Chapter 6 Checmial Reactions And Energy Flashcards
When does chemical reaction occur?
Chemical reactions occur when atoms of reactants are rearranged to form new products. (E.g, fireworks, burning candles and burning fuels are examples transferring energy as heat and light)
Understanding exothermic change
Exothermic process refers to heat releasing process with temperature of the surrounding increases. (E.g, burning candle, reaction between iron wool and oxygen, reaction between calcium oxide an water.)
During collision, energy is absorbed from the surroundings to break bonds
During the formation of new bonds, energy is released to the surroundings, usually in the form of heat.
If the energy released to the surroundings during the bond-making process is higher than the energy absorbed during the bond-breaking process, the reaction is exothermic.
Understanding endothermic change
Endothermic process refers to heat absorbing from surrounding and causing the temperature from the surroundings to decrease.
The most important endothermic reaction for life is photosynthesis. More energy is absorbed by green plants from the Sun than is given out when glucose and oxygen are made.
When citric acid and sodium hydrogencarbonate powder are mixed, the temperature decreases. Cold packs make use of this.
If the energy absorbed from the surrounding to break the reactant bonds is higher than the energy released on forming new product bonds, the process is endothermic.
Melting ice and evaporating water is a typical endothermic physical process.
The science behind a cold pack
An instant cold pack relies on an endothermic reaction. Each pack contains water and ammonium nitrate (NH4NO3) in a sealed separated chamber.
Squeezing the bag causes the water and ammonium nitrate to mix. The ammonium nitrate dissolves, and the mixture cools rapidly. This is because the energy absorbed to break the chemical bonds is greater than the energy released on forming new bonds.
All the different variables
Independent variable: the reason of why the independent variable is changed
Dependent variable: the things that you are trying to measure and what is changed in the experiment
Controlled variable: Something that is kept the same in the experiment
