In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The administration of phenobarbitone and carbamazepine for 21days caused a significant impairment of learning and memory as well as an increased oxidative stress .
### Output Text:
The administration of <span class="chemical">phenobarbitone</span> and <span class="chemical">carbamazepine</span> for 21days caused a significant impairment of learning and memory as well as an increased oxidative stress . 