C8 - Rates and Equilibrium

Question 1

What are 2 general ways to calculate the rate of a reaction?

Answer 1

-mass of reactant used up in a period of time
-mass of product formed in a period of time

Question 2

How can mass loss be used as a method to investigate rate of reaction?

Answer 2

-placing reaction container on mass balance and measuring mass decrease due to gas produced
-cover flask container with cotton wool to stop liquid spitting out (still allows gas to escape)

Question 3

Name 5 factors that affect the rate of a chemical reaction:

Answer 3

-conc. of reactants in solution
-pressure of reacting gases
-SA of solid reactants
-temperature
-presence of catalysts

Question 4

How does increasing SA affect rate of reaction?

Answer 4

-higher number of particles available to collide with
-more frequent collisions
-more frequent successful collisions, increasing RoR

Question 5

How does increasing the concentration of reactants in solution affect rate of reaction?

Answer 5

-particles are closer together per unit same volume
-available to collide more
-more frequent collisions
-more frequent successful collisions, increasing RoR

Question 6

How does increasing the pressure of reacting gases affect rate of reaction?

Answer 6

-particles are closer together per unit volume
-available to collide more
-more frequent collisions
-more frequent successful collisions, increasing RoR

Question 7

How does increasing the temperature affect rate of reaction?

Answer 7

-higher average particle speeds due to increased thermal energy
-more particles with energy above activation energy
-more frequent collisions
-more frequent successful collisions, increasing RoR

Question 8

How does introducing a catalyst affect rate of reaction?

Answer 8

-lowers activation energy
-each particle needs less energy for a successful collision
-more frequent collisions
-more frequent successful collisions, increasing RoR

Question 9

What is collision theory, and what does it explain?

Answer 9

-a theory that says chemical reactions can only occur when reacting particles collide with each other and with sufficient energy (activation energy)

-explains how various factors affect RoR

This can be applied to C7 (energy changes) too

Question 10

What is activation energy?

Answer 10

the minimum amount of energy needed for a successful collision

Question 11

What is a catalyst, and how do they work?

Answer 11

-a substance that increases the rate of chemical reaction but isn’t used up in the reaction itself

-they do so by providing an alternate pathway for the reaction to occur that has a lower activation energy

Question 12

What is a reversible reaction?

Answer 12

a type of chemical reaction where, under certain conditions, the products formed can react again to reform the original reactants

Question 13

Give the word equation for the ammonium chloride reversible reaction:

Answer 13

Question 14

Describe the energy changes in a reversible reaction:

Answer 14

if exothermic in one direction it will be endothermic in the other, and the same amount of energy is transferred either way

If it didn’t do that, it would mean that energy would is created/destroyed

Question 15

Give the word equation for the copper sulfate reversible reaction:

Answer 15

Question 16

What is dynamic equilibrium and when does it occur?

Answer 16

-when the forwards and reverse reactions occur at exactly the same rate, so the masses of the products and reactants remain constant (but not necessarily equal)

-only happens during a reversible reaction in a container where no matter enters/leaves (a closed system)

Question 17

What is Le Chatelier’s principle?

Answer 17

whenever a change in conditions (eg conc., pressure, temp.) is introduced to a system at equilibrium, the position of equilibrium shifts to counteract the change

Question 18

What does it mean for the equilibrium to lie to one side?

Answer 18

eg: if eq. lies to left, there are more reactants than products

This could mean a higher mass, concentration, or volume depending on what the reactants are (like solids, solutions, etc)

Question 19

What is the effect on the position of equilibrium if you change the concentration of the reactants?

Answer 19

-increasing reactants is counteracted upon by more forming products until eq reached (lies to the right)

-decreasing reactants is counteracted by products reforming the reactants (eq. lies to the left)

Question 20

More thiocyanate ions are added to a solution at equilibrium containing the ions below - explain what happens after:

SCN⁻ is thiocyanate

Answer 20

-position of equilibrium shifts to the right to increase the concentration of the product
-this counteracts the initial increase in SCN⁻ ions, as more of them react to form the product, which then decreases the concentration of SCN⁻ ions

Question 21

Where does the equilibrium lie if you change the concentration of the products?

Answer 21

-increasing products gets counteracted by more reforming the reactants until eq reached (lies to left)

-decreasing products is counteracted by more reactants forming the products until eq reached (lies to right)

Question 22

What does the energy symbol mean next to a chemical equation?

Answer 22

-overall energy change of the substances used in the forward reaction

-if negative, forward reaction is exothermic (products lose energy)
-if positive, forward reaction is endothermic (products gain energy)

Question 23

What happens if the temperature is changed in a reversible reaction?

Answer 23

-if temperature increased, reaction takes in more energy to counteract the change and the eq moves in the direction of the endothermic reaction

-if temperature decreased, reaction releases more energy to counteract the change and the eq moves in the direction of the exothermic reaction

Question 24

What happens if the pressure is changed in a reversible reaction?

Answer 24

ONLY WORKS WITH GASES

-if pressure increased, the eq will move to the side where there are fewer moles of gas, as less particles means lower pressure

-if pressure decreased, the eq will move to the side with more moles of gas, as more particles will increase the pressure

Question 25

How will adding a catalyst affect the eq. in a reversible reaction?

Answer 25

-no effect

(however it will bring the reactants to the eq. faster)

Question 26

If both sides of the equation have the same moles of gas, how will the eq. shift if pressure is changed (in reacting gases)?

Answer 26

-no change
-both sides have same moles, so shifting eq. has no effect

Question 27

RP5a - how would you investigate the RoR as concentration changes based on the volume of gas produced from a reaction?

Answer 27

-add HCl of a specified concentration into a conical flask
-set up apparatus as shown in diagram
-add an Mg strip to flask and quickly close bung while starting a stopwatch
-record volume of gas every 10 seconds by reading the graduations on the measuring cylinder and stop when no more gas is released
-repeat with at least 5 concentrations and plot graph of volume of gas released vs time with a new curve for each concentration

Question 28

RP5a - Name 2 pieces of apparatus that are necessary in order to find the rate production of hydrogen gas:

Answer 28

-gas syringe
-stopwatch

Question 29

RP5a - describe the disadvantages of collecting gas over water and using a gas syringe to calculate the RoR:

Answer 29

(same disadvantages for gas syringe method)

-gas could escape before bung is inserted
-some gases can dissolve in the water (CO₂, O₂, Cl₂)

Question 30

RP5b - how would you calculate the RoR as concentration changes when there is a colour change?

Answer 30

-a volume of sodium thiosulfate into conical flask, and dilute (with water) with 50-the volume of sodium thiosulfate used
-put the flask on a paper with a black cross on it and add HCl and swirl the flask
-start the stopwatch and look down from the top until you can’t see the cross anymore and record the time
-repeat but use less sodium thiosulfate and more water at least 5 times to adjust its concentration
-plot time taken vs volume of sodium thiosulfate used

Question 31

Give 2 ways you could measure the volume of gas produced from a reaction:

Answer 31

-upside down measuring cylinder with delivery tube
-gas syringe

Question 32

What does [x] mean?

eg [HCl]

Answer 32

the concentration of substance x

in moles per dm³

Question 33

When calculating rate of reaction, what should you ensure that your number at the end is?

Answer 33

-positive and a decimal (not fraction)

-rate of reaction is always positive, as a reaction is going from reactants to products

Question 34

What should you say when a question asks about equilibrium positions?

Answer 34

-say which direction the equilibrium shifts in
-don’t just be vague and say “in the endothermic direction”, say “left/right”

Question 35

A reaction between calcium carbonate chips and hydrochloric acid occurs, and the rate decreases over time. Explain why:

Answer 35

-acid particles used up so concentration decreases (and SA of calcium carbonate chips decreases)
-less frequent collisions
-less frequent successful collisions
-RoR decreases