While chance and risk both refer to events that are uncertain, the usage of the words is in different contexts. The word "chance" is used when referring to positive things and "risk" when referring to negative things. For example, "chance of survival", "risk of failure".
Introduction (from Wikipedia)
Chance refers to the probability of a (usually desirable) outcome or event.
Risk is the potential of loss resulting from a given action, activity and/or inaction. The notion implies that a choice having an influence on the outcome sometimes exists (or existed). Potential losses themselves may also be called "risks".