Thermochemistry Equations & Formulas - Lecture Review & Practice Problems

Summary

This video explains key concepts in thermochemistry, focusing on internal energy, heat transfer, and work in chemical systems. It covers equations like the change in internal energy (ΔE = Q + W), heat energy absorption and release, and work done by a system. Examples include calculating energy changes during gas expansion and phase transitions, such as melting ice. The video also explores using Hess's law to estimate enthalpy changes in reactions and provides various equations and conversion factors. It offers practical examples and calculations to deepen understanding of energy interactions in chemical processes.

Takeaways

• 😀 The change in internal energy (ΔE) of a system is calculated using the formula ΔE = Q + W, where Q is heat and W is work.
• 😀 Heat (Q) flows from hot to cold, so when heat is released by the system, Q is negative, indicating an exothermic process.
• 😀 Work (W) is given by the equation W = PΔV, where P is pressure and ΔV is the change in volume. Work is positive when done on the system, and negative when done by the system.
• 😀 When a gas expands, it performs work (W is negative), while compression of gas requires work (W is positive).
• 😀 For energy conversion, 1 liter·atm is equal to 101.3 joules, and you can use this to convert units when calculating work done by or on a gas.
• 😀 When calculating heat energy (Q), the formula Q = mcΔT is used, where m is mass, c is specific heat capacity, and ΔT is the change in temperature.
• 😀 To calculate heat energy required to change the temperature of water, use specific heat capacity values, such as 4.184 J/g°C for water.
• 😀 For phase changes, such as melting or vaporization, heat energy (Q) is calculated using Q = mΔH, where ΔH is the enthalpy of fusion or vaporization.
• 😀 The heat of fusion for water is about 6 kJ/mol, which means 6 kJ of energy is required to melt one mole of ice.
• 😀 In thermochemical equations, the enthalpy of reaction (ΔH) can be calculated by using the heat of formation of reactants and products, as in the formula ΔH = Σ(ΔH products) - Σ(ΔH reactants).
• 😀 Hess's Law allows you to estimate the enthalpy of a reaction by manipulating and combining multiple thermochemical equations.

Q & A

What does the equation ΔE = Q + W represent in thermochemistry?

This equation represents the change in the internal energy (ΔE) of a system. It is the sum of heat energy (Q) added to or removed from the system and the work (W) done by or on the system.

What does a negative value of Q indicate in thermochemistry?

A negative value of Q indicates that heat is released from the system to the surroundings, which is an exothermic process.

When is work (W) positive in thermochemistry?

Work (W) is positive when work is done on the system, causing the internal energy of the system to increase.

How does the expansion of a gas affect the work done in a system?

When a gas expands, the work done is negative because the gas exerts force and does work on the surroundings. This results in a decrease in the internal energy of the system.

What happens to the internal energy when a gas is compressed?

When a gas is compressed, the change in volume is negative, and the work done is positive. This leads to an increase in the internal energy of the system.

How is heat (Q) measured in thermochemistry?

Heat (Q) is typically measured in joules (J), with conversion factors like 1 kilojoule (kJ) = 1,000 joules and 1 calorie = 4.184 joules.

What is the specific heat capacity of water, and what does it indicate?

The specific heat capacity of water is 4.184 J/g°C. It means that it takes 4.184 joules of energy to raise the temperature of 1 gram of water by 1°C.

What is the heat of fusion, and how is it used to calculate the energy required to melt ice?

The heat of fusion is the amount of energy required to change a substance from a solid to a liquid at its melting point. For water, it is 6 kJ per mole. To calculate the energy needed to melt ice, you convert grams of ice to moles and then multiply by the heat of fusion.

What is Hess's Law, and how does it apply to calculating the enthalpy of a reaction?

Hess's Law states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps. This allows you to calculate the enthalpy of a reaction by manipulating and combining known reactions.

In a combustion reaction, how can you determine how much energy is released if you know the amount of oxygen used?

To determine the energy released, convert grams of oxygen to moles, then use the mole-to-energy ratio from the balanced combustion equation to calculate the heat energy released.