High School Chemistry  ·  Practice by Skill

Thermochemistry Practice Problems

Thermochemistry tracks how heat moves in and out of chemical and physical changes. Most problems use one of a few equations, and the main skill is choosing the right one for the situation. Practice below covers q = mcΔT, heating curves, exothermic and endothermic changes, and enthalpy.

What you’ll practice

  • Heat and temperature change: use q = mcΔT for warming or cooling a sample.
  • Heating curves: combine temperature-change steps with melting and boiling energy.
  • Exothermic and endothermic: read the direction of heat flow from the sign of ΔH.
  • Enthalpy and Hess's law: add reaction steps to find the ΔH of a reaction you cannot measure directly.

Learn the method first →

A quick example: heating water

How much heat is needed to warm 25.0 g of water by 10.0 °C? The specific heat of water is 4.18 J/g·°C.

Use q = mcΔT: q = (25.0 g)(4.18 J/g·°C)(10.0 °C).

That works out to 1.05 × 103 J, or about 1.05 kJ.

3 free thermochemistry problems

Give each one a real try before you open the solution. Your answer is checked as soon as you submit it, and the worked solution shows every step so you can see exactly where a setup went wrong.

#1q = mcΔT
How much heat (in J) is required to raise the temperature of 50.0 g of water from 20.0°C to 35.0°C? (Specific heat of water: c = 4.18 J/g·°C)
#2Exothermic vs. Endothermic
In an exothermic reaction, which of the following is true about the flow of heat?
#3Heating Curve
A 40.0 g ice sample starts at -10.0°C. It is first warmed to 0°C, then completely melted. How much total heat is required? (c_ice = 2.09 J/g·°C; ΔH_fus = 334 J/g)

Questions you might have about thermochemistry

What is specific heat?
Specific heat is the energy needed to raise the temperature of 1 gram of a substance by 1 °C. Water's is 4.18 J/g·°C, which is unusually high, so water heats and cools slowly.
What is the difference between exothermic and endothermic?
An exothermic change releases heat to the surroundings, so the surroundings warm up and ΔH is negative. An endothermic change absorbs heat from the surroundings, so they cool down and ΔH is positive.
How do I solve a heating curve problem?
Split the curve into segments. On sloped segments use q = mcΔT, and on flat segments, where the phase changes, use q = mΔHfus or mΔHvap. Then add all the segments together.
What is Hess's law?
Hess's law says the total enthalpy change of a reaction is the same no matter how many steps it takes. You can add the ΔH values of steps, flipping the sign when you reverse a step and scaling when you multiply it.
High School Chemistry · Thermochemistry Practice Problems