Endothermic Exothermic Reactions Enthalpy H Heat Enthalpy Change

Endothermic & Exothermic Reactions • Enthalpy- (H) Heat • Enthalpy Change- (ΔH) The change in heat for a reaction

• Endothermic reactions – – – Energy is absorbed Heat goes into the system from the surroundings will feel colder Temperature of surroundings goes down +ΔH • Exothermic reactions – – energy is released Heat goes out of the system into the surroundings Temperature of the surrounding will increase -∆H

Endo- and Exothermic Surroundings System Heat ENDOTHERMIC Heat System EXOTHERMIC 3

Endothermic & Exothermic Reactions

Endothermic & Exothermic Reactions

Endothermic & Exothermic Reactions • Are the following reactions endothermic or exothermic? • CO + 3 H 2 CH 4 + H 2 O H= -206 k. J • Exothermic ( H is negative) • I add magnesium metal to some hydrochloric acid. The temperature goes from 23 C to 27 C • Exothermic – temperature goes up • I mix together some vinegar & baking soda. The temperature goes from 28 C to 23 C • Endothermic – temperature goes down

Thermochemical equations • An equation that gives us information about H … S + O 2 SO 2 H = – 296. 9 k. J • If we change the equation, then the H also changes … SO 2 S + O 2 H = + 296. 9 k. J • If the reaction is reversed the sign is reversed • Also, if the coefficients in the equation change, so will the amount of energy produced/absorbed: 2 S + 2 O 2 2 SO 2 H = – 593. 8 k. J

• Given the following thermochemical equation: • CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (l) ΔH = -890. 4 k. J/mol • calculate the heat evolved when 20. 00 g of CH 4 reacts – The chemical equations are always written in terms of moles, but the problem is stated in terms of mass (g). • First, convert grams to moles. • Then, convert moles to k. J (kilojoules)

CH 4 (g) + 2 O 2 (g) → CO 2 (g) + 2 H 2 O (l) ΔH = -890. 4 k. J/mol • 20. 00 g CH 4 1 mole CH 4 -890. 4 k. J 16. 04 g CH 4 1 mole CH 4 (from the coefficient in the equation) • -1110. k. J

CH 4 (g) + 2 O 2 (g) � → CO 2 (g) + 2 H 2 O (l) ∆H = - 890. 4 k. J • How much energy is given off when 2. 00 mol of CH 4 are burned? • How much energy is released when 22. 4 g of CH 4 are burned? • If you were to attempt to make 45. 0 g of methane from CO 2 and H 2 O , how much energy would be required?

• -1780 k. J • -1240 k. J • 2. 50 x 103 k. J