please help me with chemistry

Question

The sign for ΔH of the SURROUNDINGS if an endothermic reaction is taking place is

A. surroundings is (-)ΔH

B. surroundings is (0)ΔH    

C. surroundings is (=)ΔH

D. surroundings is (+)ΔH

Answer 1

Short answer: A. For the surroundings, delta H is negative.

Endothermic does not mean heat appears. It means heat moves, and it has to come from somewhere. The system and the surroundings are trading one packet of energy, so whatever sign one of them gets, the other gets the opposite. That single relationship answers the whole question:

  delta H (system) = -delta H (surroundings)

  ENDOTHERMIC:  system      ABSORBS heat  ->  delta H is POSITIVE
                surroundings LOSE heat    ->  delta H is NEGATIVE

  EXOTHERMIC :  system      RELEASES heat ->  delta H is NEGATIVE
                surroundings GAIN heat    ->  delta H is POSITIVE

An endothermic reaction pulls heat OUT of the surroundings. The surroundings lost energy, and losing energy is negative. So the answer is A.

The check that settles it with no algebra: an instant cold pack. Ammonium nitrate dissolving in water is endothermic. Hold one and your hand feels cold. Your hand is part of the surroundings, and it feels cold because heat left it. Heat leaving is a negative delta H for your hand. If D were right, the pack would feel warm.

Why D is the trap. D is the correct answer to a different question. Every thermodynamic sign convention in your course is written from the point of view of the SYSTEM, so students learn "endothermic equals plus" and then apply the plus to whatever the question names. This question deliberately names the other side of the boundary. Read the noun before you pick the sign.

  Choice   Claim for the surroundings   Verdict
  ------   --------------------------   ---------------------------------------
    A      negative                     CORRECT
    B      zero                         wrong - zero means NO heat moved at all,
                                        which contradicts "endothermic"
    C      "equal"                      half true: equal in MAGNITUDE, opposite
                                        in SIGN. Magnitude is not a sign.
    D      positive                     wrong - that is the SYSTEM sign. This is
                                        the popular wrong answer.

One caution about B. Zero is not a compromise answer. Delta H equals zero for the surroundings would mean the reaction absorbed heat from nowhere, which is the first law being broken, not a mild reaction. There is no reaction with a small enough enthalpy to make the surroundings read zero. If the number is small, the sign is still negative.

The fine print your book is assuming. This works because delta H equals the heat exchanged at constant pressure, and because the universe here is only two pieces, system plus surroundings, with nothing leaking out to a third place. Written formally: delta H (universe) is zero for the heat transfer, so the two terms must cancel. In a real lab the "surroundings" is the water in the calorimeter, and that is worth remembering for the next question in this unit. A calorimeter never measures the reaction; it measures the water around it. A solution that gets colder is the surroundings paying for the reaction, and the temperature drop you measure is the size of the payment.

So: A, and the one line to carry into the exam is that the system and the surroundings always carry opposite signs and equal magnitudes.