Calculate the enthalpy change,,ΔH=∑ Hp - ∑ HR for the following reaction using equations 1, 2 C graphite(s) --> C diamond(s) The following is known. 1. Cgraphite(s) + O2(g) --> CO2(g) ∆H = -394 kJ 2. Cdiamond(s) + O2(g) --> CO2(g) ∆H = -396 kJ A. -763kJ B. 2kJ C. 790kJ D. -2kJ

Explanation:

We have two equations:

1. C(s, graphite) + O₂(g) ⟶ CO₂(g);  ∆H = -394 kJ  

2. C(s, diamond) + O₂(g) ⟶CO₂(g); ∆H = -396 kJ  

From these, we must devise the target equation:

3. C(s, graphite) ⟶ C(s,diamond); ΔH = ?

The target equation has C(s, graphite) on the left, so you rewrite Equation 1.

4. C(s, graphite) + O₂(g) ⟶ CO₂(g);  ∆H = -394 kJ  

Equation 4 has CO₂ on the right, and that is not in the target equation.

You need an equation with CO₂ on the left, so you reverse Equation 2.  

When you reverse an equation, you reverse the sign of its ΔH.

5.  CO₂(g)⟶ C(s, diamond) + O₂(g); ∆H = +396 kJ  

Now, you add equations 4 and 5, cancelling species that appear on opposite sides of the reaction arrows.

When you add equations, you add their ΔH values.

You get the target equation 3:

4. C(s, graphite) + O₂(g) ⟶ CO₂(g); ∆H =  -394 kJ  

5. CO₂(g) ⟶ C(s, diamond) + O₂(g); ∆H = +396 kJ

3. C(s, graphite) ⟶ C(s, diamond);   ΔH =        2 kJ

h let me think

conduction is the transfer of thermal energy (heat in transfer) due to collisions between the molecules in the object. collisions between adjacent atoms and molecules transfer kinetic energy from the warmer to the cooler object. the objects must be in physical contact.