How does the rate of reaction change over time? Why?
Reactions are fast initially, as there is a large number of reactant molecules that are reacting. The rate then begins to decrease as most of the molecules have already reacted, then eventually the reaction stops once all the reactant molecules have reacted.
What are the two ways we can measure rate?
mean rate of reaction = quantity of product formed / time taken
mean rate of reaction = quantity of reactant used / time taken
What is collision theory?
The idea that chemical reactions take place when the reacting particles collide. The collisions must have sufficient energy (activation energy).
This means reactions slow down over time as more particles have reacted, meaning there are less frequent collisions.
What does collision frequency mean?
The number of successful collisions per second
What is the relationship between concentration and rate?
Concentration is proportional to rate.
Higher concentration = higher rate as more particles can react meaning the frequency of collisions is higher. The same applies to gases: more gas particles = a higher gas pressure.
Describe the first method of the rate of reaction RP.
- Use a measuring cylinder to measure 10cm3 of sodium thiosulfate solution into a conical flask. Place the flask onto a printed black cross. The sulfur in the solution turns it cloudy - this is known as turbidity.
- Add 10cm3 of hydrochloric acid into the conical flask. Swirl the solution and start a stopwatch. Watch the solution as it turns cloudy until the cross disappears.
- Carry out the experiment again using lower concentrations of sodium thiosulfate solution. Then repeat the entire experiment again and calculate a mean value for each concentration.
Describe the second method of the rate of reaction RP.
This measures the volume of gas produced.
- Use a measuring cylinder to measure out 50cm3 hydrochloric acid into a conical flask. Attach the conical flask to a bung and a delivery tube. Place the delivery tube into a container filled with water.
- Place an upturned measuring cylinder filled with water over the delivery tube. Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch.
- Measure the volume of hydrogen gas in the measuring cylinder every 10 seconds until no more is given off. Repeat using different concentrations of hydrochloric acid.
Why is it good to use two methods?
It shows the finding(s) is reproducible. Reproducibility is whether or not the measurement can be repeated by another person or by using a different method while still getting the same results.
The problem with using the first method is that everyone has different eyesights, so they may stop seeing the cross at different points (parallax error).
What is the effect of surface area on rate of reaction?
Increasing surface area increases the rate of reaction
Why does increasing surface area increases rate of reaction?
Particles in a solution can only react with particles on the surface of a solid, meaning increasing surface area increases the number of collisions that can occur.
How do we test the effect of surface area?
Use the exact same experiment as testing the effect of concentration BUT
- Replace the magnesium strip with marble chips. This is because they contain calcium carbonate which forms CO2 when reacted with hydrochloric acid. The volume of CO2 can then be measured in the upturned measuring cylinder.
This can be made more accurate using a gas syringe.
How else can surface area be tested?
Place a conical flask containing hydrochloric acid and the marble chips on a balance. Place a piece of cotton wool in the top of the flask, which allows CO2 to escape but prevents acid from splashing out. You can measure the decrease in mass as CO2 is produced and released.
What is the relationship between rate and temperature?
Temperature and rate are proprtional
- Increasing temperature increases the kinetic energy particles have, meaning they move faster which increases the frequency of collisions.
More particles now have the energy to overcome the activation energy barrier.
What is a catalyst?
- Catalysts increase the rate of chemical reactions, but are not used up during the reaction.
- As they are not used up, they are not included in chemical equations.
- Different reactions require different catalysts
Why are catalysts useful?
They save money as they allow us to carry out reactions without increasing the temperature. They are also resuable.
How do catalysts increase rate?
They provide a different pathway for the reaction that has a lower activation energy - less activation energy is required meaning more particles can collide more frequently.
What is a reversible reaction?
A reaction whereby the reactants can form products, and those products can reform the reactants under different conditions.
What is a good example of a reversible reaction that is often asked?
Hydrated copper sulfate (blue) –> anhydrous copper sulfate (white) + water
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What is an important thing about reversible reactions?
If one reaction is exothermic, the other is endothermic and vice versa. The same amount of energy is transferred each time.
What is equilibrium?
When both the forward and reverse reactions of a reversible reaction occur at the same rate in a closed system.
What is Le Chatelier’s Principle?
If a system is at equilibrium and a change is made to the conditions, the system responds to counteract the change
What is the effect of changing concentration on equilibrium?
Increasing the concentration of the reactants means the system will react by producing more product. If the concentration of prodcuts is increased, more product will be converted back into reactant until the system reaches equilibrium.
INCREASING PRODUCT shifts equilibrium left
INCREASING REACTANT shifts equilibrium right
What happens to equilibrium when pressure is changed?
If gas presure is increased, equilibrium shifts to the side with fewer molecules. If it is reduced, equilibrium shifts to the side with more molecules. If each side has the same number of molecules, the position doesn’t change.
