Chapter 14: Introducing Energy Changes in Reactions.
Making new bonds is an: Breaking bonds is an:
exothermic reaction new bonds have to made for the products. breaking bonds need energy - energy released when new bonds are made. endothermic reaction
Endothermic reaction:
heat energy is absorbed/taken in. may be in the form of light, heat taken from the surroundings, or electrical energy. - this course focuses only on heat.
Exothermic reaction.
heat energy is given out
Examples of endothermic reactions.
heating a carbonate constantly to make it react. Most carbonates split up to give the metal oxide and carbon dioxide when you heat them. - thermal decomposition - breaking something up by heating them.
Examples of a endothermic reaction.
heating calcium carbonate - breaking up the original bonds takes a lot more energy than you get out when the new ones are made. - reaction is finished and more energy has been absorbed than is released again.
Examples of reaction of a metal with an acid.
magnesium and dilute sulfuric acid - mixture gets very warm. Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) ∆H = -466.9 kJ/mol
Describe what happens when you add water to calcium oxide.
CaO(s) + H₂O(l) → Ca(OH)₂(s) heat produced - enough to boil the water and produce steam. Calcium hydroxide produced.
Examples of endothermic reactions: copper(II)carbonate. zinc carbonate. Calcium carbonate.
CuCO₃(s) → CuO(s) + CO₂(g) copper(II)carbonate is a green powder which decomposes on heating to produce black copper(II)oxide. ZnCO₃(s) → ZnO(s) + CO₂(g) zinc carbonate - white powder decomposes on heating to give zinc oxide - yellow when it is hot, white on cooling. Calcium carbonate to decompose has to be heated at high temperatures. Important reaction - convert limestone (calcium carbonate) into quicklime (calcium oxide). CaCO₃(s) → CaO(s) + H₂(g) ∆H = +178 kJ/mol
Any reaction that produces a flame must be exothermic. Burning things produces heat energy. What type of reaction is this?
combustion reaction (burning in oxygen) Example: testing for hydrogen by lighting it and getting a squeaky pop - energy is released - exothermic change.
Example of neutralization reaction.
neutralization reactions means when an acid is added to an alkaline until neutral: sodium hydroxide (alkaline) and dilute hydrochloric acid (acid): NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(g) temperature rises. page 74 to read more.
Endothermic change on energy diagram.
products have more energy than reactants. taken from surroundings the extra energy ∆H = energy absorbed. - positive sign. for decomposition of carbonates, extra energy comes for the bunsen burner, or fuel in the lime kiln. (limestone gets heated in a limekiln to produce calcium oxide (quicklime) ).
Calcium oxide is known as ______________ Adding water to it is described as: The calcium hydroxide produced is known as:
quicklime. slaking it. slaked lime.
In an exothermic reaction, the reactants or the products : which has more energy?
reactants as reaction happens, energy is given out in the form of heat. That energy warms up both the reaction and surroundings.
Give a list of exothermic reactions:
reactions of metals with acids. neutralization reactions. adding water to calcium oxide.
Example of a reaction that involves making new bonds.
when hydrogen burns in oxygen to make water: 2H₂(g) + O₂(g) → 2H₂O(l) energy has to be supplied to break the bonds in hydrogen molecules and in oxygen molecules. new bonds formed between hydrogen and oxygen atoms in water molecules - energy released. more energy released when new bonds formed than was used to break the original ones - surplus energy = heat.
What is the symbols for amount of heat energy released?
∆H given a plus or minus sign to show wether heat is being given out or absorbed by reaction. exothermic - ∆H is negative - reactants are losing energy. - heat transferred to surroundings. ∆ = change in heat. unit of measurement = kJ/mol (kilojoules per mole)
