The carbonyl group and infrared spectroscopy (taking things a little further)
Infrared spectroscopy can tell you about the strength of a bond. The position of the peaks, or stretches, in an infrared spectrum are related to the energy required to cause the bond to vibrate or stretch. An approximation is that the higher the wavenumber (inverse wavelength in cm or cm–1) the stronger the bond. This is based on thinking of a bond as a spring connecting two masses (the atoms) and then using Hooke’s Law of springs (this law also says the mass of the atoms effects the energy but we'll ignore).
The range of values for the carbonyl stretch in various functional groups. Higher the wavenumber, the stronger the bond (and normally, more reactive).
Looking at the stretch for the carbonyl group in various functional groups, you should notice that the substituent has a large effect. Groups that enhance the polarization, such as a chlorine atom, strengthen the bond. Chlorine is electronegative and pulls electrons away making the carbon more partially positive and causing greater electrostatic attraction to the oxygen. The nitrogen of the amide has the opposite effect. Its lone pair is delocalized, feeding electrons onto the carbon and reducing the polarization of the bond. The carbonyl stretch in an amide has a lower wavenumber indicating that the carbonyl bond is weaker and less electrophilic or less reactive.