1.0M hydrochloric acid using a measuring cylinder. Pour the acid into a conical flask
- Set up your apparatus
- Add a 0.5g magnesium ribbon strip to the flask and place the bung back in the flask. Start the stopwatch at the same time.
- Record the volume of H2 given off every 10 seconds and stop when no more gas is given off.
- Clean the equipment and repeat steps 1-4 with 1.5M of hydrochloric acid
- Plot a graph of ‘volume of gas produced’ against ‘time’. Then calculate the rate.
Practical: Investigating rate using colour change/ turbidity.
- Using a measuring cylinder, measure 10cm3 of 1.0M sodium thiosulfate solution into a conical flask.
- Place the conical flask on a piece of white paper marked with a black cross.
- Measure 5cm3 of dilute hydrochloric acid using a measuring cylinder.
- Add the acid to the flask and immediately start the clock. Swirl the flask to mix the solutions.
- Look down through the top of the flask. Stop the clock when you can no longer see the cross and record the time taken.
- Repeat steps 1-6 again
- Repeat the experiment using different concentrations of sodium thiosulfate solution
- Calculate the mean time for each of the solutions
How do you calculate mean rate of reaction?
Mean rate of reaction = quantity of reactants used or quantity of product formed / time
How do you find the rate of reaction at a given time using a graph?
- Draw a tangent to the curve at that time (a straight line that just touches the curve at this point).
- Then construct a right angled triangle, using the tangent as its longest side (hypotenuse)
- Finally, calculate the gradient of the tangent, as shown on the graph below.
What are factors that affect the rate of reaction?
- the concentrations of reactants in solution,
- the pressure of reacting gases
- the surface area of solid reactants
- the temperature
- the presence of catalysts.
What is the effect of surface area on the rate of reaction?
If solid reactants are in smaller pieces, this means that they have a larger surface area, and it also means that they have a larger surface area to volume ratio. A larger surface area means more particles are exposed and available to collide. This increases the frequency of collision and therefore increases the rate of reaction.
What is the effect of pressure on the rate of reaction?
For reactions involving gases, increasing pressure increases the reaction rate because it forces gas molecules closer together, leading to more frequent collisions and thus more opportunities for successful reactions.
What is the effect of concentration on the rate of reaction?
Increasing the concentration of reactants in solution, increases the number of particles in the same volume and so increases the frequency of collisions and therefore increases the rate of reaction.
What is the effect of temperature on the rate of reaction?
Increasing the temperature of reaction increases the rate of reaction for two main reasons:
Particles collide more frequently
Particles collide with more energy
When the temperature of a reaction mixture is raised, the average kinetic energy of the reactant particles increases. This means they move faster and have more energy.
Increasing the temperature of a reaction generally increases the rate of reaction because it causes reactant particles to move faster and collide more frequently and with greater energy, leading to more successful collisions that overcome the activation energy barrier.
More of the collisions occurring in a given time results in a reaction, because a higher proportion of the particles have energy greater than the activation energy.
When can reactions take place?
Reactions can only take place when the particles of the reactants come together. The reacting particles need to collide with each other with enough energy to cause a reaction to take place. This is known as the collision theory.
What is activation energy?
The minimum amount of energy that particles must have before they can react is called the activation energy. Therefore, reactions are more likely to occur if you:
Increase the frequency of reacting particles colliding with each other
Increase the energy they have when they collide
What is a catalyst?
A catalyst is a substance that changes the rate of a reaction. However, it is not chemically changed at the end of the reaction. Catalysts are not used up in the reaction, so they can be used again and again.
What is the effect of catalysts on the rate of reaction?
Catalysts increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy, meaning more reactant molecules have enough energy to react and form products.
What are the advantages to catalysts in industry?
It is usually cheaper to use a catalyst, than to pay for the extra energy needed without one. To get the same rate of reaction without a catalyst would require higher temperatures/pressures. Therefore, catalysts also help to save the environment/reduce greenhouse gas emissions, because to achieve the higher temperatures/pressures, fossil fuels must be burnt.
