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Developing Fuels (DF) Q+A
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Questions (and answers) written by an A* A-Level Chemistry student. Diagrams included.
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1) Define 'exothermic'A reaction that gives off energy and heats the environment
2) Define 'endothermic'
A reaction that takes in energy and cools the environment
3) What happens during an exothermic reaction? Which ends up with more energy
reactants or products?
The chemical reactants are losing energy to their surroundings
The products end up with less energy than the reactants had and the surroundings end up
with more (E transferred to environment)
ΔH is negative
4) What happens during an endothermic reaction? Which ends up with more energy
reactants or products?
The reactants take in energy from the surroundings leaving the products at a higher energy
level than the reactants. Environment cooler since it lost energy that was given to the
products
ΔH is positive
5) Show the enthalpy level diagram of an exothermic reaction
6) Show the enthalpy level diagram of an endothermic reaction
, 7) Define enthalpy change (ΔH)
The energy transferred to and from the surroundings
8) Why can't you measure the enthalpy of a substance? What is measured instead?
Can't be measured directly as we cannot physically measure how much energy.
Change in enthalpy is measured (ΔH) when a reaction occurs.
9) Give the formula for change in enthalpy ΔH (kJ/mol) =
Hproducts - Hreactants
10) Give the standard conditions for measuring enthalpy changes - 298K/25℃
- 1atm/1.01x10^5 Pa
- Standard concentration of 1mol/dm3 for solutions
11) Under what states do elements exist in under standard conditions?
In their standard states
12) How to convert from Celsius to Kelvin?
Add +273.15
13) What is the SI unit for temperature?
Kelvin (K)
14) Define 'standard enthalpy change for a reaction'
The enthalpy change when molar quantities of the reactants as stated in the equation react
together under standard conditions in standard states
15) Define 'standard enthalpy change of combustion'
The enthalpy change that occurs when one mole of a substance is burnt completely in
oxygen under standard conditions in standard states
16) Q (energy transferred) =
mcΔT
17) Define 'standard enthalpy change of formation'
The enthalpy change when one mole of a compound is formed from its elements under
standard conditions in standards states
18) Define 'standard enthalpy of neutralization'
The enthalpy change when one mole of hydrogen ions reacts withy one mole of hydroxide
ions to form one mole of water under standard conditions and in solution containing
1mol/dm3
19) What is the standard enthalpy change of formation of a pure element in its standard
state?
Zero
20) State Hess's Law
The total enthalpy change of a reaction is independent of the route taken
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