Chapter 6 - Chemical reactions Flashcards
What are physical changes?
Physical changes do not produce any new chemical substances, they are easy to reverse and relatively easy to separate. A change of state is an example of a physical change.
What are chemical changes?
Chemical changes form new chemical substances that have very different properties to the reactants, most chemical changes are difficult to reverse. A colour change is an example of a chemical change.
What is activation energy?
Activation energy is the minimum energy that colliding particles must have to react.
What are five factors that affect the rate of reaction?
The concentration of solutions, the pressure of gases, the surface area of solids, temperature and the presence of a catalyst.
How does a higher concentration affect the rate of reaction?
Higher concentration means that there are more reactant particles in a given volume which leads to more collisions per second, leading to more frequent and successful collisions per second and thus the rate of reaction increases. The number of collisions is proportional to the number of particles present.
How does a higher pressure affect the rate of reaction?
Higher pressure means that there are the same number of reactant particles in a smaller volume which leads to more collisions per second, leading to more frequent and successful collisions per second and thus the rate of reaction increases.
How does a larger surface area affect the rate of reaction?
Larger surface area means that there is a greater surface area of particles that will be exposed to the reactant which leads to more collisions per second, leading to more frequent and successful collisions per second and thus the rate of reaction increases.
How does a higher temperature affect the rate of reaction?
Higher temperature means that particles have more kinetic energy which leads to more collisions per second, leading to more frequent and successful collisions per second and thus the rate of reaction increases.
How does the presence of a catalyst increase the rate of reaction?
The presence of a catalyst provides a different reaction pathway that has a lower activation energy. This means that a higher proportion of the reactant particles have energy greater than the activation energy and will result in more successful collisions per second.
What is a catalyst?
A catalyst increases the rate of reaction and is unchanged at the end of the reaction. It provides an alternative pathway, lowering the activation energy required.
What properties can change during a chemical reaction?
Properties that change during the course of a reaction could include colour, mass or volume.
How do you measure the changes during chemical reactions?
Some commonly used techniques are measuring the mass loss on a balance, measuring the volume of a gas produced or measuring a reaction where there is a colour change at the end of the reaction.
How do you show that chemical reactions are reversible?
Chemical reactions that are reversible are shown by the symbol ⇌. The forward reaction forms the products and the reverse reactions forming the reactants.
What is the reversible reaction between hydrated salts and anhydrous salts?
Hydrated salts contain water of crystallisation which affects their molecular shape and colour. The salt can be heated or dehydrated to form anhydrous salts.
e.g. hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
CuSO4*5H2O ⇌ CuSO4 + 5H2O
Describe the reversible reaction between hydrated copper(II) sulfate and anhydrous copper(II) sulfate
Hydrated copper(II) sulfate is seen as blue crystals, it can be heated or dehydrated to form anhydrous copper(II) sulfate. This reaction is endothermic as energy is taken in to remove the water. Anhydrous copper(II) sulfate is usually seen as white crystals or white powder. Adding water to the anhydrous salt forms the hydrated copper(II) sulfate. This reaction is exothermic.
hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
CuSO4*5H2O ⇌ CuSO4 + 5H2O
Describe the reversible reaction between hydrated cobalt(II) chloride and anhydrous cobalt(II) chloride
Hydrated cobalt(II) chloride is seen as pink crystals, it can be heated or dehydrated to form anhydrous cobalt(II) chloride. This reaction is endothermic as energy is taken in to remove the water. Anhydrous cobalt (II) chloride is seen as blue crystals, adding water to the anhydrous salt forms the hydrated cobalt(II) chloride. This reaction is exothermic.
hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water
CoCl2*6H2O ⇌ CoCl2 + 6H2O
How does a reversible reaction reach equilibrium?
A reversible reaction can reach equilibrium in a closed system. This is so none of the participating chemical species can leave the reaction vessel and nothing else can enter.
What two things show that a reversible reaction in a closed system is at equilibrium?
A reversible reaction in a closed system is at equilibrium when: the rate of the forward reaction is equal to the rate of the reverse reaction and when the concentrations of reactants and products are no longer changing.
How can equilibrium change?
Equilibrium is dynamic which means that the molecules on the left and right of the equation are changing into each other by chemical reactions constantly at the same rate.
Describe the Haber process as an example of a reaction reaching equilibrium
An example of a reaction reaching equilibrium is the reaction between H2 and N2 in the Haber process. At the start of the reaction, only nitrogen and hydrogen are present which means that the rate of the forward reaction is at its highest since the concentrations of hydrogen and nitrogen are at their highest. As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease and so the rate of the forward reactions will increase. However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase as ammonia will decompose to reform hydrogen and nitrogen. In a closed system, the two reactions are interlinked and none of the gases can escape. So, the rate of the forward reaction and the rate of the backward reaction will eventually become equal, and equilibrium is reached.
When does the position of equilibrium shift to the right?
The position of equilibrium is said to shift to the right when the forward reaction is favoured. This means that there is an increase in the amount of products formed.
When does the position of equilibrium shift to the left?
The position of equilibrium is said to shift to the left when the reverse reaction is favoured. This means that there is an increase in the amounts of reactants formed.
How does temperature affect equilibrium?
If the temperature is raised, the yield from the endothermic reaction increases and the yield from the exothermic reaction decreases. If the temperature is lowered, the yield from the endothermic reaction decreases and the yield from the exothermic reaction increases. When a change in temperature is made to a system, the system will oppose the change. E.g. if the temperature is increases, the system will oppose the change by decreasing the temperature and therefore favouring the endothermic reaction. So, an increase in temperature causes the equilibrium to move in the endothermic direction to reverse the change and a decrease in temperature causes the equilibrium to move in the exothermic direction to reverse the change.
How does pressure affect equilibrium?
Changes in pressure only affect reactions where the reactants or products are gases. An increase in pressure causes the equilibrium to shift in the direction that produces the least number of molecules and a decrease in pressure causes the equilibrium to shift in the direction that produces the greatest number of molecules.
How does concentration affect equilibrium?
An increase in the concentration of a reactant will cause the equilibrium to shift to the right and a decrease in the concertation of a reactant will cause the equilibrium to shift to the left. An increase in the concentration of a product will cause the equilibrium to shift to the left and a decrease in the concentration of a product will cause the equilibrium to shift to the right.
How does a catalyst affect equilibrium?
The presence of a catalyst does not affect the position of equilibrium, but it does increase the rate at which equilibrium is reached. This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway required lower activation energy. As a result, the equilibrium concentration of reactants and products is the same as it would be without a catalyst.
How is ammonia manufactured?
Ammonia is manufactured in an exothermic reactions called the Haber process.
N2(g) + 3H2(g) ⇌ 2NH3(g)
What conditions are needed in the Haber process?
20000 kPA (200atm), iron catalyst and 450 degrees celcius
What are the five stages of the Haber process?
Stage 1: hydrogen is obtained from methane and nitrogen is obtained from the air. Both gases are pumped into the compressor through a pipe.
Stage 2: inside the compressor, the gases are compressed to around 20000 kPA (200atm)
Stage 3: the pressurised gases are pumped into a tank containing layers of an iron catalyst an at a temperature of 450 degrees celcius. Some of the hydrogen and nitrogen will react to form ammonia: N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
Stage 4: unreacted H2 and N2 and the ammonia product pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels.
Stage 5: the unreacted H2 and N2 gases are recycled back into the system and start over again.
How does pressure and temperature affect percentage yield in the Haber process?
As the pressure increases (shown by the curved lines) the percentage yield increases.
As the temperature increases (shown by the vertical line) the percentage yields increase.
How do you choose the conditions required for the Haber process?
The actual conditions used must be chosen depending on a number of economical, chemical and practical considerations.
Why does the Haber process use a temperature of 450 degrees celcius?
By using a high temperature of 450oC, it favours the reverse reaction as it is endothermic, so a higher yield of reactants is made. This is a compromise for having a lower yield of products but bring made more quickly.
Why does the Haber process use a pressure of 200 atmospheres?
By using 200 atm, it is a compromise pressure between a lower yield of products being made safely and economically
Why does the Haber process use an iron catalyst?
The presence of an iron catalyst does not affect the position of equilibrium, but it does increase the rate at which equilibrium is reached. This is because the catalyst increases the rate of both the forward and backward reactions by the same amount by providing an alternative pathway that requires lower activation energy. As a result, the concentration of reactants and products is always the same at equilibrium as it would be without the catalyst. So, a catalyst is used as it helps the reaction reach equilibrium quicker. This allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required.
How is sulfuric acid manufactured?
Sulfuric acid is synthesised by the contact process. Concentrated sulfuric acid is used in car batteries, making fertilisers, soaps and detergents.
What conditions are needed in the contact process?
450 degrees celcius, vanadium(V) oxide catalyst and a pressure of 2 atmospheres
What are the two stages of the contact process?
Stage 1: the oxygen is obtained from the air and the sulfur dioxide is obtained by burning sulfur to oxidise the sulfur; sulfur + oxygen → sulfur dioxide S + O2 → SO2 or by roasting sulfide ores; metal sulfide + oxygen → metal oxide + sulfur dioxide.
Stage 2: the main stage is the oxidation of sulfur dioxide to sulfur trioxide using a vanadium(V) oxide, V2O5 catalyst. sulfur dioxide + oxygen ⇌ sulfur trioxide 2SO2 + O2 ⇌ 2SO3. The oxygen is obtained from the air. The conditions for this main stage of production are a temperature of 450oC and a pressure of 2 atm (200kPa)
Why does the contact process use a temperature of 450 degrees celcius?
By using a high temperature of 450oC, it favours the reverse reaction as it is endothermic so a higher yield of reactants will be made. This is a compromise for having a lower yield of products but bring made more quickly.
Why does the contact process use a pressure of 2 atmospheres?
By using a low pressure of 200 kPa or 2 atm, it favours the reverse reaction so a higher yield of reactants will be made. So, the reaction is carried out at just above atmospheric pressure because high pressures can be dangerous and very expensive equipment is needed and a higher pressure causes the sulfur dioxide to liquify.
What is used to indicate the oxidation number of an element in a compound?
Roman numerals re used to indicate the oxidation number of an element in a compound.
What do redox reactions involve?
Redox reactions involves simultaneous oxidation and reduction.
What is oxidation?
Oxidation is the loss of electrons, an increase in oxidation number or when oxygen is added to an element or a compound. An example of oxidation is Ag -> Ag+ + e-
What is reduction?
Reduction is a gain of electrons, a decrease in oxidation number or when oxygen is removed from an element or compound. An example of reduction is O2 + 4e- -> 2O2-
What are the oxidation number rules?
Oxygen in compounds is always -2, hydrogen is always 1, groups 1, 2, 6 and 7 all have their specific oxidation numbers (+1 for group 1) and the sum of a compound is 0.
What is an oxidising agent?
An oxidising agent is a substance that oxidises another substance and is reduced itself.
What is a reducing agent?
A reducing agent is a substance that reduces another substance and is oxidized itself.
How do you test for reducing agents?
Potassium manganate(VII) is used to test for reducing agents. If positive, its colour changes from purple to colourless.
How do you test for oxidising agents?
Potassium iodide is used to test for oxidising agents. If positive, the solution turns a red/brown colour.
