Given the balanced equation representing a reaction:
h2(g) + cl2(g) ==> 2hcl(g) + energy
which statement describes the energy changes in this reaction?
(1) energy is absorbed as bonds are formed, only.
(2) energy is released as bonds are broken, only.
(3) energy is absorbed as bonds are broken, and energy is released as bonds are formed.
(4) energy is absorbed as bonds are formed, and energy is released as bonds are broken.

(3) Energy is absorbed as bonds are broken, and energy is released as bonds are formed.

Explanation:

1) The equation given is H₂(g) + Cl₂(g) → 2HCl(g) + energy

2) The energy is shown in the product side, so energy is a product of the reaction, so energy is release.

But this is just the net energy of the process. You need more insight to deal with the energy changes in the reaction.

3) Chemical bonds store energy; this stored energy is the potential chemical energy of the molecules.

When a chemical reaction occurs, the first stage is to brake bonds.

Braking chemical bonds requires energy to overcome the bond energy. Braking bonds always absorbs energy.

On the other hand, forming bonds always release energy.

The neat energy of the reaction is the difference between the energy needed to break bonds and the energy released when new bonds form.

So, regardless the fact that, in the chemical equation that represents the reaction a net energy release is shown, you know that energy is absorbed as bonds are broken, and energy is released as bonds are formed (option 3).

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.

out of these options:

ability to burn paper

pure sodium reacting violently with water

the boiling point of water

helium does not tend to react with anything

i would say this is a physical property:

helium does not tend to react with anything

why? :

i said helium does not tend to react with anything because well, it reacts to ! but ability to burn paper, pure sodium reacting violently with water, & the boiling point of water are chemical or

physical properties: the properties of a substance that can be observed without changing the chemical makeup of the substance

chemical properties: properties of a substance relating to the chemical nature and reactivity of the substance