C8 - Rates and Equilibrium

Question 1

What does the rate of a chemical reaction tell you

Answer 1

the rate of a chemical reaction tells you how fast the reactants turn into products

Question 2

give examples of slow, moderate and fast reactions

Answer 2

Slow - chemical weathering & rusting of iron
Moderate - magnesium reacting with acid to produce a gentle stream of bubbles
Fast - burning and explosions

Question 3

How to plot a rate of reaction graph

Answer 3

Amount of reactant used up/product formed on the Y-axis and time on the X-axis

Question 4

How to find the rate of reaction from a graph

Answer 4

the gradient of the line is equal to the rate of reaction. The steeper the line, the faster the reaction and over time the line becomes less steep as the reactants are used up. The quickest reactions have the steepest lines and become flat in the least time

Question 5

rate of reaction equation

Answer 5

rate of reaction = amount of reactant used or amount of product formed / time

Question 6

units for rate of reaction

Answer 6

cm^3/s (gases) g/s (solids) mol/s (any)

Question 7

What are the three ways of measuring the rate of reaction

Answer 7

Precipitation and Colour Change
Change in Mass (gas given off)
Volume of Gas Given Off

Question 8

how to use precipitation and colour change to measure the rate of reaction

Answer 8

If the initial solution is transparent and the product is a precipitate which makes the solution opaque, observe a mark through the solution and measure how long it takes for the mark to disappear.

If the reactants are coloured and the products are colourless, time how long it takes for the solution to lose or gain colour

Question 9

Limitations of measuring rate of reaction using precipitation and colour change

Answer 9

Results are very subjective

You can’t plot a rate of reaction graph

Question 10

How to measure the rate of reaction by measuring the change in mass

Answer 10

Use a mass balance and as the gas is released, the mass reading drops. The faster the balance reading drops, the faster the reaction. Take measurements at regular intervals and plot a rate of reaction graph. This is the most accurate method as the mass balance is very accurate. Put cotton wool on the top of the flask so the gas can escape but the acid won’t

Question 11

Limitations of measure RoR using change in mass

Answer 11

If the solution is hot, some of the liquid might evaporate instead of the gas. This could make results differ
Also releasing gas straight into the room might be dangerous

Question 12

How to measure the RoR by measuring the volume of gas given off

Answer 12

Use a gas syringe to measure the volume of gas given off, the more gas given off during a given time interval then the faster the reaction. Gas syringes usually give volumes accurate to the nearest cm3 so they’re quite accurate. You can take measurements at regular time intervals and plot a graph

Question 13

Limitations of measuring volume of gas given off

Answer 13

If the reaction is too vigorous, you can blow the plunger out the end of the syringe

Question 14

what is the collision theory

Answer 14

reactions can only take place when the particles (atoms,ions or molecules) of reactants come together. The reacting particles have to collide with each other with enough (sufficient) energy to cause a reaction to take place.

Question 15

What is the activation energy

Answer 15

the minimum amount of energy that particles need to react by breaking bonds with reactants and start reactions

Question 16

What does the rate of a reaction depend on

Answer 16

the collision frequency - more collisions, the faster the reaction

the energy transferred during a collision - particles have to have enough energy for the collision to be successful

Question 17

4 factors that the rate of reaction depends upon:

Answer 17

Temperature
The concentration of solution or pressure of gas
Surface Area
The presence of a catalyst

Question 18

How temperature affects rate of reaction

Answer 18

Increasing temperature increases the rate of reaction. When temperature is increase particles move faster so they will collide more frequently. also the faster they move, the more energy they have so more collisions will have enough energy for the reaction to happen

Question 19

Concentration of solution or Pressure of gas

Answer 19

If a solution is more concentrated, there are more particles colliding in the same volume of solvent.If the pressure of a gas is increased it means there is the same number of particles occupying a smaller space. Both make collisions more frequent which increases the rate of reaction

Question 20

How does surface area affect the rate of reaction

Answer 20

Breaking a reactant up into smaller pieces increases the surface area to volume ratio. The rate of reaction increases as this ratio increases because for the same volume of the solid the particles around it will have more area to work on so there will be collisions more frequently.

Question 21

How using a catalyst affects the rate of reaction

Answer 21

A catalyst speeds up the rate of reaction without being used up in the reaction itself. They work by decreasing the activation energy needed for the reaction to occur by providing an alternative reaction pathway with a lower activation energy

Question 22

Magnesium + hydrochloric acid -> Hydrogen Gas experiment

Answer 22

1. Add a set volume of dilute HCl to a conical flask
2. Add some magnesium ribbon to the acid and quickly attach an empty gas syringe to the flask
3. Start the stopwatch and take readings of the volume of gas at regular intervals and record on a table
4. Plot a graph with time on the X-axis and volume of gas on the Y-axis

You can repeat the experiment with a number of different concentrations of acid to investigate the effect the concentration has. Plot all your results on the same graph

Question 23

Sodium Thiosulfate + HCl

Answer 23

the two chemicals are both clear solutions but react together to form a yellow precipitate of sulphur.

1. Start by adding a set volume of dilute sodium thiosulfate to a conical flask
2. place the flask on a piece of paper with a black cross drawn on it
3. add some dilute HCl to the flask and start to stopwatch
4. Watch the black cross disappeared through the cloudy sulfur and time how long it takes

NB. this reaction releases Sulfur dioxide so should be carried out in a well ventilated place

Question 24

How reversible reactions reach an equilibrium

Answer 24

As the reactants react, their concentrations fall so the forwards reaction will slow down and as more products are made the backwards reaction will speed up. After a while the forwards reaction will go at the exact same rate as the backwards one and the system will be at equilibrium where both reactions are happening but there is no overall effect. Equilibrium is only reached if the reversible reaction takes place in a ‘closed system’

Question 25

3 factors that the position of equilibrium depends on

Answer 25

1. The temperature
2. The pressure (only affects gases)
3. The concentration of reactants and products

Question 26

The thermal decomposition of Hydrated Copper Sulfate

Answer 26

• If you heat blue hydrated copper (II) sulfate crystals, it drives the water off and leaves white anhydrous copper (II) sulfate powder. This is endothermic
• If you then add a couple drops of water to the cooled white powder you get blue crystals back again. This is exothermic

Question 27

what is als Chatelier’s Principle

Answer 27

If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract or oppose the change

Question 28

How temperature affects equilibrium

Answer 28

If you decrease the temperature, the equilibrium will move in the exothermic direction to produce more heat so you will get more products for the exothermic reaction.
If you increase the temperature, the equilibrium will move in the endothermic direction to try to decrease it

Question 29

How pressure affects equilibrium

Answer 29

If you increase the pressure, the equilibrium will move in the direction where there are fewer molecules of gas to try and reduce it. If you decrease the pressure, the equilibrium tries to increase it by moving in the direction where there are more molecules of gas.

Question 30

How concentration affects equilibrium

Answer 30

If you change concentration of reactants or products, the system will no longer be at equilibrium. If you increase the concentration of reactants, the system tries to decrease it by moving to the products side and making more products. If you increase the concentration of products, the system tries to decrease it by moving to the side of the reactants