GCSE Chemistry revision

Greenhouse gases

Carbon dioxide and methane as greenhouse gases

AQA 4.9.2.1 Foundation & Higher

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Short-Wavelength Radiation Pass-Through: High-energy, short-wavelength electromagnetic radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it up.
Example: Visible light and ultraviolet rays from the Sun travel straight through atmospheric greenhouse gases without being significantly absorbed.
Long-Wavelength Infrared Emission: The warmed Earth re-emits this absorbed energy back out toward space as longer-wavelength infrared radiation (thermal energy).
Example: On a sunny day, the ground warms up and radiates heat upward as thermal infrared energy, which has a much longer wavelength than incoming sunlight.

1. Solar Radiation and Earth's Thermal Emission AQA

Short-Wavelength Radiation Pass-Through: High-energy, short-wavelength electromagnetic radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it up.
Example: Visible light and ultraviolet rays from the Sun travel straight through atmospheric greenhouse gases without being significantly absorbed.
Long-Wavelength Infrared Emission: The warmed Earth re-emits this absorbed energy back out toward space as longer-wavelength infrared radiation (thermal energy).
Example: On a sunny day, the ground warms up and radiates heat upward as thermal infrared energy, which has a much longer wavelength than incoming sunlight.

Selective Absorption of Thermal Energy: Greenhouse gas molecules—specifically carbon dioxide (CO₂) and methane (CH₄)—absorb this outgoing long-wavelength thermal radiation because its frequency matches their molecular bonds.
Example: Methane molecules in the atmosphere absorb thermal radiation very efficiently, making methane a highly potent greenhouse gas per molecule compared to carbon dioxide.
Omnidirectional Re-Emission: After absorbing thermal energy, carbon dioxide and methane molecules re-emit infrared radiation in all directions, including back down toward the Earth's surface and lower atmosphere.
Example: Burning fossil fuels releases CO₂ into the atmosphere, creating a denser layer of gas molecules that absorb outgoing thermal energy and re-radiate a portion of it back to the surface.

2. Absorption and Re-Emission by Greenhouse Gases AQA

Selective Absorption of Thermal Energy: Greenhouse gas molecules—specifically carbon dioxide (CO₂) and methane (CH₄)—absorb this outgoing long-wavelength thermal radiation because its frequency matches their molecular bonds.
Example: Methane molecules in the atmosphere absorb thermal radiation very efficiently, making methane a highly potent greenhouse gas per molecule compared to carbon dioxide.
Omnidirectional Re-Emission: After absorbing thermal energy, carbon dioxide and methane molecules re-emit infrared radiation in all directions, including back down toward the Earth's surface and lower atmosphere.
Example: Burning fossil fuels releases CO₂ into the atmosphere, creating a denser layer of gas molecules that absorb outgoing thermal energy and re-radiate a portion of it back to the surface.

Trapping Heat in the Atmosphere: By continuously absorbing and re-emitting thermal energy, greenhouse gases reduce the overall rate at which thermal energy escapes into space, keeping heat locked within the atmosphere.
Example: Much like a blanket trapping body heat, higher concentrations of CO₂ from industrial processes and CH₄ from livestock farming decrease heat loss to space.
Enhanced Greenhouse Effect and Warming: As human activities increase the atmospheric concentrations of carbon dioxide and methane, more thermal energy is trapped than can escape, causing global average temperatures to rise (global warming).
Example: Decomposing waste in landfills releases methane gas into the atmosphere, contributing directly to the enhanced greenhouse effect and driving rising global temperatures.

3. Reducing Thermal Energy Loss to Space and Global Warming AQA

Trapping Heat in the Atmosphere: By continuously absorbing and re-emitting thermal energy, greenhouse gases reduce the overall rate at which thermal energy escapes into space, keeping heat locked within the atmosphere.
Example: Much like a blanket trapping body heat, higher concentrations of CO₂ from industrial processes and CH₄ from livestock farming decrease heat loss to space.
Enhanced Greenhouse Effect and Warming: As human activities increase the atmospheric concentrations of carbon dioxide and methane, more thermal energy is trapped than can escape, causing global average temperatures to rise (global warming).
Example: Decomposing waste in landfills releases methane gas into the atmosphere, contributing directly to the enhanced greenhouse effect and driving rising global temperatures.