reaction kinetics + rate equations

Question 1

what 3 things must occurs for a chemical reaction to happen?

Answer 1

1.Reactions can only occur when collisions take place
between particles having sufficient energy
2.The energy is
usually needed to break the relevant bonds in one or either of the reactant molecules. This minimum energy is called the activation energy
3. must collide with the correct orientation

Question 2

what are the 2 types of collision?

Answer 2

1. ineffective collision - when particles collide in the wrong orientation or when they don’t have enough energy bounced off each other without causing a chemical reaction. they give no reaction
   2.successful (effective) collision - occurs when particles collide with sufficient energy to result in a reaction

Question 3

what are collisions and reaction rate?

Answer 3

• the collision frequency is the number of collisions per unit time
• when more collisions per unit take place, the number of particles with energy greater than the Ea increases
• this leads to more successful collisions
• this causes an increase in the rate of reaction

Question 4

what is activation energy? (Ea)

Answer 4

• for a reaction to take place , the reactant particles need to overcome a minimum amount of energy (Ea)
  -in exothermic reactions the reactants are higher in the energy than the products
  -in endothermic reactions the reactants are lower in energy than the products
• therefore, the Ea in endothermic reactions is relatively larger than in exothermic reaction

Question 5

what happens to the atoms of a reactant in a chemical reaction?

Answer 5

-In a chemical reaction, the atoms of the reactants are separated and rearranged to form the products. there is always an energy changed in a chemical reaction
- the rate of reaction is how quickly reactants are broken and rearranged into products

Question 6

what happens to the enthalpy of an exothermic reaction?

Answer 6

• the enthalpy of the reacting system decreases
• enthalpy change, ∆H is negative
  -energy is transferred from the reacting system to the surroundings

Question 7

what happens to the enthalpy of an endothermic reaction?

Answer 7

• the enthalpy of the reacting system increases
• enthalpy change, ∆H is positive
• energy is transferred to the reacting system from the surroundings

Question 8

what does the enthalpy profile diagram for exothermic and endothermic look like?

Answer 8

Question 9

what is the collision theory?

Answer 9

-For a reaction to happen, the reacting particles must collide with sufficient energy called the activation energy with the correct orientation.
- different factors effect the rate of reaction by affecting the frequency of particle collisions, and/or the proportion of collisions that have enough energy to react

Question 10

how does increasing the temperature increase the rate of reaction?

Answer 10

• increasing the temperature increases kinetic energy of particles
• this increases the frequency of collisions and a greater proportion of collisions will have the energy greater than the activation energy required to react.
• this gives more successful collisions between particles
  -As the temperature increases, the graph shows
  that a significantly bigger proportion of particles
  have energy greater than the activation energy, so the frequency of successful collisions increases.

Question 10

what is the maxwell-Boltzmann distribution?

Answer 10

-The Maxwell-Boltzmann energy distribution shows the spread of energies that molecules of a gas or liquid have at a particular temperature
- in a sample of gas, a few particles will have very low energy, a few particles will have very high energy, but most particles will have energy in between
- the graph shows that only a small proportion of molecules in the sample have enough energy for an effective collision and for a chemical reaction the take place

Question 10

How can a reaction go to completion if few particles have energy greater than EA?

Answer 10

Particles can gain energy through collisions

Question 11

how does increasing the concentration of a solution/pressure increase the rate of reaction?

Answer 11

• increasing the concentration/pressure increases more reactant particles and collisions per unit volume (forces the particles to be closer together)
• leads to more frequent collision between the reactant particles and will be more successful which increases the rate of reaction
  -Note: If a question mentions a doubling of concentration/rate
  then make sure you mention double the number of particles
  per unit volume and double the frequency of effective collisions.
  -If concentration increases, the shape of the energy distribution
  curves do not change (i.e. the peak is at the same energy) so
  the Emp and mean energy do not change.
  -The curves will be higher, and the area under the curves will
  be greater because there are more particles.
  -More molecules have energy > EA (although not a greater proportion)

Question 11

how does increasing the surface area increase the rate of reaction?

Answer 11

• increasing the surface area of solid reactants increases the exposed surface of the reactants.
• this increases the number of successful collisions which increases the rate of reaction

Question 12

how does a catalyst increases the rate of reaction?

Answer 12

-Catalysts increase reaction rates without getting used up without being changed in chemical
composition or amount.
-They do this by providing an alternative route or mechanism with a lower activation energy
- If the activation energy is lower, more particles will have
energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster.
- Ecat = catalysed energy

Question 13

how does temperature changes the maxwell-Boltzmann distribution diagram?

Answer 13

-As the temperature increases the distribution shifts towards having more molecules with higher energies
-At higher temps both the Emp (most probable energy- the highest peak of the curve) and mean energy shift to higher energy values, although the number of molecules with those energies decrease.
-The total area under the curve should remain constant because the total number of particles is constant
-At higher temperatures the molecules have a wider range of energies than at lower temperatures.

Question 14

what does the area under the distribution curve represent?

Answer 14

total number of molecules present

Question 15

what is the definition of the rate of reaction?

Answer 15

-the change in concentration of the reactants/products per unit time.
-The usual unit is mol dm-3s-1

Question 16

how to measure a rate from graph?

Answer 16

-When a graph of concentration of reactant is plotted vs
time, the gradient of the curve is the rate of reaction.
-The initial rate is the rate at the start of the reaction where it is fastest and all concentrations are known
-Reaction rates can be calculated from graphs of
concentration of reactants or products, by drawing a
tangent to the curve (at different times) and calculating
the gradient of the tangent.

Question 17

how to compare rate curves?

Answer 17

Question 18

what is the aim of using a continuous monitoring method on Mg and HCL?

Answer 18

the aim of this experiment is to use a continuous monitoring method to investigate how the rate (speed) of reaction of magnesium with hydrochloric acid changes over time

Question 19

what is continuous monitoring?

Answer 19

taking measurements of rates at specific intervals

Question 20

describe and explain the rate of reaction graph

Answer 20

The gradient represents the rate of reaction. The reaction is fastest at the start where the gradient is steepest. The rate drops as the reactants start to get used up and their
concentration drops. The graph will eventual become
horizontal and the gradient becomes zero which
represents the reaction having stopped.

Question 21

what can the Mg and HCL experiment be used to investigate?

Answer 21

This reaction can be used to investigate the effect of varying the concentration of the acid while keeping the temperature constant.

Question 22

what can you measure when the gas is released in the reaction in the Mg and HCL practical?

Answer 22

-either try to measure the volume of the concentration of the acid given off or the mass change in the reaction flask

Question 23

why can you not measure the mass change in the change of volume of gas experiment?

Answer 23

the gas hydrogen is too low in density so the mass change will be far too small to register on a laboratory balance

Question 24

how can you measure the volume?

Answer 24

-displacement of water into an inverted measuring cylinder
-or by using a gas syringe

Question 25

what is rate measured by?

Answer 25

:1000/time
- in many experiments, a gas is produced and we often measure the rate is gs^-1 or cm^3 s^-1

Question 26

how can insoluble gases be collected?

Answer 26

for gases that are not very soluble in water, such as oxygen and hydrogen, the gas may be collected under water in an inverted measuring cylinder or burette

Question 27

how can gas be collected if its soluble?

Answer 27

if a gas is soluble in water like carbon dioxide then a gas syringe may be attached to a sealed reaction vessel to measure the volume of gas produced

Question 28

how could you check that your apparatus is gas tight before the procedure?

Answer 28

• this can be done by assembling everything without the acid or magnesium and trying to move the plunger
• if you feel resistance the apparatus is gas tight

Question 29

why would the acid go into the flask first before the magnesium?

Answer 29

because it is quicker to drop a piece of magnesium ribbon in that to pour in the acid

Question 30

how would you vary the concentration of the acid?

Answer 30

• dilute the acid by measuring portions of acid in a measuring cylinder and then portions of distilled water in another measuring cylinder and adding them to the conical flask

Question 31

why do you need to choose a suitable volume of acid to match the size of the flask?

Answer 31

-if the volume of acid is too small, it may not react effectively with other substances in the flask, leading to incomplete reactions.
-if the volume is too large, it can cause overflow during reactions, especially if gas is produced or if the solution foams.

Question 32

why does the plunger need to be fully inserted before you start the experiment?

Answer 32

you may have a volume error

Question 33

why do you need to make sure the MG look new and shiny?

Answer 33

-because the surface condition of the magnesium affects its reactivity. When magnesium is oxidized, it forms a layer of magnesium oxide, which can prevent further reactions with acids.
-By using clean, shiny magnesium, you ensure that the maximum surface area is available for the reaction, allowing it to proceed efficiently and giving a more accurate measurement of the gas released during the reaction
- you do this by cleaning the surface with sandpaper

Question 34

why must the plunger be secure and does not fall out of the barrel if the volume exceeds 100cm^3?

Answer 34

If the plunger is not secured, an increase in volume could cause the plunger to be ejected forcefully due to pressure build-up.
-This could lead to spills, accidents, or injuries.
-if the plunger falls out, it would disrupt the measurements resulting in inaccurate readings of the volume of liquid or gas being measured.

Question 35

how could you reduce the systematic error of gas escaping due to the time lag between adding the Mg and replacing the bung?

Answer 35

• this could result in a decrease in measured volume of gas
  -to reduce this error, the Mg could be suspended by a string above the acid and the stopper loosened just enough to release the thread, dropping the Mg into the acid
  -Mg could be placed in a small tube in the conical flask, acid added, bung replaced and then the flask swirled to mix the reactants

Question 36

what do you need to do beforehand with a gas syringe?

Answer 36

-a typical gas syringe only measures 100ml of gas so you don’t want a reaction to produce more than this volume.
-before carrying out an experiment you need to check by moles calculation that the mass of solid used produces less than 100cm^3 of gas

Question 37

what is the typical method for the measurement of the change in volume of a gas

Answer 37

• Measure 50 cm3 of the 1.0 mol dm–3 hydrochloric acid and add to conical flask.
• Set up the gas syringe in the stand
• Weigh 0.20 g of Mg.
• Add the Mg ribbon to the conical flask, place the
  bung firmly into the top of the flask and start the timer.
• Record the volume of hydrogen gas collected every 15 seconds for 3 minutes.

Question 38

what happens if theres a large excess of reactants?

Answer 38

In reactions where there are several reactants, if the
concentration of one of the reactant is kept in a large
excess then that reactant will appear not to affect rate and will be pseudo-zero order . This is because its concentration stays virtually constant and does not affect rate

Question 39

what is the initial rate of the reaction?

Answer 39

The initial rate is the rate at the start of the
reaction, where it is fastest. It can be calculated
from the gradient of a continuous monitoring
conc vs time graph at time = zero. A measure of
initial rate is preferable as we know the concentrations at the start of the reaction.

Question 40

how can you calculate rate with the measurement of change of mass?

Answer 40

-this works if there is a gas produced which is allowed to escape.
-works better with heavy gases such as CO2

Question 41

how can you calculate rate with titrating samples of reaction mixture with acid, alkali, sodium thiosulphate etc?

Answer 41

-small samples are removed from the reaction mixture, quenched (which stops the reaction) and then titrated with a suitable reagent

Question 42

how can you calculate rate from colorimetry?

Answer 42

-if one of the reactants or products is coloured then colorimetry can be used to measure the change in colour of the reacting mixtures

Question 43

how can you calculate rate when measuring change in electrical conductivity?

Answer 43

can be used if there’s a change in the number of ions in the reaction mixture

Question 44

how can rate be calculated by measuring optical activity?

Answer 44

if there’s a change in the optical activity through the reaction this could be followed in a polarimeter

Question 45

what happens when the temperature is increased using the maxwell-Boltzmann distribution?

Answer 45

-at higher temperatures, the particles are moving faster, so collide more frequently. A higher number of collisions in total mean a higher number of successful collisions
-at higher temperatures, a higher proportion of the molecules have the activation energy or more. This means that a higher proportion of collisions are successful

Question 46

what happens with the concentration and rate of reaction using the MB distribution?

Answer 46

-the more concentrated a solution is, the greater the number of particles in a given volume of solvent
-an increase in concentration causes an increased collision frequency and therefore increases rate of reaction

Question 47

what happens with pressure and rate of reaction using the MB distribution?

Answer 47

-an increase in pressure in reactions that involve gases has the same effect as an increase in the concentration of solutions
-when the pressure is increased, the molecules have less space in which they can move
-this means that the number of effective collisions increase due to an increased collision frequency
-an increase in pressure therefore increases the rate of reaction

Question 48

what are the two types of catalysts called?

Answer 48

1. homogeneous catalyst
2. heterogeneous catalyst

Question 49

what is homogeneous catalyst?

Answer 49

-catalyst is in the same physical phase/state as the reactants
e.g. the reactants and the catalysts are all in solution
-enzyme reactions within organisms (aq)

Question 50

what is heterogeneous catalyst?

Answer 50

-are in different physical state from the reactants
-e.g. the reactants are gases but the catalyst used is a solid
-most industrial process. (I) or (g) reacting upon surface of (s)

Question 51

why would you use a catalyst?

Answer 51

• allow reactions to take place at lower temperatures decreasing energy costs
  -enable different reactions to be used with better atom economy reducing waste
  -are often enzymes generating specific products
  -can reduce pollution (e.g. use of catalytic converters)
  -catalysts provides a different reaction pathway with a lower activation energy. catalysts do not change ΔH for a reaction

Question 52

explain why a small increase in temperature leads to a large increase in the rate of reaction

Answer 52

many more molecules have an energy greater than/equal to Ea at a higher temperature which causes a large increase in frequency of successful collisions and so the rate of reaction increases.

Question 53

what is the equation for the disappearing cross experiment for sodium thiosulfate and HCL?

Answer 53

Question 54

what is the key product to make the disappearing cross experiment work?

Answer 54

sulfur as its a solid which causes the solution to become opaque

Question 55

what are the two factors which can be investigated using the disappearing cross reaction?

Answer 55

1. changing the temperature
   2.changing the concentration of the HCL

Question 56

what is the measure of time taken used for in the disappearing cross experiment?

Answer 56

the time is taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur. This is an approximation for the rate of reaction as it does not include concentration. We can use this because we can assume the amount of sulfur produced is fixed and constant

Question 57

how can you plot a disappearing cross experiment graph?

Answer 57

• plot graphs of concentration against time if the concentration of HCL was changed or plot 1/t against temperature if temperature was changed
• you plot 1/t as the rate is proportional to 1/t and the graph you want is rate VS temperature

Question 58

what would you do during the disappearing cross experiment when the temperature is changing?

Answer 58

• the temperature of the reaction mixture will change as the reaction proceeds
  -calculate the average temperature for the rate of reaction using start and end temperature
  -OR place the flask in a thermostatically controlled water bath as the desired temperature

Question 59

how would you handle with sulfur dioxide during the disappearing cross experiment?

Answer 59

-sulfur dioxide is a toxic gas that aggravates asthmatics.
-use fume cupboard
-as soon as the reaction is complete, pour the solutions away into the fume cupboard sink
-wash away with plenty of water
-very important with solutions used at higher temperatures

Question 60

why are light sensors sometimes used in a disappearing cross experiment?

Answer 60

a light sensor is placed under the flask and linked to a computer instead of the the experimenter’s eye to remove errors determining end point

Question 61

what are the problems with measurements?

Answer 61

-no physical quantity can be measured with perfect certainty
-there are always errors in any measurement
-if we measure some quantity and repeat the measurement we will almost certainly measure a different value the second time

Question 62

define experimental error

Answer 62

the difference between a measurement and the value or between two measured values

Question 63

define uncertainty

Answer 63

an estimate to a measurement which characterises the range of values within the true value lies. we use ± to show uncertainty

Question 64

how do you use a burette?

Answer 64

1.record all figures that are known for certain
2.read to half a division
3.for a burette, record values to two decimal places ending in a 5 or 0
4.aim for concordant results (within 1cm^3)
5.remove anomalous results
6.taking averages doesn’t make the results more precise
-with a burette we measure a change in volume
-this means there will be an error in our first and second reading

Question 65

what does accuracy measure?

Answer 65

-how close a measured value is to the true value or accepted value

Question 66

what does precision measure?

Answer 66

-how closely two or more measurements agree with other
-precision is sometimes referred to as repeatability or reproducibility

Question 67

what is the equation for percentage uncertainty?

Answer 67

percentage uncertainty= uncertainty/reading x 100

Question 68

what is the equation to experimental error?

Answer 68

experimental error= 100 x (real answer-experiment answer) divide real answer

Question 69

how to determine whether the experimental error calculation is accurate?

Answer 69

• if experimental error is smaller than the total apparatus error, then you have an accurate result

Question 70

why are catalyst often used in powder form or coated on a mesh?

Answer 70

to increase surface area