rates of reactions physical chemistry

Question 1

what is the rate of a reaction?

Answer 1

the speed at which the amount of reactants decreases or the amount of products increases

Question 2

what does the rate of reaction measured as?

Answer 2

the change in the concentration or amount) of reactants or products per unit of time (per min,per sec, etc..)

Question 3

what is the equation for rate of reaction?

Answer 3

RoR = (change in conc., volume, or mass) / time

Question 4

what is needed for a reaction to take place?

Answer 4

• collisions of particles, however not all collisions end up in a reaction. Many particles just bounce off each other.
• In order to for a reaction to take place, the particles have to collide with a minimum amount of energy, called activation energy.

Question 5

what are successful collisions?

Answer 5

the collisions with energy greater than or equal to the activation energy

Question 6

what happens between calcium carbonate and HCl in a reaction?

Answer 6

the particles in the acid have to collide with the particles at the surface of the marble chips.
As the acid particles are getting used up, the collision rate decreases, and so the reaction slows down.
if the marble is in excess, the shape wont change very much in the reaction

Question 7

what does a conc.-time graph show?

Answer 7

it shows the fall in concentration of on of the reactants in a reaction.
where the graph is falling most quickly (steepest), the reaction is at its fastest.

Question 8

what does collision theory state?

Answer 8

Collision theory states that in order for a reaction to occur:
The particles must collide with each other
The collision must have sufficient energy to cause a reaction i.e. enough energy to break bonds

Question 9

what does it mean of collisions can be successful or unsuccessful

Answer 9

Collisions can be described as successful or unsuccessful
A successful collision means that the reactant particles collide and rearrange to form the products
This happens when the particles have sufficient energy (i.e. energy greater than the activation energy) to react

An unsuccessful collision means that the reactant particles just bounce off each other and remain unchanged
This happens when the particles do not have sufficient energy to break the necessary bonds or do not collide in the correct orientation

Question 10

what does number of successful collisions depend on?

Answer 10

The number of successful collisions depends on:
The number of particles per unit volume - more particles in a given volume will produce more frequent successful collisions
The frequency of collisions - a greater number of collisions per second will give a greater number of successful collisions per second
The kinetic energy of the particles - greater kinetic energy means a greater proportion of collisions will have an energy that exceeds the activation energy and the more frequent the collisions will be as the particles are moving quicker, therefore, more collisions will be successful
The activation energy - fewer collisions will have an energy that exceeds higher activation energy and fewer collisions will be successful

Question 11

what is RoR dependent on?

Answer 11

no. successful collisions

Question 12

what does Surface area do to rate of reactions?

Answer 12

as surface area increases, the RoR increases as there are more particles and higher overall collisions because there are more particles available for collisions. So, collisions occur more frequently, so the frequency of collisions increases as surface area of the solid increases.

Question 13

what is activation energy? why is it needed?

Answer 13

the minimum amount of energy that particles need to react.
this is needed because the particles need energy to break the bonds in the reactants to start the reaction.

the greater the activation energy, the more energy needed to start the reaction.

Question 14

what different forms does activation energy come in?

Answer 14

heating

Question 15

what is rate of reaction dependent on

Answer 15

• temperature
• conc of solution or pressure of gas
• surface area
• the presence of a catalys

Question 16

how does temperature affect rate of reaction?

Answer 16

as temperature increases, RoR increases because the particles have more kinetic energy collide more frequently, and move faster. Each collision will also be more energetic, so more likely to exceed the activation energy. hence, frequency of successful collisions increases and rate of reaction increases

Question 17

how does concentration affect rate of reaction?

Answer 17

as concentration increases, RoR increases as there are more particles to collide and there are more particles in a given volume, so they will collide more frequently. Hence, increasing overall successful collision frequency and overall RoR

Question 18

how does a catalyst affect rate of reaction?

Answer 18

a catalyst increases RoR without being used up. A catalyst provides an alternate reaction pathway with a lower activation energy. this means that the collisions that will occur will exceed the activation energy. This will increase the frequency of successful collisions and increase the rate of reactions.

Question 19

what is a catalyst?

Answer 19

a substance that increases the rate of reaction, but is chemically unchanged at the end of the reaction

Question 20

what does increasing the pressure of a gas do to the rate of reaction?

Answer 20

the particles are forced closer together and so the frequency of collisions increases and so RoR increases.
you can increase pressure by putting the same mass of gas into a smaller volume

Question 21

how can you use precipitation to follow the rate of reaction? what are some limitations to it?

Answer 21

1. This method works for any reaction where mixing two see-through solutions produces a precipitate, which clouds the solution.
2. You mix the two reactant solutions and put the flask on a piece of paper that has a mark on it.
3. Observe the mark through the mixture and measure how long it takes for the mark to be obscured. The faster it disappears, the faster the reaction.
4. The result is subjective - different people might not agree on exactly when the mark ‘disappears’.

Question 22

how can you use change in mass (usually gas given off) to follow the rate of reaction? what are some limitations to it?

Answer 22

1. You can measure the rate of a reaction that produces a gas using a mass balance.
2. As the gas is released, the lost mass is easily measured on the balance.
The quicker the reading on the balance drops, the faster the reaction.
3. You know the reaction has finished when the reading on the balance stops changing.
4. You can use your results to plot a graph of change in mass against time.
5. This method does release the gas produced straight into the room
   — so if the gas is harmful, you must take safety precautions.
   The cotton wool lets gases through
   e.g. do the experiment in a fume cupboard.

Question 23

how can you use the volume of gas given off to follow the rate of reaction? what are some limitations to it?

Answer 23

1. This involves the use of a gas syringe to measure the volume of gas given off.
2. The more gas given off during a set time interval, the faster the reaction.
3. You can tell the reaction has finished when no more gas is produced.
4. You can use your results to plot a graph of gas volume against time elapsed.
5. You need to be careful that you’re using the right size gas syringe for your experiment though — if the reaction is too vigorous, you can blow the plunger out of the end of the syringe.

Question 24

describe an investigation where you measure the rate of reaction between marble chips and dilute HCl

Answer 24

Marble chips are made of calcium carbonate and react in hydrochloric acid to produce carbon dioxide gas. Calcium chloride solution is also formed.
CaCOs(s) + 2HClaq) → CaClaq) + H20(l) + CO2 (g)
Method:
apparatus:cylinder, conical flask, measuring scale, cotton wool, acid, marble chips. (everything end up on the scales)
1. Use a measuring cylinder to measure 25 cm of 2.00 mol/dm dilute hydrochloric acid.
2. Add 5.00 g of large marble chips to a conical flask and place a piece of cotton wool at the opening of the flask. The cotton wool is there to allow the carbon dioxide to escape during the reaction, but to stop any acid spitting out. The marble is in excess - some of it will be left over when the acid is all used up.
3. Place everything on a balance and reset it to zero.
4. Add the acid to the marble chips and record the reading on the balance every 30 seconds.
5. The acid has been poured into the flask and everything has been replaced on the balance.
6. Once the reaction starts, the balance shows a negative mass. The mass goes down because the carbon dioxide escapes through the cotton wool.

Question 25

how can we measure how fast the reaction is going at any point?

Answer 25

We can measure how fast the reaction is going at any time point by finding the slope (gradient) of the line at that point. This is the rate of the reaction at that point (rather than the average). This is done by drawing a tangent to the line at the time you are interested in and finding its slope (gradient). For example, at 5 minutes the carbon dioxide is being lost at the rate of about 0.05 g per minute

Question 26

how can you investigate the effect of the changing area of marble chips on the rate of reaction between the marble chips and dilute hydrochloric acid?

Answer 26

Marble chips are made of calcium carbonate and react in hydrochloric acid to produce carbon dioxide gas. Calcium chloride solution is also formed.
CaCOs(s) + 2HClaq) → CaClaq) + H20(l) + CO2 (g)
Method:
apparatus:cylinder, conical flask, measuring scale, cotton wool, acid, marble chips. (everything end up on the scales)
1. Use a measuring cylinder to measure 25 cm of 2.00 mol/dm dilute hydrochloric acid.
2. Add 5.00 g of large marble chips to a conical flask and place a piece of cotton wool at the opening of the flask. The cotton wool is there to allow the carbon dioxide to escape during the reaction, but to stop any acid spitting out. The marble is in excess - some of it will be left over when the acid is all used up.
3. Place everything on a balance and reset it to zero.
4. Add the acid to the marble chips and record the reading on the balance every 30 seconds.
5. The acid has been poured into the flask and everything has been replaced on the balance.
6. Once the reaction starts, the balance shows a negative mass. The mass goes down because the carbon dioxide escapes through the cotton wool.
7. repeat this experiment using smaller chips (or just different sized chips)
8. Notice that the same mass of carbon dioxide is produced because we are using the same quantities of everything in both experiments. However, the reaction with the smaller chips starts off much faster and finishes sooner.

Question 27

how can you investigate the effect of changing the concentration of the acid on the rate of reaction between marble chips and dilute hydrochloric acid

Answer 27

Marble chips are made of calcium carbonate and react in hydrochloric acid to produce carbon dioxide gas. Calcium chloride solution is also formed.
CaCOs(s) + 2HClaq) → CaClaq) + H20(l) + CO2 (g)
Method:
apparatus:cylinder, conical flask, measuring scale, cotton wool, acid, marble chips. (everything end up on the scales)
1. Use a measuring cylinder to measure 25 cm of 2.00 mol/dm dilute hydrochloric acid.
2. Add 5.00 g of large marble chips to a conical flask and place a piece of cotton wool at the opening of the flask. The cotton wool is there to allow the carbon dioxide to escape during the reaction, but to stop any acid spitting out. The marble is in excess - some of it will be left over when the acid is all used up.
3. Place everything on a balance and reset it to zero.
4. Add the acid to the marble chips and record the reading on the balance every 30 seconds.
5. The acid has been poured into the flask and everything has been replaced on the balance.
6. Once the reaction starts, the balance shows a negative mass. The mass goes down because the carbon dioxide escapes through the cotton wool.
7. repeat the experiment but dilute the acid by adding distilled water to the original 2.00 mol/dm3 solution but making sure the total volume of water and acid remains at 25 cm^2. if you att the same amount of distilled water the conc is halved. if you add 20 cm^3 of distilled water and 5cm^3 of acid then is is 1/5 of the original conc.
8. In this experiment we are going to calculate the average rate during the first 30 seconds, so we record the mass of carbon dioxide lost in 30 seconds.
We can calculate the average rate of reaction within the first 30 seconds by dividing the mass loss by 30
9. To identify the effect of changing concentration on rate, we can plot the results for the different concentrations on a piece of graph paper with rate on the y-axis and concentration of acid on the x-axis. you shoudl get a straight line

Question 28

describe a practical investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide

Answer 28

set up the apparatus like this:
- conical flask with a gas syringe attached branching off to the side
- bung on lid
- manganese powder in hydrogen peroxide solution
method:
1. Measure 100cm of 2 vol hydrogen peroxide and transfer to a 250 cmg conical flask.
2. Weigh out 0.20 g of manganese(IV) oxide on a balance.
3. Add the manganese(IV) oxide to the hydrogen peroxide and quickly replace the bung with the gas syringe already attached. Swirl the reaction mixture at a constant speed.
4. Record the amount of oxygen produced every 20 seconds for 3 minutes and plot a graph of volume of oxygen versus time.
5. Repeat the reaction with 0.20 g of lead (V) oxide and copper(Il) oxide but keep everything else the same.
6. plot this out and you shoudl see that manganeselV oxide is a very effective catalyst for the decomposition of hydrogen peroxide. In comparison, leadIV oxide is less effective as the rate of the reaction is much slower and copper(Il) oxide does not act as a catalyst for this reaction at all.

Question 29

what gas is produced in the practical investigating the effect of different solids on the catalytic decomposition of hydrogen peroxide

Answer 29

oxygen

Question 30

what are catalysts?

Answer 30

substances that speed up chemical reactions but aren’t used up in the process as they remain chemically unchanged

Question 31

how do bombarider beeltes defend themselves?

Answer 31

they spray a lot of hot, unpleasant liquid on attackers. part of the reaction involves splitting hydrogen peroxide into water and oxygen, using the enzyme catalase. This reaction happens almost explosively and produces a lot of heat.

Question 32

what is an enzyme?

Answer 32

a biological catalyst that that speeds up a chemical reaction in a cell without being destroyed or permanently altered

Question 33

what catalyses the decomposition of hydrogen peroxide?

Answer 33

manganese IV oxide aka manganese dioxide, Mn)2, Lead IV oxide, PbO2

Question 34

what is they symbol for hydrogen peroxide?

Answer 34

2H2O2

Question 35

how can you prove something is a catalyst?

Answer 35

by weighing it before and after a practical to show that it remains unchanged. it it is in a liquid at the end you can remove the liquid by filtering it through a weighted filter paper, allowing the paper and residue to dry, and then reweighing to calculate the mass of the remaining catalyst

Question 36

how can you show the effect of a catalyst?

Answer 36

using a profile diagram/reaction profile. it’s like a graph with energy on the y axis and the progress of the reaction on the x axis.
the shape of the line is kind of like Ω in the sense that it begins flat and then the peak is the activation energy without a catalyst and then it drops back down (but below the initial line - this is called enthalpy change) and that line is the products. with a catalyst, you have the same thing but the bump bit is a lot smaller as there is a lower activation energy on the alternative route provided . this is for exothermic reactions

for endothermic reactions the end line is higher up than the initial reactant line. other than that it is very similar

look at p.237 for reference.

Question 37

how does concentration affect reaction rate? (on graph)

Answer 37

• the lines have varying steepness but all end up the same place because higher conc. gives a faster reaction so it will finish sooner so the gradient will be steeper

Question 38

how can you measure how rate is affected by temperature?

Answer 38

1. sodium thiosulfate and HCl are both clear solutions. When they react together, a yellow precipitate of sulphate is formed
2. you can measure the rate by watching a black mark diasappear through the cloudy sulfur and timing how long it takes to go
3. the reaction can be repeated for solutions at different temperatures.
4. in practice it is hard to do accurately and safely ( not safe to heat acid directly)
5. the best way to do it is use a water bath to heat both solutions to the right temp before you mix them
6. the depth of liquid must be the same each time
7. results should show that higher temp = quicker RoR
8. you can also use this to test the effects of concentration

Question 39

how does surface area affect reaction rate? (on graph)

Answer 39

using finer particles = steeper gradient

Question 40

on a reaction profile, what will the highest point show?

Answer 40

(the activation energy ) without a catalyst

Question 41

endothermic vs exothermic reaction?

Answer 41

Endothermic reactions
Absorb energy from the surroundings, causing the temperature to decrease. This happens when more energy is required to break the bonds in the reactants than is released when new bonds form in the products.
Exothermic reactions
Release energy to the surroundings, causing the temperature to increase. This happens when the products have less energy than the reactants.

The difference in energy between the reactants and products is called the enthalpy change, or ∆H. For exothermic reactions, the enthalpy change is always negative.

Question 42

draw out a reaction profile/a profile diagram of catalyst/w/o catalyst

Answer 42

pg 61 in cgp