2.1 Flashcards
What is enthalpy, H
Heat content of a system at constant pressure
What is enthalpy change
Head added to a system at constant pressure
Equation to measure enthalpy change
Enthalpy of products - enthalpy of reactants
Exothermic reaction
- releases energy to surroundings
- temperature rise
- enthalpy change is negative
Endothermic reactions
- absorb energy from surroundings
- temperature drop
- enthalpy change is positive
What is the principle of conservation of energy
energy cannot be created or destroyed only changed from one form to another.
Conditions for standard enthalpy change
- all substances in their standard states
- 298 K (25 c)
- 1 atm (101000 Pa)
What is the standard enthalpy change of formation
the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states under standard conditions.
What is the standard enthalpy change of formation of all elements in their standard state
0 kJ mol-1
Standard enthalpy change of formation of water
H2 (g) + 1/2 O2 (g) —–> H2O (l) /\fH = -286 kJ mol-1
What is the standard enthalpy change of combustion
enthalpy change when one mole of a substance is completely combusted in oxygen under standard conditions.
What is the standard enthalpy change of combustion of methane
CH4 (g) + 2O2 (g) —-> CO2 (g) + 2H2O (l) /\cH = -891 kJ mol-1
Equation for the standard enthalpy of reaction
/\rH = sum/\fH(products) - sum/\fH(reactants)
What is Hess’s law
the total enthalpy change for a reaction is independent of the route taken from the reactants to the products
What is bond enthalpy
the enthalpy required to break a covalent X-Y bond into X atoms and Y atoms, all in the gas phase
What is average bond enthalpy
Average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species
How to calculate bond enthalpy change
BURP
Sum of reactants - sum of products
What can be done to measure enthalpy due to not being able to measure it directly
Measure the heat transferred to its surroundings
What is a calorimeter
Insulated container that helps calculate the chemical change of a reaction
• change in temperature inside the calorimeter caused by the enthalpy change is measured with a thermometer
What information is needed to calculate the enthalpy change when doing it physically
- temperature change
- mass
- specific heat capacity of contents of calorimeter
Equation that represents the relationship between the temperature change and the amount of heat transferred
q= mc/\t
c- specific heat capacity
m- mass of solution in cup (volume)
What is done to obtain the maximum temperature change
allowances are made for heat lost to the surroundings
• temperatures of the solution are taken for a short period before mixing and some time after mixing
• graph of temperature against time is plotted and maximum temperature is obtained by extrapolating the graph back to the mixing time
Look
Equation to calculate the enthalpy change of reaction per mole
/\H = -q/n
reactant that is not in excess has to be accurately measured - mass of solid or concentration of solution must be known
