What Is the Geometric Difference in Intersection Forms Q(a,b) vs Q(b,a)?

[WWGD]

Hi, everyone:

This should be easy, but I am having trouble with it. I am rusty and trying

to get back in the game:

Let Q(a,b) be an intersection form in the middle homology class

of some 2n-manifold.

What is the geometric difference between Q(a,b) and Q(b,a).?

If n is even, they are of the same sign, opposite sign

if n odd, but I am not clear on what the geometric

difference is with the different orders.


2) Also: Am I missing something really obvious here:

If H_n==0 for a 2n-manifold M . Does it follow (by bilinearity)

that Q==0.?. Since the only class is the zero class, it

would seem to follow right away. What is the geometry behind

this.?. I understand that this does not imply that there is

no actual intersection, but that the (signed) net intersection

is zero. (If above is correct) Anyone have an insight on the

geometry behind this.?

Thanks.