Acceleration is the rate at which velocity changes, which means that acceleration tells you how fast the velocity is changing. A large acceleration tells you that the velocity is changing quickly - a small acceleration tells you that the velocity is changing slowly - an acceleration of zero tells you that the velocity is not changing at all.
Acceleration tells you how the velocity changes - it doesn't tell how how much the velocity is! An object can have a large velocity and a small (or zero) acceleration - and vice versa.
The chemical reaction that would represent the combustion of methane would be CH4 + 2O2 = CO2 + 2H2O. Combustion reactions are reactions which involves a substance burning in oxygen.
