duration of a bond with a par value

What is the duration of a bond with a par value of $10,000 that has a coupon rate of 3.5 percent annually and a final maturity of two years? Assume that the required rate of return is 4 percent compounded semiannually. What is the duration of a two-year zero coupon bond that pays $10,000 at maturity and is priced to yield 4 percent with semiannual compounding? Why do the durations differ?