Coming up with the equations is grand, and finding out the heat of reaction. But I'm always confused about the signs? How do you know whether the heat of formation is a plus or a minus. Eg. when they give you the heats of formation of carbon dioxide, water, butane: -393.5, -285.8, -125.7. Why are carbon dioxide and water values negative but the butane sign is changed to +125.7. Thanks in advance.