chapter 10 - speed of a reaction

Question 1

What is rate?

Answer 1

rate is a measure of how fast or slow something is.
it is a measure of the change that happens in a single unit of time.
* you are encouraged to use the term “rate” rather than “speed”.

Question 2

how would you measure the rate of the rxn between zinc and sulfuric acid?
*demonstrate knowledge and understanding of a practical method for investigating the rate of a reaction involving gas evolution.

Answer 2

zinc + sulfuric acid —> zinc sulfate + hydrogen
- the amount of zinc used up per minute.
- the amount of sulfuric acid used up per minute.
- the amount of zinc sulfate formed.
- the amount of hydrogen produced per minute.
it is easiest to measure the amount of hydrogen —> it is a gas —> can be collected in a gas syringe as it bubbles of —> volume can be measured.

Question 3

in general, how would you measure the rate of a reaction?

Answer 3

• measure the amount of a reactant used up per unit time.

- measure the amount of product produced per unit time.

Question 4

what are the ways you can use to change the rate of a reaction?

Answer 4

1- change the concentration
2- change the temperature
3- change the surface area
4- catalysts

Question 5

*describe the effect of concentration on the rate of reactions

Answer 5

a reaction goes faster when the concentration of a reactant is increased.

Question 6

*describe the effect of temperature on the rate of reactions

Answer 6

a reaction goes faster when the temperature is raised. when the temperature increases by 10C, the rate generally doubles.

Question 7

*describe the effect of particle size (surface area) on the rate of reactions

Answer 7

the rate of a reaction increases when the surface area of a solid reactant is increased.

Question 8

*what should you include when interpreting data from experiments concerned with rate of reaction?

Answer 8

• steepness of the curves.
• how long the reaction lasts.
• how much product is produced at the end of the reaction.

Question 9

*describe the application of the above factors to the danger of explosive combustion with fine powders (flour mills) and gases (methane).

Answer 9

in flour mills:
- flour particles are so tiny —> large surface area
- a spark from a machine could cause an explosion because there is a lot of dust in the air.
- this could also happen in wood mills, silos, factories that make custard powder and dried milk.
in coal mines:
- methane and other flammable gases collect in the air.
- at certain conc they form explosives mix with the air. a spark is enough to set off an explosion (ex: from candles).

Question 10

*devise and evaluate a suitable method for investigating the effect of concentration on the rate of a reaction

Answer 10

• clean a strip of magnesium with sand paper.
• put dilute HCl in the flask flask.
• drop the magnesium into the flask.
• insert the stopper and syringe immediately.
• start the clock at the same time.
• hydrogen begins to bubble off, it rises up the flask and into the gas syringe, pushing the plunger out.
• volume of the gas in the syringe is noted at intervals (ex: every half a min).
• repeat the experiment but double the concentration of HCl.

Question 11

*devise and evaluate a suitable method for investigating the effect of temperature on the rate of a reaction

Answer 11

• mark a cross on a piece of paper.
• place a beaker containing sodium thiosulfate solution on to of the paper, so that you can see the cross through it, from above.
• quickly add HCl, start a clock at the same time, and measure the temperature of the mixture.
• the cross grows fainter as the precipitate forms. Stop the clock the moment you can no longer see the cross. Note the time.
• repeat those 4 steps several times, changing only the temperature. do this by heating the sodium thiosulfate solution to different temps, before adding the acid.

Question 12

*devise and evaluate a suitable method for investigating the effect of surface area on the rate of a reaction

Answer 12

• place marble in the flask and add the acid.
• quickly plug the flask with cotton wool to stop any liquid splashing out.
• weight it, starting the clock at the same time. not the mass at regular time intervals until the reaction is complete.
• repeat these same steps twice more, but use marble chips of greater surface area (crush it).

Question 13

what is the collision theory?

Answer 13

in order for 2 substances to react together:

• the particles must collide with each other
• the collision must have enough energy to break bonds to allow the reaction to occur.

Question 14

what does the rate of a reaction depend on?

Answer 14

how many successful collisions there are in a given unit of time.

Question 15

*explain the effect of concentration on the rate of reactions

Answer 15

in a concentrated substances, there are more particles so there is more chance of successful collisions.
(*an increase in concentration only causes an increase in collision rate)

Question 16

*explain the effect of temperature on the rate of reactions

Answer 16

increasing temperature gives particles more KE so they move faster. they collide more often and the collisions have more energy so more successful.
(*an increase in temp causes and increase in collision rate and more of the colliding molecules have sufficient energy (activation energy) to react)

Question 17

*explain the effect of surface area on the rate of reactions

Answer 17

in powdered substances, many more particles ae exposed so the chance of collision increases.

Question 18

what is a catalyst?

Answer 18

a substance that speeds up chemical reactions, but remains chemically unchanged itself.

Question 19

*describe and explain the effect of catalysts on the rate of reactions

Answer 19

when a catalyst is present, the reactants are able to react in a way that requires less energy.—> lower activation energy.—> so more more collisions have enough energy to be successful, so the reaction is sped up.
(not all catalysts work for all reactions).

Question 20

what catalysts are used in the chemical industry?

Answer 20

often transition elements or their oxides:

• iron in the manufacture of ammonia
• vanadium IV oxide in the manufacture of sulfuric acid.

Question 21

what is an enzyme?

Answer 21

proteins made by cells, to act as biological catalysts.
found in every living thing.
- if the temp gets too high, the enzyme gets denatured and loses its shape.
- each enzyme works best in a specific pH range. you can denature it by adding acid or alkali.

Question 22

how are enzymes put to use?

Answer 22

• they speed up reactions in our bodies, which otherwise would happen far too slow and we would die.
• bacteria make enzymes which can be used in biological detergents.

Question 23

what name is given to reactions that need light?

Answer 23

photochemical reactions

Question 24

• describe what photosynthesis is

* Describe and explain the role of light in photochemical reactions

Answer 24

carbon dioxide + water –(light and chlorophyll)–> glucose + oxygen
chlorophyll is a catalyst for the reaction
sunlight provides the energy for the endothermic reaction

Question 25

how can we change the rate of the photosynthesis reaction?

*the effect of light on the rate of photochemical reaction

Answer 25

placing a bulb closer to a plant increases light intensity
—> increases rate of photosynthesis.
a photochemical reaction can be speeded up by increasing the light intensity.

Question 26

• Describe the use of silver salts in photography

* Describe and explain the role of light in photochemical reactions

Answer 26

film is covered with a coating of gel that contains tiny grains of silver bromide.
light causes it to break down:
- 2AgBr(s) —-> 2Ag(s) + Br2(l)
- it is a photochemical reaction and redox.
- silver ions are reduced: 2Ag+ + 2e- —-> 2Ag
- bromide ions are oxidized: 2Br- —> Br2 + 2e-

Question 27

• Describe the use of silver salts in photography

* Describe and explain the role of light in photochemical reactions

Answer 27

film is covered with a coating of gel that contains tiny grains of silver bromide.
light causes it to break down:
- 2AgBr(s) —-> 2Ag(s) + Br2(l)
- it is a photochemical reaction and redox.
- silver ions are reduced: 2Ag+ + 2e- —-> 2Ag
- bromide ions are oxidized: 2Br- —> Br2 + 2e-
where brighter light strikes (from brighter parts of the scene), decomposition is faster, giving more silver.