Here’s the matching of the definitions with the correct idioms:

1. the death penalty - **e** capital punishment
2. a drink (alcoholic) taken before one goes to bed - **l** a nightcap
3. a short sleep in a chair (not in a bed) - **f** a catnap
4. a success that only lasts a short time - **j** a flash in the pan
5. someone who is blamed for other people's mistakes - **n** a scapegoat
6. someone who is very conceited - **a** a bighead
7. someone who attends a party without being invited - **k** a gatecrasher
8. a difficulty – often hidden or unexpected - **o** a snag
9. a heavy shower of rain - **h** a downpour
10. someone who offers to work when the regular workers are on strike - **c** a blackleg
11. unnecessary bureaucracy - **m** red tape
12. a sudden, clever thought or idea - **d** a brainwave
13. something which is unexpected but very much needed and appreciated - **g** a godsend
14. a search for something which has no hope of being successful - **p** a wild goose chase
15. something very unpleasant to look at (e.g., a building) - **i** an eyesore
16. a place in a road where accidents often happen - **b** a black spot

Feel free to ask if you need further explanations!