In an isothermal reaction (either reversible or irreversible), DELTA T is constant..hence zero
since temperature remains constant, can we also assume that heat is also zero?
if so, how then is an isothermal reaction different from an Adiabatic one?
Heat input is not necessarily zero in an isothermal process.
An example would be heating a gas and letting it expand enough to keep the temperature the same. This is one of the steps of a Carnot cycle engine.
Another example would be melting ice!