chem speed revision Flashcards
(23 cards)
Explain physical and chemical trends in properties of group 0 elements.
As go down group, boiling point increases in noble gases, because:
they become larger, the intermolecular forces between the elements become stronger and so more energy is needed to overcome these forces.
The elements in Group 0 of the periodic table are called the noble
gases. They are unreactive and do not easily form molecules
because their atoms have stable arrangements of electrons. The
noble gases have eight electrons in their outer shell, except for
helium, which has only two electrons.
What are the physical properties of group 7 elements?
They are soft
They all have relatively low melting points
They have low density
As element goes down group, reactivity increases, melting point decreases.
Why does reactivity increase as you go down the group?
The reactivity of Group 1 elements increases as you go down the group because:
the atoms get larger
the outer electron gets further from the nucleus
the attraction between the nucleus and outer electron gets weaker – so the electron is more easily lost
Describe what happens in reactions of water with lithium, sodium and potassium?
Lithium Li Fizzes steadily; slowly becomes smaller until it disappears
Sodium, Na Fizzes rapidly; melts to form a ball; quickly becomes smaller until it disappears
Potassium, K Burns violently with sparks and a lilac flame; quickly melts to form a ball; disappears rapidly, often with a small explosion
metal + water –> metal hydroxide + hydrogen
metal hydroxide is alkali (blue/purple in indicator)
Group 1 reactions with oxygen
The group 1 elements react with oxygen from the air to make metal
oxides
.
At room temperature, oxygen reacts with the surface of the metal. This forms a white oxide, which covers the surface. The metal below the surface does not react.
The
reactivity
of the group 1 elements increases down the group, so their reactions with oxygen get more vigorous.
Group 1 reactions with chlorine
The group 1 elements react vigorously with chlorine. The products of the reactions are chlorides. At room temperature the chlorides are white solids. They dissolve in water to form colourless solutions.
The reactions with chlorine get more vigorous going down the group.
Explain the physical properties of group 7 elements and its trend as you go down the group
The halogens exist as simple molecules. Each molecule is made up of a pair of halogen atoms joined by a single covalent bond. In all groups of the periodic table, the further down the group an element is, the higher its relative molecular mass
In group 7, the further down the group an element is, the higher its melting point and
boiling point. This is because, going down group 7:
the molecules become larger
the intermolecular forcesbecome stronger
more energy is needed to overcome these forces
Explain the chemical properties of group 7 elements and its trends.
In Group 7, the reactivity of the elements decreases going down the
group.
The halogens react with metals to produce
salts.
A more reactive halogen can displace a less reactive halogen from
an aqueous solution of its salt.
Reactivity decreases down Group 7 (halogens) because the attraction between the nucleus and the outer electrons becomes weaker as the atomic radius increases. This weaker attraction makes it harder for the atoms to gain an electron, which is what halogens need to achieve a full outer shell and become stable.
Explain reaction of bromine, chlorine, and iodine with metals and non-metals
Metals:
Chlorine Hot iron wool burns vigorously to produce orange-brown iron(III) chloride
Bromine Hot iron wool burns quickly to produce red-brown iron(III) bromide
Iodine Hot iron wool reacts slowly in iodine vapour to produce grey iron(II) iodide
Non-metals:
Chlorine Explodes with a flame or in sunlight, forming hydrogen chloride
Bromine Vigorous reaction when warmed with hydrogen, forming hydrogen bromide
Iodine Very slow reaction when heated strongly, forming some hydrogen iodide
Physical properties of transition metals and their comparison with group 1 metals
The transition elements share some
physical properties with all metals:
they conduct electricity in the solid and liquid states
they are shiny when freshly cut
Some properties of transition elements are different from those of the metals in group 1. Compared to other metals, most transition metals have:
higher melting points
higher densities
greater strength
greater hardness
Chemical properties of transition elements
Most transition elements react slowly, or not at all, with oxygen at room temperature.
Most transition elements react slowly with cold water, or not at all.
Transition elements form ions with different charges.
Metals that are not transition elements usually form white compounds. Transition elements form coloured compounds.
Transition metals are useful as catalysts. Eg.:
iron is the catalyst in the Haber process, which makes ammonia
manganese(IV) oxide increases the decomposition of hydrogen peroxide to oxygen and water
Examples of exothermic and endothermic reactions
An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
Exothermic reactions include combustion, many oxidation reactions and neutralisation.
Everyday uses of exothermic reactions include self-heating cans and hand warmers.
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. Some sports injury packs are based on endothermic reactions.
RP4
Measure 25cm3 of hydrochloric acid into a polystyrene cup.
2. Place the cup inside the beaker to make it more stable.
3. Measure and record the temperature of the hydrochloric acid.
4. Measure 5cm3 of sodium hydroxide and add it to the polystyrene cup.
5. Quickly put a lid on the cup and gently stir the solution with the thermometer through the hole of lid.
6. When the reading on the thermometer stops changing and becomes fairly constant, record the temperature.
7. Repeat steps 4 and 5 to add further 5 cm3 amounts of sodium hydroxide to the cup. A minimum total of 40 cm3 needs to be added.
8. Repeat steps 1–7 to ensure reliability of results.
9. Calculate the mean maximum temperature reached for each of the sodium hydroxide volumes
What’s activation energy
The minimum amount of
energy that particles must have to react is called the activation
energy.
What are cells, batteries, and what does voltage depend on
Cells contain chemicals which react to produce electricity.
The voltage produced by a cell is dependent upon a number of factors including the type of electrode and electrolyte.
A simple cell can be made by connecting two different metals in contact with an electrolyte.
Batteries consist of two or more cells connected together in series to provide a greater voltage.
Difference between rechargeable and non rechargeable cells
In non-rechargeable cells and batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable.
Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.
Half equations at each electrode for fuel cells
At the negative electrode: 2H2 + 4OH- → 4H2O + 4e-
At the positive electrode: O2 + 2H2O + 4e- → 4OH-
Combined:
2H2 + 4OH- + O2 + 2H2O + 4e- → 4H2O + 4e- + 4OH-
This cancels out to make
2H2 + O2 → 2H2O
What is the overall equation and reaction for hydrogen fuel cells
Hydrogen-oxygen fuel cells are an alternative to rechargeable cells and batteries. In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage. Water is the only product. The overall reaction in a hydrogen-oxygen fuel cell is:
hydrogen + oxygen → water
2H2(g) + O2(g) → 2H2O(l)
The overall reaction in a hydrogen fuel cell involves the oxidation of
hydrogen to produce water.
Brief description of how fuel cells work
Fuel cells are supplied by an external source of fuel (eg hydrogen)
and oxygen or air. The fuel is oxidised electrochemically within the
fuel cell to produce a potential difference.
Evaluate use of different cells
Alkaline cell Cheaper to manufacture May end up in landfill sites once fully discharged; recyclable though it is expensive
Rechargeable cell Can be recharged many times before being recycled, reducing the use of resources Costs more to manufacture
Hydrogen fuel cell Easy to maintain as there are no moving parts; small size; water is the only chemical product Very expensive to manufacture; need a constant supply of hydrogen fuel, which is a flammable gas
common reasons for loss of percentage yield
loss of product in apparatus
reactants might be impure
reaction is reversible
side reactions
Why is good atom economy important
high atom economy is good for profits and environemnt
reactions with low atom economy makes lots of waste materials that have to be disposed of. This coudl be bad for the nevironment and may be expensive to dispose of them responsibly
reactions with low atom economy use up resoures very quickly. If the resouces are non-renewable they will run out, raw materials can be expensive and so will reduce profits.
