Tuesday, 30 June 2015
Saturday, 28 February 2015
Sunday, 30 November 2014
The Chemistry of Christmas
The Chemistry of Christmas
Noses are red, candles ignite, your puds on fire, its Christmas right?!
Candles
I can’t help but notice the growing trend of candles. People just seem obsessed with them. This Christmas I am sure you will have a Christmas themed candle on your dinner table. Fancy impressing your family with a fascinating and awesome trick?
Candles are made from a mixture of long-chain alkanes, which combine to form the paraffin wax. Alkanes are hydrocarbons which are saturated. As you light the candle, you will trigger an exothermic chemical reaction. Remember an exothermic reaction is one that produces heat and takes in energy! This is why we feel warmth from a candle! The chemical reaction is between the wax and the oxygen in the air. As this is a complete combustion reaction, it would of course produce water (in the form of steam) and carbon dioxide. But what is the smoke from a candle (you may ask)? The smoke is made in the yellow part of the flame where not enough oxygen is available for perfect combustion to take place. The smoke consists of tiny particles of solid, unburned carbon from the wax. The steam produced in the reaction is made from the blue part of the candle flame. This is because the blue area is where the wax burns cleanly with plenty of oxygen.
And now for the trick to excite your unknowing friends and family… First blow out your candle. You should notice the smoke rising from the extinguished wick. But amongst the smoke you won’t be able to see the paraffin vapour, which also rises from the hot wax. This vapour is flammable, meaning you could hold a lit match over the smoke (about an inch away from the wick) and poof! The flame will make an impressive return as it catches the paraffin vapour and the candle reignites!
Christmas pudding
Once your food baby is at least six months old after indulging in the
honey roasted turkey, roast potatoes and parsnips, you are all set to proceed
in the pregnancy. The dessert! No amount of fullness can tempt you out of
eating a bit of Christmas Pud! It is traditional for brandy to be poured all
over the pudding and then ignited with a flame. This leaves a blue glow to
mesmerise all your family feasters. One of the things that has always puzzled
me though is: why does the pudding not burn?
The reason: The brandy is 60% water so the energy from burning the
alcohol is absorbed by this water. The water then evaporates, which prevents
the pudding from burning to a crisp (hence why the temperature of the pudding
is kept low). This is similar to our own body when it sweats. The sweat
evaporates allowing you to stay cool. (Ok ok, enough with the analogy between
food and sweat). Finally, the alcohol burns off before the water has
evaporated, meaning you get left with a somewhat damp, soggy pudding. Still
appetized?
Another question you may have is: Why is the flame blue? The brandy burns this colour due to the sugar infused in it. As you may have learnt in your science lessons, different chemicals burn to produce different colours. For example, potassium chloride burns purple and copper sulphate burns green! But don’t go adding these chemicals to your food, they are toxic to the human body…
Brussel Sprouts
No Christmas dinner is compete without the good old Brussel sprouts. As a vegetable, it is no shock that they are jam packed with nutrients like Vitamins A and C, fibre and folic acid. Have your family chefs ever overcooked these healthy emeralds, leaving a pungent smell? This smell is created due to the organic compound ‘glucosinolate sinigrin’ which contains sulphur. It is the sulphur in the Brussels that leave the unpleasant smell. But next time your Mum tells you to eat your Brussels on Christmas day remember that it is the smelly sulphur chemical that is also responsible for its cancer-fighting characteristics. Brussels are a bit like marmite. But whether you love it or hate them, it is down to your genes. Yes you heard me right. This almighty veggie has a chemical in it which tastes bitter to a person with one version of a gene, and people with a different gene can’t even taste it at all. Weird huh? So which gene do you have?
Rudolf’s Nose
Most of us can’t remember all those reindeer names, but anyone who forgets Rudolf must be one mighty scrooge. If I asked you to describe Rudolf to me, I bet you would say: “He has a bright red nose!” But why is his nose so bright? Does he have the flu? Well Reindeers have a lot of membranes in their nose which act as heat exchangers. This means that the air is rapidly warmed as it enters Rudolf’s nose, and cooled when it leaves. This allows Rudolf and his pals to retain heat and reduce moisture. Handy for when you are stuck in the North Pole 364 days a year… But beware, because this moist and warm environment is home to bacteria and nasty parasites. This could give the reindeers an infection, providing us the reason as to why Rudolf has a bright red nose. Aww poor Rudolf.
Sunday, 9 November 2014
Lattice Enthalpy
Lattice Enthalpy
The enthalpy change when ONE MOLE of a solid ionic lattice is separated into its GASEOUS IONS under standard conditions.
- There is a huge release in energy when two
ions of opposite charge combine to form a solid. (Bonds are being made)
- Lattice enthalpy indicates the strength of
an ionic bond.
- It is a highly EXOthermic reaction as it
has strong electrostatic attraction between two oppositely charged ions.
- The larger the negative lattice enthalpy
shows, the larger the electrostatic forces of attraction there are between the oppositely charged ions in the lattice.
Example:
Na⁺(g) + Cl⁻(g) → NaCl(s)
- Lattice enthalpy cannot be measured
directly because it is impossible to form one mole of an ionic lattice from
gaseous ions.
- As you can’t calculate lattice enthalpy
directly, you use Hess’ Law. This is called a Born-Haber Cycle.
Factors that affect Lattice Enthalpy
· Lattice Enthalpy becomes less exothermic and less
negative as the size of the negative ions increases.
· The charge density decreases
· The attraction gets weaker between the ions, hence the
weaker ionic bonding.
② Ionic Charge
Also
the charge density increases.
Enthalpy change of
solution
The
standard enthalpy change of solution is the enthalpy change that takes place
when ONE MOLE of a COMPOUND is completely DISSOLVED in WATER under standard
conditions.
Example - This process can be endothermic or exothermic:
KCl(s) + aq → K+(aq)
+ Cl-(aq)
When
an ionic solid dissolves in water, two processes take place:
- Overcoming the attractive forces between the oppositely charged
ions requires energy (it is an endothermic reaction – opposite to lattice
enthalpy)
KCl(s) + aq
→ K+(g)
+ Cl-(g)
2. Hydration
Definition: The standard enthalpy change of hydration is the enthalpy change when ONE MOLE of ISOLATED GASEOUS IONS is dissolved in water forming ONE MOLE of AQUEOUS IONS under standard conditions.
Example: K+(g) + aq → K+(aq)
Cl-(g) + aq → Cl-(aq)
- Hydration involves the GASEOUS IONS bonding with H2O molecules.
- The +vely charged ions will be attracted to the slightly negative oxygen atoms in water.
- The –vely charged ions will be attracted to the slightly positive hydrogen atoms in H2O
- It is an EXOthermic process as it is MAKING BONDS.
Q: Why is the enthalpy change of hydration of magnesium ions
more exothermic than the enthalpy change of hydration of calcium ions. Explain
why. (2)
A: The
Mg2+ ion is smaller than the Ca2+ ion. This means Mg2+
has a stronger attraction to H2O.
Q: Explain the differences in the lattice enthalpies of
magnesium fluoride (-2957), sodium fluoride (-918) and sodium chloride (-780).
(3)
A: * Cl- is larger than F- in ionic size. This makes NaCl less exothermic.
* Mg2+ has a greater charge than Na+, this makes Mg2+ more exothermic. Also Mg2+ is smaller than Na+. This means Mg2+ is more exothermic as there is greater attraction.
* F- has greater attraction for Na+ ions and Mg2+ has greater attraction for F- ions.
Born
Haber Cycles
Enthalpy of formation = Sum of all the other
enthalpy values
Steps:
1. Standard enthalpy change of formation, ∆Hf:
2. The standard enthalpy change of atomisation, ∆Hat
3. First Ionisation energy, ∆HI1:
4. (Possible) Second Ionisation energy, ∆HI2:
5. First Electron affinity, ∆HEA1:
6. Second Electron affinity, ∆HEA2:
Example of a born Haber Cycle for Sodium Oxide:
Entropy
Entropy (s) is a measure of the degree of
disorder/randomness in a chemical system.
- All substances possess some degree
of disorder as particles are always in constant motion
- There is a tendency for entropy to
increase, i.e. for things to become more disordered.
- At 0K, perfect crystals
have zero entropy.
Entropy,
S (unlike enthalpy, H) of a substance can be measured directly. It is measured
in J K-1 mol-1
∆S can
be +ve or –Ve, whereas S is always +ve.
Free Energy
v Spontaneous reactions
occur on their own and move to products with a lower energy and more stability.
Exothermic Reactions
- Many occur spontaneously at room temperature
- The enthalpy content of the chemical system decreases during the reaction as the energy is released to the surroundings.
- This increases stability.
Endothermic Reactions
- Can also take place spontaneously at room temperature
- The enthalpy content of the chemical system increases during the reaction as energy is being taken in from surroundings.
- This must also increase stability.
- But this is dependent on entropy.
Gibbs Free Energy
- Spontaneous reactions (feasible reactions at a given temperature) occur if the value for ∆G is NEGATIVE!
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