# Hot Packs, Cold Packs and Airbags How chemical reactions absorb or release heat, from a cold pack and hand warmer to activation energy, collisions and an airbag inflating\. By Numa Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/vYxj0LOGvBkY) ## Transcript ### 1. Hot Packs, Cold Packs and Airbags The kit on this bench runs on chemical energy\. A reaction moves heat between the system, which is the chemicals, and the surroundings, which is everything else\. The enthalpy change, ΔH, is the heat a reaction takes in or gives out\. Negative ΔH means heat leaves the system\. Positive ΔH means heat flows in\. ### 2. Squeeze the cold pack Drag across the cold pack until the water bag bursts\. Ammonium nitrate dissolves, and this takes in heat from the water, the plastic and your hand\. A process that takes in heat is endothermic, and its ΔH is positive\. Cold does not flow into your hand: heat flows out of it\. ### 3. Measure the heat: q = mcΔT A calorimeter is a foam cup that keeps heat in, so the temperature change measures the heat\. The heat q equals m, the mass, times c, the specific heat capacity \(the heat that warms 1 g by 1 °C\), times ΔT, the final minus the starting temperature\. Here, 30 g of ammonium nitrate dissolves in 100 g of water\. The solution cools, so its ΔT and q are negative: it loses the heat the dissolving takes in\. ### 4. Shake the hand warmer Drag the hand warmer back and forth to shake it\. Air reaches the iron powder inside, and it rusts very fast\. Making the new bonds in iron oxide releases more energy than breaking the old bonds takes in\. A reaction that gives out heat is exothermic, and its ΔH is negative\. Salt water speeds up the rusting, and charcoal holds air and water close to the iron\. A warmer stays warm for 8 to 10 hours\. ### 5. The energy hill Watch the marble roll back, then turn on Catalyst\. Every reaction needs a push to start breaking bonds: the activation energy, Ea\. The marble gets the same push every time\. A catalyst lowers Ea and is not used up, so the marble goes over\. Car catalytic converters use platinum as a catalyst, and enzymes are catalysts in your body\. ### 6. Hotter means harder collisions Drag the temperature slider to see what heating does\. Particles react only when they collide hard enough to reach Ea and facing the right way\. This is collision theory\. A flash marks each collision hard enough\. Concentration, the number of particles in a given space, changes how often they meet\. Many real reactions double their speed for every 10 °C of heating\. ### 7. Sugar cube or sugar dust? Watch the sugar cube drop, then choose powdered sugar and compare\. Surface area is how much of a solid is open to the air\. More surface means more collisions each second\. The two samples have the same mass\. Each sample holds about 83 kJ of chemical energy\. ### 8. An airbag in 30 milliseconds Press the red crash button\. An igniter breaks sodium azide into sodium and nitrogen gas: 2NaN₃ → 2Na \+ 3N₂\. About 60 litres of nitrogen fill the bag in 30 milliseconds, slowed down here\. ΔH says which way heat flows\. Ea and surface area say how fast\. Further reactions lock away the reactive sodium, and the bag vents at once to cushion you\.