# Enduring Understanding 5.A.2: Kinetic Energy Transfer

• In two bodies at different temperatures, the molecules/particles in the warmer body have a higher average kinetic energy than those in the cooler body.
• When the two bodies are in contact, collisions between molecules transfer kinetic energy.
• On average, more energy is transferred from the warmer body to the cooler one, until they reach the same temperature. This is called thermal equilibrium.
• This is described as 'energy transferred as heat'.
• Different substances have different heat capacities - the amount of energy required to increase a given amount of a substance by a given amount of temperature.
• Therefore, a given amount of energy can produce different temperature changes in the same amount of different substances.
• Specific heat capacity is the amount of energy required to increase the temperature of 1 g of a substance by 1 °C.
• The kinetic energy content of a solid consists of vibrational motion, while that of liquids and gases consists of vibrational translational, and rotational motion.
• Sample question 1: 50 g of water (heat capacity: 4.2 J/g.°C) at 0 °C and 50 g of gold (heat capacity: 0.13 J/g.°C) at 100 °C are placed in thermal contact. Once thermal equilibrium is reached, will the final temperature of the system be closer to 0 °C or 100 °C?
• The heat capacity of water is much higher than that of gold, so it takes more heat to increase one gram of water one degree Celsius than would be released by one gram of gold decreasing by one degree Celsius. So the final temperature of the system will be much closer to 0 °C than to 100 °C.
• Sample question 2: The temperature of 48 g of a metal increases from 20 °C to 45 °C after absorbing 400 J of energy. What is the heat capacity of the metal?
• Heat capacity is defined as Cp = q/(m•ΔT)
• Temperature change ΔT is 45 - 20 = 25 °C
• Therefore, 400 J / (48 g • 25 °C)
• Cp = 400/1200 = 0.333 J/g•°C
• The heat capacity of the metal is 0.333 J/g•°C

 Related Links: Chemistry Chemistry Quizzes AP Chemistry Notes Temperature

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