Chemistry answers big questions. And yes, this includes questions as big as the T. rex — did dinosaurs suntan to warm their body like lizards do?
Since the 1960s, there has been a heated debate about whether dinosaurs were cold-blooded or warm-blooded.
On one side of the debate, scientists hypothesised that dinosaurs were cold-blooded because they bore superficial resemblance to reptiles. However, an analysis of the oxygen isotopes found in their bone suggests that they were warm-blooded.
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Different Speed of Reaction of Isotopes
We are taught that isotopes are chemically identical. This is true, in the sense that isotopes can undergo the same reactions to form the same products.
But the devil is in the detail. Isotopes react at different speed due to their different atomic mass. Second, when temperature rises, the speed of reaction increases to a greater extent for the lighter isotope.
Oxygen Isotopes in Bone Phosphate
The bone contains a complex ionic compound with multiple cations and anions. The major ions are calcium ions, Ca2+, and phosphate ions, PO43-.
The phosphate ion is an oxyanion. It contains oxygen, which can either be oxygen-16 or oxygen-18. When temperature increases, the percentage of oxygen-18 decreases.
Mass Spectrometry to Determine Isotopic Abundance
To calculate the percentage of oxygen-18, mass spectrometry was used. This technique involves passing gaseous ions through electric and magnetic fields. Depending on the mass and charge of ions, their paths curve differently in the fields. This difference is detected and the results are printed out as a graph called a mass spectrum.
But first, the phosphate ions in the solid sample had to be converted into gaseous ions that could be analysed:
- The bone sample was reacted to form silver phosphate, Ag3PO4.
- Carbon was then oxidised by silver phosphate, Ag3PO4, to form carbon monoxide, CO.
- An electron was removed from carbon monoxide, CO, to form ionised carbon monoxide, CO+. Sometimes, side products like C+ and O+ are also produced.
The ionised carbon monoxide derived from the ribs of a Giganotosaurus was analysed to produce the mass spectrum below.
The position of each peak shows the relative mass of each ionised gas.
The height of each peak is proportional to the relative amount of each ionised gas, with respect to the most abundant one.
In the spectrum, the tallest peak is cut off and not shown fully. Its relative intensity is actually 100%.
Analysis of Multiple Mass Spectra
The same technique of mass spectrometry was repeated on the bones from other body parts of Giganotosaurus.
The percentage of oxygen-18 was found to be relatively constant, reflecting the constant body temperature at which the bones were formed. This is indicative of a warm-blooded animal.
However, the debate is far from over. There are other lines of evidence that point to dinosaurs occupying the middle ground, being neither completely warm-blooded nor cold-blooded.
Data-Based Questions à la Paper 2 Section B
QUESTION 1: ATOMIC STRUCTURE
Oxygen-16 and oxygen-18 are two stable isotopes of oxygen.
Explain, in terms of atomic structure, why oxygen-18 is the heavier isotope. [2 marks]
Oxygen-18 has the same number of protons but 2 more neutrons as compared to oxygen-16. As a neutron has a relative mass of 1, oxygen-18 is heavier than oxygen-16.
QUESTION 2: ATOMIC STRUCTURE
Explain, in terms of atomic and electronic structure, why we often assume that isotopes of an element are chemically identical. [3 marks]
Isotopes of the same element have have the same number of protons. Consequently, they have the same number of valence electrons in the same electron shell. Therefore, they lose, gain, or share electrons in the same way to achieve the noble gas electronic configuration.
QUESTION 3: ATOMIC STRUCTURE
Explain how this article challenges the idea that isotopes are chemically identical. [1 mark]
Isotopes undergo the same reaction at different speeds.
When temperature rises, the speed of reaction increases to a greater extent for the lighter isotope.
QUESTION 4: SPEED OF REACTION
When bones are formed at a higher temperature, the percentage of oxygen-18 in bone phosphate decreases.
Suggest, in terms of speed of reaction, why the percentage of oxygen-18 decreases with increasing temperature. [2 marks]
At higher temperature, speed of reaction increases to a greater extent for the lighter isotope, oxygen-16. More products containing oxygen-16 are formed for the same duration of bone formation.
At higher temperature, phosphate ions containing oxygen-18 precipitates even more slowly than those containing oxygen-16. Therefore, less hydroxyapatite containing oxygen-18 is formed for the same duration of bone formation.
Examiner’s comment: Your answer looks different? Don’t panic! This is an inference question that accepts a wide range of plausible answers, so long as you have compared the speed of reaction and relate it to the relative amount of product formed.
QUESTION 5: MOLE CONCEPT
The phosphate ions, PO43-, in a bone sample underwent multiple reactions to form carbon monoxide, CO. Through the reactions, all the oxygen atoms in the phosphate ions were transferred to carbon monoxide.
Calculate the relative molecular mass of a carbon monoxide molecule that contains an oxygen-18 atom derived from the phosphate ions. [1 mark]
Relative molecular mass of C18O = 12 + 18 = 30
Examiner’s comment: Do not use the mass number of oxygen as shown in the Periodic Table. That is the average mass number of all the oxygen atoms of different isotopes. It is rounded off to 16.
QUESTION 6: MOLE CONCEPT
The mass spectrum of the ionised carbon monoxide has four peaks.
Explain why there are four peaks. [2 mark]
Two of the peaks are due to the side products formed: C+ and 16O+. The remaining two peaks are due to carbon monoxide with different oxygen isotopes: C16O+, and C18O+.
QUESTION 7: MOLE CONCEPT
The number of moles of ionised carbon monoxide containing oxygen-16 was 0.000500 mol.
Use the mass spectrum to calculate the number of moles of ionised carbon monoxide containing oxygen-18. [2 marks]
From the spectrum, relative amount of C18O = 0.2% of C16O
No of moles of C18O = (0.2/100) × 0.000500 = 1×10-6 mol
Note: The relative amount shown in the mass spectrum is with respect to the most abundant particle, which is the carbon monoxide containing oxygen-16 as represented by the peak at relative mass = 28. Note that the relative amount is given in percentage.