C8

Question 1

What does rates of reactions mean?

Answer 1

The speed that the reactants get turned into products.

Question 2

Give an example of a slow reaction?

Answer 2

Rusting of iron (could take up to years/ decades).

Question 3

Give an example of a more typical rate reaction?

Answer 3

Reacting of magnesium and an acid, producing a gentle stream of H2 bubbles.

Question 4

Give an example of very fast reaction?

Answer 4

Explosions such as fireworks.

Question 5

How do we measure the RoR?

Answer 5

Measure how fast the reactants are being used up.
OR
Measure how fast the products are being formed.
Because the faster the rate of reaction, the faster the reactants are being used up/ products being formed.

Question 6

Equation for RoR?

Answer 6

• Quantity of products formed/ time taken
• Quantity of reactants used up/ time taken

Quantity= grams OR cm3/ moles/ dm3
Time= seconds/ minutes

Question 7

What does using the RoR formula tell us?

Answer 7

The MEAN rate of reaction only.

Question 8

What are the four factors that affect RoR?

Answer 8

1) Temperature
2) concentration/ pressure
3) surface area
4) catalysts

Question 9

What is the collision theory?

Answer 9

For particles to react, they have to collide with each other with SUFFICIENT energy (activation energy)

Question 10

According to the collision theory, what does the rate of a reaction at any particular point depend on?

Answer 10

1) Amount of energy the particles have (more energy they have -> more energy transferred during collision-> more likely to surpass activation energy)
2) Frequency of collisions (not all collisions are successful since they don’t always reach activation energy BUT more often collisions -> more successful collisions OVERALL)

Question 11

How does increasing the amount of energy particles have and the frequency of collisions result in increase in rate of reaction?

Answer 11

Increased the number of successful collisions as they reach activation energy.

Question 12

How does temperature increase RoR?

Answer 12

• temp increase -> particles gain more energy
• more energy -> move faster -> collide more often
• more energy -> collide with more energy -> more likely to exceed activation energy.
• overall higher rate of successful collisions -> higher RoR.

Question 13

How does concentration/ pressure increase RoR?

Answer 13

Higher conc/ pressure -> more particles per unit of volume -> more frequent collisions -> increased RoR.

Question 14

How does surface area increase RoR?

Answer 14

• Reactant can be: a solid block, small chunks, or powder (all the same mass & volume).
• The powder form has the highest surface area to volume ratio.
• Higher SA -> more area for more collisions with other reactants to occur.
• Increased frequency of collisions -> increased RoR.

Question 15

How do catalysts increase RoR?

Answer 15

Lower the activation energy by providing an alternative reaction pathway -> higher proportion of successful collisions (as larger proportion of particles posses necessary energy when they collide).
- When catalysis is present particles require less energy to cross the activation energy barrier -> more successful collisions per second -> increased RoR.
MORE ON THIS?!

Question 16

What does concentration and pressure refer to?

Answer 16

• Number of particles per unit of volume.
  Gasses= pressure
  Concentration= solutions

Question 17

How to calculate ACTUAL RoR using a graph?

Answer 17

E.g. if asked for RoR at 2 mins:
- Find 2 mins on x-axis
- Draw a line up from 2 mins to find where it intersects the curve.
- Draw a tangent to that point (outside the curve).
- Calculate the gradient (change in y/ change in x).

Question 18

How do we calculate average RoR using a graph?

Answer 18

“What is the mean RoR in the first 3 minutes?”
- Find 3 on x-axis.
- Find where going up from 3 intersects the curve.
- Go across from that point to the y-axis to find mass of reactants remaining/ volume of products produced.
- Use the average RoR equation.

Question 19

What does the double needed arrows tell us in about a reaction?

Answer 19

That the reaction is reversible.

Question 20

Describe the rates of the forward and backwards reaction at the start, as the products form and a while after the products form.

Answer 20

The forward and backwards reactions can take place at different rates.
In the beginning the forward reaction is fast, but the backwards reaction has not started yet.
As the products form, the forward reaction is slower and the backwards reaction speeds up.
After a while the rate evens out and the forward and backwards reactions will be going at the same speed.

Question 21

What does it mean when a reaction is at equilibrium?

Answer 21

1) When the concentration of reactants and products wont change anymore (they are CONSTANT)
2) and the forwards and backwards reactions happen at the same speed as the rate evens out.
(Both reactions are still happening, but they cancel eachother out so no overall change in conc. of reactants)
3) requires a closed system.

Question 22

Does the fact that the concentrations of reactants being constant at equilibrium mean they’re the same?

Answer 22

No. (There could be lots of products and little reactants or little product and lots of reactants)

Question 23

What determines the position of equilibrium?

Answer 23

More products than reactants - equilibrium lies to the right.
More reactants than products- equilibrium lies to the left

NH4Cl —>
<— NH3 + HCl

Question 24

How does heat and cold affect position of equilibrium?

Answer 24

Heat (conditions) - encourages forward reaction -> position of equilibrium moves to the right.
Cool - encourages backwards reaction and moves position of equilibrium to the left.

Question 25

What is the main condition that has to be set for equilibrium to occur?

Answer 25

Reaction must be happening in a closed system. (So no reactants and products can escape)

Question 26

Do reversible reactions release heat?

Answer 26

They are always exothermic in one direction, and endothermic in the other.

Question 27

What does hydrated and anhydrous mean?

Answer 27

Hydrated- water is present Anhydrous- no water present

Question 28

What is Le Chatelier’s principle?

Answer 28

If we change the conditions of a reversible reactions, the position of equilibrium will shit and try to counteract the change.

Question 29

What happens if we decrease the temperature in a closed system?

Answer 29

The position of equilibrium moves in the exothermic direction to release heat. (We would have more of what’s on that direction than the opposite)

Question 30

What happens if we increase the temperature in a closed system?

Answer 30

Position of equilibrium moves to the endothermic direction to counteract the change in the system and take in heat. (We would have more of what’s on that direction than the opposite)

Question 31

What happens if we increase the pressure of a reaction happening in a closed system?

Answer 31

- Pressure= how many particles there are per unit of volume. - equilibrium moves to reduce pressure and counteract the increase. - pressure lowered by equilibrium moving to the side with the least number of molecules.

Question 32

What happens if we decrease the pressure of a reaction happening in a closed system?

Answer 32

Equilibrium shifts to the side with more molecules. To increase pressure again. (Count how many molecules sing the reaction equation)

Question 33

What happens if we increase the conc of something in a reaction in a closed system?

Answer 33

Example: N2 + 3H2 —> <— 2NH3 - Increased conc nitrogen: equilibrium shift to the (OPPOSITE SITE) right and forms more ammonia to counteract the increase in nitrogen.

Question 34

What do reversible reactions involve?

Answer 34

The formation of products from reactants and the BREAKING DOWN of the products back into reactants.

Question 35

In a reversible reactions, changes in pressure will only effect substances that are in the:

Answer 35

Gaseous state.

Question 36

What is a hypothesis?

Answer 36

A proposal that could explain a fact/ observation. In science, a hypothesis must be testable.

Question 37

What reaction is involved in the disappearing cross experiment?

Answer 37

Sodium thiosulfate solution + HCl —> sulfur (solid)

Question 38

What does the sulfur do in the experiment? What is that observation used for?

Answer 38

It makes the solution go cloudy (turbidity?). To see how long a reaction takes to finish.

Question 39

What are the steps of the disappearing cross experiment (7)?

Answer 39

1) use a measuring cylinder to put 10cm3 of sodium thiosulfate solution into a conical flask. 2) Place conical flask onto a printed black cross. 3) Measure 10 cm3 of HCl and add to the sodium thiosulfate solution and start a timer immediately. 4) Look down through the top of the flask. 5) Stop the timer when solution goes cloudy and cross disappears. 6) Repeat experiment with LOWER concentrations of sodium thiosulfate solution. 7) Repeat whole experiment and calculate mean values for each conc. of sodium thiosulfate solution.

Question 40

What do we NOT include when calculating a mean?

Answer 40

Anomalous results.

Question 41

What does a reproducible measurement mean?

Answer 41

A measurement is reproducible if it can be repeated by another person or using a different technique/ equipment to still get the same results.

Question 41

What is the issue with the disappearing cross experiment?

Answer 41

Different people have different eyesights, they might see the cross for longer so may not get the same results.

Question 42

What is something that might reduce the effect of the problem raised in the disappearing cross experiment?

Answer 42

All students using the same sized printed cross -> problem may not be too great.

Question 43

What reaction is involved in the experiment where we want to measure volume of gas produced by reaction (2nd method to disappearing cross)?

Answer 43

Magnesium + HCl -> magnesium chloride + hydrogen Measure volume of hydrogen gas produced.

Question 44

How do we setup the equipment for the measuring volume of gas produced experiment?

Answer 44

- Measuring cylinder: 50 cm3 of HCl into a conical flask. - attach conical flask to a bung and a delivery tube. - place the end of delivery tube in a container filled with water. - place an upturned measuring cylinder also filled with water over delivery tube.

Question 45

What are the steps for the measuring volume of gas produced practical?

Answer 45

- set up equipment. - add 3 cm strip of Mg and start a stopwatch. - reaction produces gas which is trapped in the measuring cylinder. Every 10 seconds measure the volume of hydrogen gas in the measuring cylinder, continue until o more hydrogen given off. - repeat experiment with different concentrations of HCl. -( repeat entire experiment to calculate mean values for each concentration).

Question 46

How can we tell that the finding -the greater the concentration of a chemical in a reaction, the faster the RoR- is reproducible?

Answer 46

Because both the disappearing cross experiment and the measuring volume of a gas experiment results are the same which follow the finding.

Question 47

How does increased surface area to volume ratio result in increased RoR?

Answer 47

- Smaller sized blocks of solid reactant -> greater SA to VOL ratio than larger blocks. - More particles on surface of smaller blocks -> more collisions per second -> increased RoR.

Question 48

What is the activation energy?

Answer 48

Minimum amount of energy the particles must have to react (collide successfully).

Question 49

What is a benefit of using catalysts?

Answer 49

We can increase RoR without needing to increase temp, and they are reusable as they’re not used up in the reaction -> saves money.

Question 51

Do we include catalysts in chemical equations for a reaction?

Answer 51

No. They’re not used up and aren’t reactants.

Question 52

Do different reactions need different catalysts?

Answer 52

Yes.