Chemistry (Rates of Reaction) Flashcards
(24 cards)
2 General ways you can use to measure the RoR
1) Measuring the amount of reactant used
2) Measuring the amount of product formed over time
How to calculate the RoR
RoR = amount of reactant used/time OR amount of product formed/time
2 Concepts that RoR depends on
1) The frequency of collisions between particles
2) The energy in which the particles collide
4 Main factors of RoR
1) Concentration (of solutions)
2) Temperature
3) Surface Area (or reactants in the solid state)
4) Catalyst (presence of absence)
(5) Pressure (of reactants in the gaseous state))
Activation Energy
(Definition)
The minimum amount of energy required for a reaction to take place. The activation energy is different for different chemical reactions.
How to calculate the RoR on a graph
Use a Tangent
RoR = Tangent = Change in product/Change in time = y/x
Collision Theory
(Definition)
The idea that a reaction only occurs when two particles collide together with enough energy (equal to or greater than the activation energy)
Two Methods you could use to measure the RoR
Calcium Carbonate (marble chips) and Hydrochloric Acid Reaction
1) Gas Collection: Measures the volume of CO2 produced. Gas syringe or upside down water filled measuring cylinder. The acid is added to the carbonate, the bung quickly replaced and the volume of gas produced recorded every 10 secs.
2) Mass Loss: Measures the decrease in mass. A conical flask is placed on a balance. The acid and carbonate are mixed and a cotton wool plug is placed in the top of the conical flask (this allows the CO2 to escape but prevents loss of mass due to evaporation). Readings are taken every 10secs as the mass drops due to the lost CO2.
Catalyst
A substance that speeds up a reaction without being used up in the reaction. A catalyst does this by providing an alternative route for the reaction that has a lower activation energy than the uncatalyzed reaction.
Examples of Catalysts
Transition metals and Transition metal compounds are often good catalysts.
Platinum, palladium and rhodium are catalysts that are used in catalytic converters in cars.
Iron in the Haber process - a reaction in which nitrogen combines with hydrogen to make ammonia.
Manganese (IV) oxide is catalyst used in the decomposition of hydrogen peroxide.
Biological catalysts are called enzymes.
Explain the shape of the gradient of the line in RoR Graphs
At the start, the reaction has just started. The graph is the steepest and therefore the reaction is fastest at this point because the reactants are at their greatest concentration.
In the middle, the reaction has slowed (the curve is less steep) because some of the reactants have been used up and therefore the concentration of the reactants is lower.
At the end, the reaction has stopped. No more product is being made and the curve is horizontal/flat.
Why does an increase in Temperature increase the Rate of Reaction?
An increase in temperature - particles have more kinetic energy -
1) particles move faster - more frequent collisions - more frequent successful collisions in the same period of time - increased rate of reaction
2) more particles have more energy than activation energy - larger proportion of collisions are successful - more successful collisions in the same period of time - increased rate of reaction
Why does an increase in Concentration increase the Rate of Reaction?
An increase in concentration - more particles in the same volume - more frequent successful collisions in the same period of time - increased rate of reaction
Why does reduced Particle Size increase the Rate of Reaction?
Reduced particle size - increase in surface area - more contact between reactants - more successful collisions in the same period of time - increased rate of reaction
Why does the use of a Catalyst increase the Rate of Reaction?
Using a catalyst - reduces activation energy - more successful collisions in the same period of time - increased rate of reaction
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide
Method
Set up Equipment, fill a 100cm3 measuring cylinder with water, invert it and clamp it in place in the trough.
Add large marble chips into Hydrochloric acid (in flask) and replace the bung as fast as possible.
Read the measuring cylinder every 10 secs ad keep taking results until you get the same result three times, the volume of gas does beyond the scale or 180 seconds (3 mins) has gone.
Repeat for medium-sized marble chips, small chips and calcium carbonate powder.
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide
Equipment
- Marble Chips
- Dilute Hydrochloric Acid
- Delivery Tube
- Trough
- Water
- Clamp
- Measuring Cylinder
Write a balanced symbol equation for the reaction:
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide Experiment
CaCO3 + 2HCl => CaCl2 + H2O + CO2
Which reaction was the slowest? How can you tell from the graph?
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide Experiment
The reaction with the large chips - it’s line of best fit was the least steep.
As the marble chips get smaller, their RoR increases - why?
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide Experiment
In conclusion, the higher the surface area to volume ratio, the faster the rate of reaction.
Explain your results in terms of collision theory
Calcium carbonate + Hydrochloric Acid => Carbon Dioxide Experiment
Increasing the surface area to volume ratio increases the amount of particles present. This increases the frequency of collisions, and therefore the rate of reaction.
Sodium thiosulphate + hydrochloric acid => sodium chloride + sulphur + sulphur dioxide + water IN A BALANCED SYMBOL EQUATION
Na2S2O3 (aq) + 2HCl (aq) => 2NaCl (aq) + S (s) + S2 (g) + H2O (g)
Sodium thiosulphate + hydrochloric acid => sodium chloride + sulphur + sulphur dioxide + water
Method
Use a measuring cylinder to measure out separately sodium thiosulphate solution and water. Put these solutions into a conical flask.
Use a thermometer to measure the temperature of the solution in the conical flask. Record this temperature.
Place the flask over a cross.
Use a measuring cylinder to measure out HCl
Add the acid to the thiosulphate and time how long it takes for the mixture to become so cloudy that the cross is no longer visible. record the time in seconds.
Repeat the experiment several time but each time use a Bunsen Burner to heat the thiosulphate and water.
Sodium thiosulphate + hydrochloric acid => sodium chloride + sulphur + sulphur dioxide + water
Conclusion - How does it work?
Sulphur is a yellow solid and is one of the products in the reaction. We can monitor the rate of reaction by measuring how long it takes the sulphur to block our view of a cross beneath a conical flask.
