Unit 3: Rates of reactions Flashcards
Define exothermic reactions
Exothermic:
During an exothermic reaction, more energy is energy is given out when new bonds are made than the energy taken in to break bonds. This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative.
Describe bond breaking as an __________
process and bond forming as an _________
process
endothermic, exothermic
- Draw and label energy level diagrams for
exothermic and endothermic reactions using
data provided - Interpret energy level diagrams showing
exothermic and endothermic reactions and the
activation energy of a reaction
answer is on pg 2 of this: https://drive.google.com/drive/u/0/folders/1_ooDvqn-mh_ya45sjhJwQhuq1rldeTD1
note that the horizontal distance from the y axis to the vertex of the parabola (i.e. x coordinate of parabola) is called the “transition state”
Describe the effect of concentration, particle
size, catalysts and temperature on the rate of
reactions (collision theory)
catalysts: It provides an alternative reaction pathway of lower activation energy. Increasing rate of reaction.
Smaller particle size = greater surface area to volume ratio
Describe how concentration, temperature
and surface area create a danger of explosive
combustion with fine powders (e.g. flour mills)
and gases (e.g. methane in mines)
Suggest suitable apparatus, given information,
for experiments, including collection of gases and
measurement of rates of reaction
Describe the need for nitrogen-, phosphorus- and
potassium-containing fertilisers
- Nitrogen makes chlorophyll and protein and promotes healthy leaves
- Potassium promotes growth and healthy fruit and flowers
- Phosphorus promotes healthy roots
Describe the displacement of ammonia from its
salts
Imagine a lump of soil in front of you. Now, this soil has certain fertilisers in in that actually make it a little bit acidic. This is of course NOT good for the crops that are growing in it. So what do we do? Well, this farmer comes in, and he chucks in some lime (CaO) into the lump. This is all good because it neutralises this acidity. However, by mistake, he adds in a little more, and more, and more. This is NOT good. Why? Well, the CaO basically reacts with water in the soil. This is normal, but when there is TOO much CaO reacting, we will get a lot of SLAKED LIME. Slaked lime is Ca(OH)2 / calcium hydroxide and it is formed by the reaction of CaO and H2O. Alrighty. Now, the bad thing about this is that calcium hydroxide is a strong alkaline. And what happens is that it takes part in this ‘displacement reaction’, where by it displaces ammonia from its ammonium salts. Well what does that mean? Allow me to explain this using the word equation:
Ammonium chloride + calcium hydroxide –> ammonia + calcium chloride + water
What happened is that calcium hydroxide displaced ammonia from its ammonium salt. The ammonia produced escapes as a gas. Why is this bad? Well the plants needed the ammonium chloride! Now its gone! Not good for their growth.
Describe and explain the essential conditions
for the manufacture of ammonia by the Haber
process including the sources of the hydrogen
(reaction of methane/natural gas with steam)
and nitrogen (from the air)
Ammonia is an important industrial product used to make fertilisers, explosives and dyes. It is an exothermic reaction. The haber process involves a reversible reaction between nitrogen and hydrogen:
N2(g) + 3H2(g) ⇌ 2NH3(g)
The reaction can reach a dynamic equilibrium.
Steps of the haber process:
- nitrogen (extracted from the air) and hydrogen (hydrogen is obtained by reacting steam with natural gas, mostly methane, in the presence of a catalyst to speed up the operation.) are pumped through pipes
- the pressure of the mixture of gases is increased to 200 atmospheres
- the pressurised gases are heated to 450°C and passed through a tank containing an iron catalyst
- the reaction mixture is cooled so that ammonia liquefies and can be removed
- unreacted nitrogen and hydrogen are recycled
Name the use of sulfur in the manufacture of
sulfuric acid
Sulfuric acid is produced from sulfur, oxygen and water via the contact process.
Describe the manufacture of sulfuric acid by the
Contact process, including essential conditions
and reactions
S + O2 → SO2
2SO2 + O2 ⇌ 2SO3
H2O(l) + SO3(g) → H2SO4(aq)
H2SO4 + SO3 → H2S2O7
H2S2O7 + H2O → 2H2SO4
The contact process, for making sulfuric acid, is a process which involves a reversible reaction.
The raw materials needed to make sulfuric acid are:
- sulfur
- air
- water
Stage one – making sulfur dioxide
In the first stage of the contact process, sulfur is burned in air to make sulfur dioxide:
sulfur + oxygen → sulfur dioxide
S(l) + O2(g) → SO2(g)
This is not a reversible reaction – (l) means liquid and (g) means gas.
Sulfur dioxide should not be released into the atmosphere as it contributes to acid rain.
Stage two – making sulfur trioxide
In the second stage, sulfur dioxide reacts with more oxygen to make sulfur trioxide:
sulfur dioxide + oxygen ⇌ sulfur trioxide
2SO2(g) + O2(g) ⇌ 2SO3(g)
This reaction is reversible. The conditions needed for it are:
- a catalyst of vanadium(V) oxide, V2O5
- a temperature of around 450°C (chosen as a compromise temperature, giving a decent yield with a good rate of reaction)
- a pressure of approximately 2 atmospheres (the increased pressure favours the formation of SO3, but if the pressure is too high the risk of explosion is too great, given that SO3 is a highly acidic gas)
Stage three – making sulfuric acid
In the final stage, sulfur trioxide reacts with water to make sulfuric acid:
H2O(l) + SO3(g) → H2SO4(aq)
This is not a reversible reaction, just like the first stage – (aq) means aqueous, or dissolved in water.
The direct addition of sulfur trioxide to water is highly exothermic and leads to the formation of clouds of sulfuric acid. This is highly dangerous. In industry, sulfur trioxide is added to highly concentrated sulfuric acid to form a highly corrosive liquid called oleum to minimise the risk:
SO3 + H2SO4 → H2S2O7
Water is then added to the oleum to form sulfuric acid again:
H2S2O7 + H2O → 2H2SO4
Online notes for this unit:
https://drive.google.com/drive/u/0/folders/1_ooDvqn-mh_ya45sjhJwQhuq1rldeTD1
What is enthalpy?
It is essentially the amount of energy released or absorbed during a reaction at a constant pressure, on a energy profile of a reaction it is the vertical height/difference between the energy of reactants and products
The reaction between methane and oxygen is ___________
The reaction between nitrogen and oxygen is ____________
exothermic (the burning of methane first involves the breaking bonds BUT this is then followed by the formation of the new bonds of the products)
endothermic (it is one of the reactions that takes place when fuel is burnt in car engines. The product is nitrogen monoxide (gas).
Give examples of endothermic reactions
- Thermal decomposition
- Decomposition of calcium oxide when heated,
produces CO2 and Calcium Oxide - Electrolysis
- First stages of photosynthesis
- Citric acid + Sodium hydrocarbonate
Give examples of exothermic reactions
- combustion reactions
- Many Oxidisation reactions (the pairing of oxygen or hydrogen with something)
- Most neutralization reactions (between acids and alkali)
Define endothermic reactions
Endothermic: During an endothermic reaction, more energy is taken in to break bonds than the energy given out to make bonds
This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive.
Note: In endothermic reactions, the reactants have higher bond energy (stronger bonds) than the products. Strong bonds have lower potential energy than weak bonds.
Explain the words system and surroundings in chemistry
System: Chemicals involved in the reaction
Surroundings: Everything else/chemicals surrounding the reaction but not involved (e.g. air/water)
Examples:
In a
What would be system and the surroundings in each case:
- A Bunsen burner.
- A neutralisation reaction in solution.
- A bunsen burner would be the surroundings (provides energy for an endothermic reaction)
- reactants in neutralization reaction = system
In order for a reaction to take place:
- Particles must collide
- With enough energy
- With the correct orientation
What is activation energy?
The minimum amount of energy required to for collisions to be successful
The greater the ________ of _______ collisions the greater the _____ of reaction
OR
The more _____ per unit ____ the greater the ____ of reaction
frequency, successful, rate
OR
collisions, time, rate
What is a common mistake when measuring average rate of reaction and how can you stop it?
How do you measure rate at a specific point?
Define a catalyst
substances which speed up the rate of a reaction (by lowering activation energy) without themselves being altered or consumed in the reaction.
When you are using the investigation method where you view how long it takes for a cross at the bottom of a flask to disappear as the mixture gets cloudy, how do you calculate the rate of reaction?
1/time(s) it took for cross to dissapear
When you are investigating the effect of some factor (e.g. temperature) on the rate of reaction, what goes on the x axis and what goes on the y axis
y axis: rate of reaction
x axis: factor
What are the steps to writing an investigation method?
State:
Independent variable:
Dependent variable:
Control variable:
Range: (5)
Apparatus:
Diagram (if you wish)
Pencil
ruler
Method:
Safety precautions:
How you will process your results:
How you will use your results to draw a conclusion
What is a reversible reaction?
A chemical reaction which can go both ways
What is equilibrium?
In chemical reactions, a situation where the forward and backwards reaction happen at the same rate, and the concentration of the substances stays the same.
Why is iron used in the haber process?
Iron is a catalyst for the reaction. It increases the rate of the reaction without being used up in the reaction.
Explain why unreacted hydrogen and nitrogen are recycled in the haber process
Recycling the unused reactants saves money and increases the effective (overall) yield.
Why is ammonia created in the haber process?
It can be used to create nitrogen based fertilisers.
Why is the haber process done at 450*C
Well for one it is an exothermic reaction, this means that it is better for it to occur in a system with a lower temperature to favour the forward reaction and increase the %yield. An exothermic reaction produces heat. If we decrease the temperature the equilibrium will tend to counteract this effect and favor the reaction that produces heat to increase the temperature of the system.
BUT ON THE OTHER HAND
To increase the rate of reaction the temperature should be high.
So to come to a compromise, 450 degrees was chosen. Also generating heat is expensive, using a lot of heat would be too expensive.
Why is the haber process done at 200 atmospheric pressure (atm)?
If the pressure is increased, the equilibrium position moves to the right, so the yield of ammonia increases.
The rate of reaction also increases because the gas molecules are close together, so successful collisions are more frequent.
So what is limiting us from maintaining high pressures? It’s cost and safety(explosions). So 200 atm turns out to be the best pressure.
What are the uses of sulfuric acid?
The majority of sulfuric acid that is produced is used to make fertilisers. This is often achieved by reacting the sulfuric acid with ammonia, or ammonium hydroxide solution, to make ammonium sulfate, (NH4)2SO4.
ammonia + sulfuric acid → ammonium sulfate
ammonium hydroxide + sulfuric acid → ammonium sulfate + water
Sulfuric acid is also used in the production of:
- detergents
- paints
- dyes
- plastics
- artificial fibres
