gcse chemistry higher - bond energies and exo/endothermic reactions

Exothermic Reactions

-Release (heat) energy to the surroundings (e.g., combustion, acids and metals reacting).

- chemical energy converts into heat energy

-The products have lower energy than the reactants. EXOTHERMIC REACTIONS INCREASE IN TEMPERATURE.

Endothermic Reactions

-Absorbs energy from the surroundings (e.g., photosynthesis, thermal decomposition).

-The products have higher chemical energy than the reactants at the start.

-heat energy is converted into chemical energy. DECREASE IN TEMPERATURE.

Exothermic reaction graph

Reactants are at a higher energy level than products. The energy change (ΔH - how much energy given out) is negative as the products have less energy than reactants.

Endothermic reaction graph

Reactants are at a lower energy level than products. The energy change (ΔH - how much energy taken in) is positive, as products have more energy than reactants.

Activation Energy

-The minimum energy required to break chemical bonds in the reactants.

BREAKING IS ALWAYS ENDOTHERMIC AND POSITIVE (chemical bonds are strong so require energy to break)

-To MAKE chemical bonds is ALWAYS EXOTHERMIC (energy is released when chemical bonds are formed)

Calorimetry

A method for measuring energy changes in chemical reactions, typically using a calorimeter.

General energy formula

Q = mcΔT, where Q = heat/energy released (in J), m = mass of the water (in grams), c = specific heat capacity (4.2 J/g°c), ΔT = change in temperature (in °C).

Energy change in a reaction

Energy change = Energy required to break bonds - Energy released when new bonds form.

Endothermic reaction condition

If the energy required to break bonds is greater than the energy released, the reaction is endothermic.

Exothermic reaction condition

If the energy required to break bonds is smaller than the energy released, the reaction is exothermic.

making chemical bonds…

ALWAYS RELEASES ENERGY.

bond energy graphs

• activation energy with a CATALYST is lower.

• a catalyst provides an alternative reaction pathway with lower activation energy

products…

The substances formed when reactants have a chemical reaction

reactants…

The substances involved in a chemical reaction