What must be true of the bonds for a reaction to be endothermic and what must be true for the reaction to be exothermic?

When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?

Will mark as brainliest ! how many grams of iron metal do you expect to be produced when 245 grams of an 80.5 percent by mass iron (ii) nitrate solution react with excess aluminum metal? show all work needed to solve this problem!

What sources of error may have contributed to the percent yield not being 100 percent? think about things that may have led to inaccurate measurements or where mass of the product could have been lost if this experiment was conducted in a physical laboratory.