energy changes during chemical reactions

Energy Changes During Chemical ReactionsChemical reactions always result in the breaking of bonds between atoms and the formation of new bonds.Breaking bonds consumes energy (stores it)

Making bonds releases energy. By comparing the energy consumed by bond breaking to the energy released from bond making a determination can be made whether the net result is a storing of energy or a release of energy.

When NaOH is combined with water a large increase in temperature occurs. Where does the energy to heat up the water come from?NaOH(s) --------> NaOH(aq) + heatWhat kind of a solid is NaOH?It is an ionic solid made up of ions. This can be represented by the following diagram.

Na1+ OH1- Na1+ OH1- Na1+

Na1+ OH1- Na1+ OH1-OH1-



When crystals of NaOH are dropped into water, the polar H2O molecules are attracted to the ions in the crystal.

This pulls apart these ions forming a solution.

OH1-




Na1+ OH1- Na1+OH1-




Na1+ OH1- Na1+OH1-

Na1+



Na1+ OH1- Na1+ OH1-

Na1+ OH1- Na1+OH1-



Na1+ OH1- Na1+ OH1-

Na1+ OH1- Na1+OH1-

Na 1+



Na1+ OH1- Na1+ OH1-

Na1+ OH1- Na1+OH1-

Na 1+

Energy was consumed to break these bonds

Energy was released when these bonds are formed



Na1+ OH1- Na1+ OH1-

Na1+ OH1- Na1+OH1-

Na 1+

exceeds energy consumed to break these bonds this reaction is exothermic.Since energy

released by forming new bonds

Chemical reactions in which energy is released are called exothermic.Chemical reactions in which energy is stored are called endothermic.When ammonium nitrate is dissolved in water the temperature drops significantly. Explain this observation in terms of bond making and bond breaking and the energies involved in each process.

The energy required to break the bonds between ammonium ions and nitrate ions is very large and this quantity of energy exceeds the energy released when the freed ions become surrounded by water molecules (hydrated). Since the energy of bond breaking exceeds the energy of bond making the net result is a decrease in the temperature of the solution. In effect energy is being stored in the hydrated ions of ammonium and nitrate.

This storing and releasing of energy in bond breaking and bond making respectively can be visualized by a simple elastic band.Breaking bonds is like stretching an elastic band, energy is being stored.Forming new bonds is like releasing a stretched out elastic band.

Energy is being stored bonds are being brokenEnergy is released new bonds are being formed

The amount of total energy stored inside substances cannot be determined, however the differences in the amounts of energy stored in reactants and products can be determined in a number of ways. One experimental method is called calorimetry. Energy change is deduced by measuring temperature change.

Example - when 50.0 mL of a 0.50 kmol/m3 hydrochloric acid solution is added to 50.0 mL of a 0.50 kmol/m3 sodium hydroxide solution a temperature change occurs which can be measured. The difference in the amount of stored energy in the reactants and the products can be deduced from this temperature change.If the change in temperature (T) was 4.5 oC the change in energy (E) can be calculated by using this formula:

ΔE = mcΔt where c is the specific heat capacity of the substance absorbing the heatm is the mass of the substance absorbing the heat Δt is the change in temperature in Kelvin

In this example the substance absorbing the heat is the salt water mixture. Its specific heat capacity, c is 4.18 J/gK and its density is 1.12 g/mL.

ΔE = mcΔtΔE = (100 mL x 1.12 g/mL)(4.18 J/gK)(4.5 oC)ΔE = 2107 Joules (J)Molar Heat is the quantity of energy released per mole of reactant. In this reaction:

NaOH + HCl ======> H2O + NaClcalculate the number of moles of each reactant used. The nHCl isn = CV = 0.5 mol/L x 0.050 L = 0.025 molnNaOH isn = CV = 0.5 mol/L x 0.050 L = 0.025 mol

If 0.025 moles of each reactant produces 2107 Jthen 1.0 mole of reactant would produce 2107 J / 0.025 moles = 84 kJ / mol of reactant

This quantity can be included in the balanced chemical equation

NaOH + HCl ======> H2O + NaCl

If the temperature change is positive (temperature rises) it means stored energy was released, and there is less energy stored in the products than in the reactants. In this case the energy is written on the product side of the balanced chemical equation creating what is called a thermochemical equation.

NaOH + HCl ======> H2O + NaCl + 84 kJ

Practice Problem:When 60.0 mL of a 0.50 kmol/m3 sulfuric acid solution is added to 75.0 mL of a 0.70 kmol/m3 potassium hydroxide solution a temperature increase of 5.2 oC occurs. Write a balanced thermochemical equation. Density of mixture is 1.12 g/mL and c = 4.23 J/gK.

Write balanced thermochemical equations for the following:1. If 355 mL of a 0.15 mol/L solution of Ca(OH)2 is combined with

245 mL of a 2.4 mol/L solution of phosphoric acid a temperature increase of 5.8 oC results. Density of mixture is 1.12 g/mL and c = 4.23 J/gK.

(9.3 x 102 kJ released)2. If 35 mL of a 0. 45 mol/L solution of ammonium thiocyanate is

combined with 65 mL of a 0.76 mol/L solution of barium hydroxide a temperature decrease of 14.8 oC results. Density is 1.2 g/mL c = 4.28 J/gK(9.7 x 102 kJ absorbed)

3. If 855 mL of a 0.15 mol/L solution of Ca(OH)2 is combined with 245 mL of a 2.4 mol/L solution of perchloric acid a temperature increase of 6.8 oC results. Density is 1.17 g/mL and c = 4.22 J/gK(288 kJ released)

4. When 4.5 g of ammonium nitrate is dissolved in 45 mL of aqueous solution a temperature decrease of 9.2 oC occurs. Density of mixture is 1.07 g/mL and c = 4.18 J/gK(33 kJ absorbed

• When 6.5 g of NaOH is dissolved in 76 mL of aqueous solution a temperature increase of 14.4 oC occurs. Density is 1.1 g/mL and c = 4.1 J/gK• (30 kJ released)

Molar Heat of Combustion of Paraffin WaxDesign an experiment.Submit the procedure for assessmentParaffin is C25H52

energy changes during chemical reactions

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