C8 Flashcards
slow reaction
rusting of iron
ways to wok out the rate of a chemical reaction
find out how quickly the reactants are used up or how quickly the products are made
mean rate of reaction =
quantity or reactant used or product formed / time taken
collision theory
reactions can only happen if particles collide with enough energy to change into new substances
activation energy
the minimum energy particles must have in order to react.
how to increase the rate of reaction
increase the energy the particles have when the collide
increase the frequency of reacting particles colliding with each other
factors that increase these increase the rate of reaction
factors that affect the rate of reaction
temperature, concentration, pressure of gases, surface area, catalysts.
Why do powders react faster than a large piece of solid?
Breaking into smaller pieces exposes new surfaces so increases the surface area and SA:V ratio. Causes more collisions in the same time.
Why does increasing the temperature increase the rate of reaction
particles collide more often and particles collide with more energy
Why does increasing the concentration increase the rate of reaction?
There are more particles of the reactants in the same volume of solution so there are more collisions.
Why does increasing the pressure increase the rate of reaction?
More particles of gas in a given space so more frequent collisions.
disappearing cross method
sodium thiosulfate solution + hydrochloric acid —> sulfur (solid)
sulfur makes solution go cloudy- cloudiness is turbidity. Insoluble solid = precipitate.
1. use measuring cylinder to measure 10cm^3 of sodium thiosulfate solution into a conical flask
2. place the conical flask onto a printed black cross
3. add 10cm^3 of hydrochloric acid into the conical flask
4. swirl the solution and start a stopwatch
5. look down through the top of the flask. After a certain time, the solution will go cloudy. Stop the clock when you can no longer see the cross. Record time.
6. Repeat steps 1-5 with a lower concentration of sodium thiosulfate solution
7. Repeat whole experiment and calculate a mean for each concentration of sodium thiosulfate solution.
reproducibility
a measurement is reproducible if it can be repeated by another person using a different technique or equipment and still get the same result.
problem with disappearing cross practical
People have different eyesights. Some people can see the cross for longer than others and may not get the same result. Not reproducible. Use same size printed cross to reduce this problem.
measuring the volume of as given off practical for rate of reaction
magnesium + hydrochloric acid —> magnesium chloride + hydrogen(gas)
- Use a measuring cylinder to add 50cm^3 of hydrochloric acid into a conical flask
- Attach the conical flask to a bung and delivery tube. Place the delivery tube into a container filled with water.
- Place an upturned measuring cylinder also filled with water over the delivery tube.
- Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch.
- The reaction produces hydrogen gas which is trapped in the measuring cylinder. Every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder. Continue until no more hydrogen gas is given off.
- Repeat steps 1-6 using different concentrations of hydrochloric acid.
- Repeat experiment and calculate a mean.
concentration affecting rate of reaction practicals conclusions
Both experiments show the greater the concentration of a chemical in a reaction, the faster the reaction takes place.
Result shown by 2 experiments- reproducible.
catalysts
change the rate of reaction. Not used up or chemically changed during a reaction. Different catalysts are needed for different reactions.
How do catalysts increase the rate of reaction
Provide an alternative reaction pathway with a lower activation energy. With a catalyst, a higher proportion of the reactants have sufficient energy to react. This means the frequency of effective collisions increases and the rate of reaction speeds up.
catalysts in industry
Catalysts are used in many industrial processes because they can reduce the energy and time needed for reactions. This helps to reduce costs and impacts on the environment.
ammonium chloride
When ammonium chloride is heated, it decomposes to produce ammonia gas and hydrogen chloride gas. When the gases cool down, they react to form ammonium chloride again. Reversible reaction.
ammonia chloride NH4Cl(s)
ammonia NH3 (g)
hydrogen chloride HCl(g)
reversible reactions
forward and reverse reactions involve equal but opposite energy transfers.
Reversible reaction exothermic in one direction and endothermic in the other. Amount of energy leased by exothermic reaction = amount of energy taken in by endothermic reaction.
copper sulfate reversible reaction
hydrated copper(II) sulfate (blue)–> anhydrous copper(II )sulfate(white)+water
