"7.2.2 I can deduce the extent of a reaction from the magnitude of Kc"

Blog Tasks

1. What does the word magnitude mean?
The size or amount
2. Explain why the three reactions above do not have units for Kc
The moles of the reactants equals the product so there is no units for K
Note: [units Kc]n
3. Deduce the extent of the reaction if Kc is
a. significantly larger than 1 – this means that there are more product produced compared the the reactants input. The (C+D) must be huge compared to (A+B). We can deduce that the reaction has taken place and almost go to completion
b. between 0.01 and 100- this means that there are There will be significant concentration of both the concentration and product. Normal reaction.
c. extremely – this means that there are less product produced compared to the reactants input. The (A+B) must be huge compared to (C+D). We can deduce that the reaction hardly proceeds.

7.2.1 I can deduce the equilibrium constant Kc for homogenous reactions

a. What can change the value of K
TEMPERATURE
b. The reaction must be at EQUILIRBIUM for the value of Kc to be calculated
c. Define the term homogeneous. WHEN THE REACTANTS AND PRODUCTS ARE IN THE SAME STATE. i.e. all gases, or liquid, or solid, or aqueous.
d. What is the rule to determine the units for Kc?

aA+bB = cC+dD

Kc= [C]cx[D]d
[A]ax[B]b

If the number of moles of the reactants equals the number of moles of the product then there is no unit. (c+d) - (a+b) = no units

If the no.moles of product > no.moles of reactant
(c+d) – (a+b) = 1 then Kc=moldm-3

If the no.moles of product < no.moles of reactant
(c+d) – (a+b) = -1 then Kc=mol-1dm3 because its 1/-1

Kinetics (topic6) test - mistakes

Multiple choice number 5-
Ammonia decomposes on a tungsten catalyst according to the equation:
2NH3 (g) --> N2 (g) + 3H2O (g).
If the rate of dissapearance of NH3 is 0.030moldm-3s-1, what is the rate of appearance of H2 (in moldm-3s-1)?
A.0.020
B.0.030
C.0.045
D.0.068

rate of dissapearnace = rate of appearance.
2 ratio of NH3 while 3 ratio of H20.
0.030/2 = 0.015 per ratio
0.015X3 = 0.045 (3 ratio of H2O)

14a) when writing the numbers in the table, the numbers should have the same decimal point. i.e. if its 2 decimal, all of it needs to be 2dp including 0 (0.00).

14c) relate back to the mol per unit time

topic 6 pretest

1) B x D you can also measure rate of reaction by pH
2) A
3) D
4) D x B II is right, but I is wrong- same mass of magnesium (reactant) will have the same amount of product
5) C
6) B
7) C
11a) they could measure the gas of carbon dioxide produce over time and plot on a graph- collect using gas syringe/inverted measuring cylinder and stopwatch. They could also measure the mass of the beaker and the content as carbon dioxide will escape so the weight will decrease. Using a top pan balance and stopwatch.
b) The rate of this reaction could increase by using more concentrated acid=more particles within the same volume so the chance of successful collision increases, increase the surface area of magnesium carbonate (decrease size) reactions occur on the surface so by increasing surface area there will be more reaction going on= speeds up the reaction, it could increase temperature because it will increase the kinetic energy which more of if will exceed the activation energy/ increase collision frequency.
ci) It would stay the same as lumps of magnesium carbonate is already in excess
cii) The volume would stay the same as the temperature only increases the rate of reaction. You haven’t change anything else. Same quantities used.

7.1.1: I can outline the characteristics of chemical and physical systems in a state of equilibrium.

6.2.5 - 6.2.7

6.2.5 Sketch and explain qualitatively the Maxwell-Boltzmann energy distribution curve for a fixed amount of gas at different temperatures and its consequences for changes in reaction rate.

The moving particles in a gas or liquid do not travel with the same velocity; some faster, some slower. The faster they move, the more kinetic energy they have. The distribution is shown by a Maxwell-Boltzmann curve.
This shows how activation energy distinguishes between particles that have greater or lesser values of kinetic energy. So at lower temperature (black) there are fewer particles that exceed the activation energy. When temperature rises, the curve broadens and at the same Ea (red vertical line) there will be more particles reacting as the area is bigger. The average kinetic energy is the peak of each line, which is also the mean. Change in temperature will basically change the position of the average KE.
The area under the curve shows the number of particles so it never changes. So as the temperature increases, the area under the curve does not change.

6.2.6 Describe the effect of a catalyst on a chemical reaction
A catalyst is a substance that increases the rate of reaction without itself undergoing permanent change. It never changes chemically but it could change physically. The catalyst is ‘recycled’. The catalyst brings the reactive parts of the reactant particles into close contact therefore lowers the Ea.
A catalyst provides an alternative route for the reaction which as a lower activation energy (Ea). Note that it is a completely different reaction.
As you can see, the reactants and the product remain the same but the activation energy required without a catalyst is higher.
This means that without increasing the temperature, a larger number of particles will now have kinetic energy over the Ea therefore more successful collisions.

6.2.7 Sketch and explain Maxwell-Boltzmann curves for reactions with and without catalyst.
As said earlier, a catalyst lowers the activation energy which can be shown by a leftward shift on a Maxwell-Boltzmann curve.
You can see that the area when catalyzed is bigger so this means that more particles are able to react.

6.2.4.- Predict and explain, using the collision theory, the qualitative effects of particle size, temperature, concentration and pressure on the rate of a reaction.

Question Answer
What was the independent variable The surface area of the Calcium Carbonate
What was the dependent variable Volume of gas produced
What variables were controlled The volume and concentration of acid used, the mass of Calcium Carbonate used. The same room temperature. Identical equipment used (same size: flask, delivery tube).
Using collision theory explain the following the shape of the graphs at the start of the reaction The smallest chip has the steepest slope suggesting it has the fastest rate of reaction. This is because as surface area increases, rate of reaction increases. - In a solid substance only the particles on the surface can come into contact with a surrounding reactant. The powder is supposed to be the fastest but in our reaction; the powder is blocking the gas from reaching the delivery tube so it doesn’t follow the theory. Our results for small chips are an anomaly as it doesn’t follow the trend. The big chip has the slowest rate of reaction as it takes the longest time until the reaction is over and the gradient is gentle.
What does the gradient of the graph at any one point represent The steeper the gradient, the faster the rate of reaction.
What are the units for the gradient of the graph Volume/unit time
Discuss the reasons for the differences in the shape of the graphs The steeper the gradient the faster the reaction. If we use the same amount of reactants assuming there is one that is in excess, we should get the same amount of volume produced. In this case, it is different so there must be something wrong.

Factors affecting the rate of reaction:
• Temperature- as the temperature increase (temperature is a measure of the average kinetic energy so if temp increases, the energy increases), the particles will move faster so there will be more chances of success collisions per time. However, the main reason why an increase in temperature increases the rate is that more of the colliding particles will possess the necessary activation energy (energies exceed Ea) resulting in more successful collisions. For many reactions, the rate approximately doubles for every 10K temp rises and there is an exponential relationship.
• Surface area (particle size) - In a solid substance only the particles on the surface can come into contact with a surrounding reactant. If the surface area increases/particle size decreases (e.g. in powder form), the rate of reaction should increase. Similarly, if the surface area decreases/ particle size increases (e.g. in big chips), the rate of reaction should decrease.
• Concentration- The more concentrated the reactants are, the more collisions there will be per second per unit volume. As the reactants got used up their concentration decreases. This explains why the rate of most reactions gets slower as the reaction proceeds. If there are particles in a volume then there are more chances of successful collisions.
• Catalyst- Catalysts increase the rate of a chemical reaction without themselves being chemically changed at the end of the reaction. They work essentially by bringing the reactive parts of the reactant particles into close contact with each other, this provides alternative pathway for the reaction with a lower Ea. If Ea is lowered, it means that the energy required for a reaction to take place is lowered.
• Pressure- For reactions involving gases, increasing pressure increases the rate of reaction. This is because the higher pressure compresses the gas, effectively increase its concentration. This will increase the frequency of collisions. The Haber process illustrates the effect of high pressure on reactions that involve gaseous reactants.

• light (EMR)- can promote some reactions e.g. dye fading, photosynthesis, skin tanning, methane/chlorine explosion, skin cancer, photography, vitamin D formation in skin and hydrogen peroxide photodecomposition (it is kept in brown bottles.