Exambusters Chemistry Study Cards 10

A chemical reaction which evolves heat
A chemical reaction which absorbs heat
The heat content of a system
Enthalpy Change (State the equation)

The difference in het content between the products and the reactants


ΔH = ΣHProducts – ΣHReactants

Exothermic Reaction: ΔH is (positive or negative)
Exothermic Reaction: Enthalpy is (increasing or decreasing)
Endothermic Reaction: ΔH is (positive or negative)
Endothermic Reaction: Enthalpy is (increased or decreased)
List in the order of increasing enthalpy: Liquid, Solid, Gas

Solid (least enthalpy)


Gas (most enthalpy)

Thermochemical equation

A balanced chemical equation which includes the enthalpy change.

H2(g) + ½O2(g) →H2O(l)

Δ H=-285kJ

In a thermochemical equation, what happens to ΔH when the moles of reactants double?
Enthalpy is directly proportional to mass. Therefore, when the moles double, so does ΔH.

Reverse Reaction What is the reverse reaction of:

C + O2 → CO2

Products becoming reactants

CO2 → C + O2

How are ΔH for a forward and ΔH for a reverse reaction related?
ΔH forward is equal in magnitude but opposite in sign to ΔH reverse.
Hess’s Law
;H for a reaction is the same regardless of the path traveled from reactants to products
Molar Heat of Formation
The molar heat of formation of a compound is equal to ;H when 1 mole of compound is formed from its elements at 1 atm. and 25;C.
Write the general equation to calculate ;H for a chemical reaction.

;H = ;(;HF(products)) – ;(;HF(reactants))


;HF = Heat of Formation

What is the heat of formation of an element in its most stable state?
A device used to measure the quantity of heat liberated or absorbed in a chemical reaction or physical change
Specific Heat
The amount of heat required to raise one gram of substance 1;C
Give the specific heat value for liquid water in calories and Joules.
1 calorie/gram ;C
Give the specific heat value for liquid water in Joules.
4.18 Joules/gram ;C
Write an equation to calculate energy change when a fixed mass of substance changes temperature.

;H = mCp;T


m = mass in grams

T = temperature change

Cp = specific heat

;H = heat absorbed, released

How many calories to raise 100 grams of water from 20;C to 30;C

;H = mCp;T


Heat = (100g)(1 cal/g;C)(10;C)

Heat = 1000 cal