Hess's Law and Enthalpy Cycles · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Adding detours to find the heat

Science · Chemistry · ages 16-17
Name ______________________   Date ____________

  1. The standard formation enthalpy of an element in its standard state is...

    • Always negative
    • Zero
    • Impossible to define

  2. What does Hess's law say?

    • The same start and finish give the same enthalpy change by any route
    • Shorter routes release more heat
    • Enthalpy depends on the path taken

  3. With combustion data, cycle arrows point up to the shared products.

    Circle one:   True   False

  4. With formation data, arrows point...

    • Up to the combustion products
    • Sideways between reactants
    • Down from elements to compounds

  5. Products sum to minus 1029 and reactants to minus 1207. What is delta H in kJ per mol? Type the number.

    Answer: ______________

  6. You flip one arrow in the cycle. What happens to its value?

    • It doubles
    • It changes sign
    • Nothing changes

  7. A student works out reactants minus products with formation data. What is wrong?

    • The arrows should point up
    • Formation cycles use products minus reactants, so the sign flips
    • Zero should replace every product

  8. Why find the calcium carbonate decomposition via a cycle, not a beaker?

    • Beakers melt at that temperature
    • Cycles are always exact
    • It cannot be measured cleanly, so measurable steps stand in
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Adding detours to find the heat W1-mt_530V9czLOb-s1

  1. Zero · Elements are the zero line every cycle measures from.
  2. The same start and finish give the same enthalpy change by any route · Only the endpoints matter, never the detour.
  3. True · Burning turns everything into the same products above.
  4. Down from elements to compounds · Formation builds each compound down from its elements.
  5. 178 · Subtract: minus 1029 minus minus 1207.
  6. It changes sign · Reversing a step reverses heat flow, flipping the sign.
  7. Formation cycles use products minus reactants, so the sign flips · Backwards subtraction gives the same size with the wrong sign.
  8. It cannot be measured cleanly, so measurable steps stand in · Hess lets measurable steps answer for an unmeasurable one.
Worksheet · LightMySky