Hess's Law Lab

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Enthalpy change of magnesium in combustion
Rui Shen
Chemistry 4U – Ms .Brazier

Hess's law is a terminology in chemistry named after Germain Hess, a Swiss-born Russian chemist and physician who published it in 1840. The law states that the total enthalpy change during the complete course of a reaction is the same whether the reaction is made in one step or in several steps. Enthalpy cannot be directly measured, but rather the change in enthalpy. Enthalpy is described as the change in kinetic and thermal energy. Hess’s law states that the enthalpy change of a reaction will result in the same value regardless of what pathway is taken to achieve the products. In other words, only the start and end states matter to the reaction, not the individual steps between. This allows the change in enthalpy for a reaction to be calculated even when it cannot be measured directly. Chemical equations may be multiplied (or divided) by a whole number. When an equation is multiplied by a constant, its ΔH must be multiplied by the same number as well. If an equation is reversed, ΔH for the reaction must also be reversed. The addition of the equations can lead to the resulting net equation. If the net enthalpy change is negative, the reaction will be exothermic as it is releasing heat to the surrounding. If the net enthalpy change is positive, the reaction will be endothermic as it is absorbing heat from the surrounding. Calorimetry is the scientific experimental measurement of the change of heat of an object or substance between its system and the surrounding. A calorimeter is a tool (usually a container) in which the heat exchange experiment is conducted, and is used to measure the quantity of heat transferred to or from the object. Heat exchange is the process that describes how the average molecular kinetic energy of a system is transferred to another system. Using proper terminology regarding heat exchange from above, the equations can be deduced as ΔH = -Q for endothermic reactions, and -ΔH = +Q for exothermic reactions. Q in this case is the measure of the amount of heat released/absorbed, or rather the heat/energy gained by the surrounding and lost by the system, or vice versa. The equation to calculate the heat capacity is Q=mcΔT, where M is the mass, C is the specific heat capacity and Δt is the change in temperature. For the lab, four different reactions were listed. The net reaction, along with three other reactions was given: Net reaction: Mg(s) +½O2(g) → MgO(s)

2. Mg(s) +2HCl(aq) →H2 (g) + MgCl2(aq)
3. MgO(s) + 2Hcl(aq) → H20(l) + MgCl2(aq)
4. H2(g) + ½O2(g) → H2) (l) The enthalpy change for reaction two and three must be obtained experimentally, and the enthalpy change for reaction four was already given. To proceed, the first part of Hess’s law must be applied to solve for the change in enthalpy of the net reaction using reaction two, three and four. Overall, the goal of this lab is to determine the enthalpy change in the net reaction by using the subsequent three reactions.

To determine the molar enthalpy change of the combustion of magnesium in the reaction Mg(s) +½O2(g) MgO(s) through experimentally obtaining the enthalpy change for reaction two and three listed below. Then, using the enthalpy change of reaction two, three and four and applying the first part of Hess’s law to determine the enthalpy change of magnesium in the net equation. (2) Mgs+ 2HCl(aq)→H2(g)+MgCl2(aq)

(3) MgO(S)+2HCl(aq)→ H2Ol+MgCl2(aq)
(4) H2(g)+12O2(g)→H2Ol ∆H°f= -285.5KJ

The enthalpies of reaction two and three can be found experimentally by measuring the change in temperature of the reaction and using the equation:mc∆T. The mass can be determined by using the volume of HCl used and since hydrochloric acid is almost the same density as water which is this equation can be...
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