Chemistry: Heat
Endothermic
describing a process that involves the absorption of energy from the surroundings by substances undergoing change
Exothermic
describing a process that involves the release of energy into the surroundings by substances undergoing change
Which description explains events that take place during the mixing? Select all that apply.
-Collisions result in transfer of energy from molecules of higher kinetic energy to molecules of lower kinetic energy. -Warm water molecules slow down while cold water molecules speed up.
Which statement about exothermic reactions is accurate?
As reactants form products, the potential energy decreases.
Why does pressure change in this way? Select all that apply.
Kinetic energies of molecules increase. Speeds of molecules increase. Number of collisions per second increase.
The diagram below shows the temperature dropping from 80°C to 20°C. Molecules that have longer arrows are moving faster. Which statement describes what is happening in this system?
Kinetic energy decreases as temperature decreases.
A student touches a freshly baked cake that was taken out of the oven. Which best explains why the cake feels hot?
Molecules in the cake are moving faster than molecules in the skin.
Why does a hot water bottle feel warm?
Molecules in the hot water bottle are moving faster than molecules in the skin.
A baker touches a pie right after taking it out of the oven. Which statement best explains why the pie feels hot?
Molecules in the pie move faster than molecules in the skin, so heat is transferred to the skin.
Why does a glass containing an iced beverage feel cold?
Molecules in the skin are moving faster than molecules in the glass.
Which reaction is endothermic?
N2O4 + 59 kJ mc019-4.jpg →2NO2
Heat that flows by conduction is the transfer of thermal energy between substances in contact. What must occur for this to happen?
One system must have higher kinetic energy than the other system.
The temperature of a gas is increased. Which statement best explains the effect that this has on the motion of gas particles?
The average kinetic energy increases, and the particles collide more frequently.
Two different gases are stored in separate closed containers. If both gases have a temperature of 50°C, which statement is true?
The gas molecules in both containers have the same average kinetic energy.
The temperature of a sample of liquid water changes from 50°C to 30°C. Which statement best explains the change that must happen at the molecular level?
The molecules move more slowly and their average kinetic energy decreases.
How does the pressure change if you heat a gas enclosed in a rigid container?
The pressure increases.
Conduction
a heat transfer that involves direct contact between two substances so there can be collisions between molecules
Thermal Energy
the kinetic energies of the molecules making up a substance; energy that can flow as heat
Temperature
the measure of the average kinetic energy of the molecules making up a substance
Which event is endothermic?
the melting of ice
Heat
the transfer of kinetic energy between molecules as faster-moving molecules collide with slower-moving molecules; energy that flows from a warmer object or substance to a cooler object or substance
If temperature is varied for a fixed quantity of gas...
volume varies if pressure is constant pressure caries if volume is constant both can vary simultaneously
Four variables that describe a gas
volume, pressure, temperature, quantity of particles
