We present results from a time-dependent gas-phase chemical model of a hot core based on the physical conditions of G305.2+0.2. While the cyanopolyyne HC₃N has been observed in hot cores, the longer chained species, HC₅N, HC₇N and HC₉N, have not been considered as the typical hot-core species. We present results which show that these species can be formed under hot core conditions. We discuss the important chemical reactions in this process and, in particular, show that their abundances are linked to the parent species acetylene which is evaporated from icy grain mantles. The cyanopolyynes show promise as 'chemical clocks' which may aid future observations in determining the age of hot core sources. The abundance of the larger cyanopolyynes increases and decreases over relatively short time-scales, ∼10²·⁵ yr. We present results from a non-local thermodynamic equilibrium statistical equilibrium excitation model as a series of density, temperature and column density dependent contour plots which show both the line intensities and several line ratios. These aid in the interpretation of spectral-line data, even when there is limited line information available. In particular, non-detections of HC₅N and HC₇N in Walsh et al. are analysed and discussed.